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A&A565,A33(2014) Astronomy DOI:10.1051/0004-6361/201321757 & (cid:2)c ESO2014 Astrophysics A stellar population synthesis model for the study of ultraviolet star counts of the Galaxy AnantaC.Pradhan1,DevendraK.Ojha1,AnnieC.Robin2,SwarnaK.Ghosh1,3,andJohnJ.Vickers(cid:2),4 1 TataInstituteofFundamentalResearch,HomiBhabhaRoad,400005Mumbai,India e-mail:[email protected] 2 InstitutUtinam,CNRSUMR6213,OSUTHETA,UniversitédeFranche-Comté,41bisavenuedel’Observatoire25000Besançon, France 3 NationalCentreforRadioAstrophysics,TataInstituteofFundamentalResearch,411007Pune,India 4 Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr, 12-14, 69120 Heidelberg, Germany Received23April2013/Accepted11March2014 ABSTRACT Context.GalaxyEvolutionExplorer(GALEX),thefirstallskyimagingultraviolet(UV)satellite,hasimagedalargepartofthesky providinganexcellentopportunityforstudyingUVstarcounts.Combiningphotometryfromthedifferentwavelengthsintheinfrared (fromWide-fieldInfraredSurvey(WISE)andTwoMicronAllSkySurvey(2MASS))toUVallowsustoextractarealstarcatalogue fromtheGALEXsourcecatalogue. Aims.TheaimofourstudyistoinvestigateindetailtheobservedUVstarcountsobtainedbyGALEXvis-à-visthemodelsimulated cataloguesproducedbytheBesançonmodelofstellarpopulationsynthesisinvariousGalacticdirections,andtoexplorethepotential for studying the structure of our Galaxy from images in multiple near-UV (NUV) and far-UV (FUV) filters of the forthcoming UltravioletImagingTelescope(UVIT)tobeflownonboardAstrosat. Methods.Wehave upgraded the Besançon model of stellar population synthesis to include the UV bands of GALEX and UVIT. Depending ontheavailabilityofcontiguous GALEX,SloanDigitalSkySurvey(SDSS),WISE,and2MASSoverlapping regions, wehavechosenasetof19GALEXfieldswhichspreadoverarangeofGalacticdirections.Weselectedasampleofobjectsfrom theGALEXdatabaseusingtheCASjobsinterfaceandthencross-matchedthemwiththeWISE+2MASSandSDSScatalogues.The UVstarsintheGALEXcatalogueareidentifiedbychoosingasuitableinfrared(IR)colour,J−W1(W1isaWISEbandat3.4μm), whichcorrespondstoatemperaturerangefrom1650Kto65000K.TheIRcolourcutmethod,whichisusedforthefirsttimefor separationofstars,isdiscussedincomparisonwiththeGALEX+SDSSstarcountsmethod. Results.WepresenttheresultsoftheUVstarcountsanalysiscarriedoutusingthedatafromGALEX.WefindthattheBesançon modelsimulationsrepresenttheobservedstarcountsofboththeGALEXAll-skyImagingSurveyandMediumImagingSurveywell withintheerrorbarsinvariousGalacticdirections.BasedontheanalysisofthemodelFUV−NUV colour,weseparatedoutwhite dwarfsofthediscandbluehorizontalbranchstarsofthehalofromtheobservedsamplebyselectingasuitableFUV−NUVcolour. Conclusions.TheBesançonmodelisnowreadyforfurthercomparisonsintheUVdomainandwillbeusedforprospectivestudies fortheUVITinstrumenttobeflownonboardAstrosat. Keywords.stars:general–ultraviolet:stars–Galaxy:disk–Galaxy:stellarcontent–Galaxy:halo 1. Introduction in predicting the different structural parameters of the Galaxy, such as stellar densities, scale length,scale height,etc. Among The Milky Way is the best studied Galaxy in the universe; its themodelsdevelopedtounderstandtheGalacticstructureusing structureandevolutionhavebeenstudiedusingavarietyoftech- starcountingmethods,onecancite:Bahcall&Soneira(1980), niques. In the early 20th century, Kapteyn (1922) first studied Gilmore & Reid (1983), Robin & Crézé (1986), Robin et al. the geometrical structure of the Galaxy using the star counts (2003), Girardi et al. (2005) and Juric´ et al. (2008). However, methodwherebyhecountedstarsonthephotographicplatesin theaboveGalaxymodelsarepredominantlybasedonthevisible selectedareasofthesky.Sincethenthestarcountsmethodhas and infrared (IR) photometric surveys. Very few attempts had beenusedasoneofthepreferredmethodstoconstrainthestruc- been made to study the star counts in ultraviolet (UV) prior to turalparametersoftheGalaxyeffectively.Severalreviewshave GalaxyEvolutionExplorer(GALEX) becauseof lack of avail- discussedtheconnectionofstarcountstotheGalacticstructure ability of UV photometric surveys (Brosch 1991; Cohen et al. (Bahcall 1986; Freeman 1987; Gilmore et al. 1988; Majewski 1994).TheadventofGALEX,whichprovidedawideskycov- 1993; Helmi 2008; Ivezic´ et al. 2012). The advent of instru- eragein UV, nowallowsfora newanalysisofthe UVsky(Xu mentswithbetterresolutionandgreatersensitivityhaveenabled etal.2005;Bianchietal.2011a,b,2014, amongothers).Anat- ustoobtainphotometricobservationscoveringlargepartsofthe tempthasalsobeenmadetopredictthestarcountsintheX-ray skyinseveralwavelengthbands.Thepopulationsynthesismod- band(Guilloutetal.1996)byextendingtheBesançonmodelof els of the Milky Way are well supportedby these observations stellarpopulationsynthesis(Robin&Crézé1986)totheROSAT (cid:2) Member of the International Max-Planck Research School for positionsensitiveproportionalcounterenergybands. Astronomy&CosmicPhysicsattheUniversityofHeidelberg. ArticlepublishedbyEDPSciences A33,page1of13 A&A565,A33(2014) Fig.1. Effective area versus wavelength plots for the GALEX FUV and NUV bands are shown in relation to the FUV and NUV filters of UVIT/Astrosatforimagingmode.TheleftpanelshowsthefiveFUVfiltersofUVIT/AstrosatindifferentcoloursalongwiththeGALEXFUV filter(dashedline)andtherightpanelshowsthesamefortheNUVfilters. Indeed, the UV surveys, among others, could help in trac- andNUVB4(NUV)wavebands,foranexposuretimeof200s, ingthespiralstructureswhichmainlycontainveryyoungstars. are 20.0and 21.2 mag, respectively(Astrosat Handbook2013; The UV surveysalso help in constraining the shape of the ini- priv.comm.). tialmassfunction(IMF)towardsthehigh-massstarendaswell It is worth mentioning here that throughout the paper we aselucidatingthe recentstar formationhistory.Moreover,they haveusedABsystemfortheGALEX,UVIT,andSloanDigital also trace very blue populations such as white dwarfs (WDs) SkySurvey(SDSS)datasets,whereastheTwoMicronAllSky and blue horizontalbranch stars (BHBs) deep in the halo pop- Survey (2MASS) and Wide-field Infrared Survey (WISE) data ulation, which in turn trace the streams and relics of ancient setsareintheJohnsonsystem(seeSect.2). accretion in the Milky Way halo. The GALEX has covered a WegivedetailsoftheobservationsandselectionofUVstars large partof the sky, which providesan opportunityto explore in Sect. 2. We describe the Besançon Galaxy model in Sect. 3 and characterise these hot sources in the far-UV (FUV: 1344– 1λ7eff86=Å2,3λ1e5ff.7=Å1)5w3a8v.6ebÅan)dasnwdintheabr-eUtteVr r(eNsUolVu:tio1n77a1n–d28g3re1atÅer, aGnAdLdEisXcu+sSsDthSeScostmarpacroiusonntsoifnthSeecGt.A4L.EWXe+pWreIsSeEn+t2thMeAcoSmSpaanrd- ison of the modelwith the observationsin Sect. 5, and discuss sensitivity than the previous surveys. A vivid description of thedistributionofthemodelstarcountsinSect.6.Wemention the source selection, FUV and NUV magnitudeerror cuts, and theidentificationofWDsandBHBsusingFUV −NUV colour the statistical analysis of the GALEX catalogue is providedby inSect.7.Finally,wesummariseourconclusionsinSect.8. Bianchietal.(2007),Bianchi(2009),andBianchietal.(2011a, 2014).DetectionofWDsandBHBsisoneofthemainachieve- mentsofGALEXasthesesourcesareelusiveintheotherwave- lengthbandsof the electromagneticspectrumdue to their high temperature.ThepopulationofWDsandBHBsisintegraltothe 2. Observationsandcross-correlationofGALEX studyofstellarevolutionandstructureoftheMilkyWayasthey sources belongtodifferentstellarpopulationsoftheGalaxy. We have upgraded the Besançon model of stellar popula- 2.1.GALEXdata tion synthesis to include the UV bands of GALEX and the upcoming Ultraviolet Imaging Telescope (UVIT1) (which will TheGALEXwasanorbitingspacetelescopelaunchedinApril, be flown onboard Astrosat) to predict star counts in different 2003, which was terminated in mid-February,2012. The satel- parts of the sky (Todmal et al. 2010). The UVIT will image lite and on-orbit performance are described in Martin et al. the sky in the FUV (1300–1800 Å) and NUV (2000–3000 Å) (2005) and Morrissey et al. (2005, 2007). It observed the sky channels, each having five filters, at a high resolution of 1.8(cid:3)(cid:3) in two UV bands, FUV, and NUV, simultaneously, with a spa- (Postma et al. 2011; Kumar et al. 2012a,b). Better positional tial resolutionof 4.2(cid:3)(cid:3) and 5.3(cid:3)(cid:3), respectively.The field of view accuracy of UVIT as compared to GALEX will enable more is1.25◦ indiameterandthe imagesaresampledwith1.5(cid:3)(cid:3) pix- reliable cross correlation with other catalogues, which will be els. The typical AB magnitude limits (5σ depth) met by All- of greatutility in inferringthe Galactic structure using the star sky Imaging Survey (AIS) for an exposure time of 100 s and counts technique. The transmission curves (effective area ver- MediumImagingSurvey(MIS)foranexposuretimeof1500s suswavelength)forthe FUV and NUV bandsof GALEX with are 19.9/20.8(FUV/NUV) and 22.6/22.7(FUV/NUV), respec- each of the five FUV (left panel)and NUV (rightpanel)filters tively(Morrisseyetal.2007).TheAIShasthelargestskycover- of the upcoming UVIT/Astrosat are shown in Fig. 1. We have agewhencomparedtotheotherGALEXsurveys.TheGALEX included the effective area curves of both the GALEX and all observationshavecovereda largepartofthesky(∼75%),with the UVIT/Astrosat bands in the modelto simulate the UV star the exception of the Galactic plane and some regions of the counts in these bands. Apart from the GALEX bands, we will Magellanic Clouds due to safety concerns of the detectors. discuss the model simulated star counts of the BaF2 (FUV: In this paper, we have used the GALEX GR6 data which is 1370–1750Å,λeff =1504Å)andNUVB4(NUV:2505–2780Å, available in Multi-mission ArchiveatSpace TelescopeScience λeff =2612Å)bandsofUVIT/Astrosat.Theexpectedsensitivity Institute(MAST2). limits (5σ)in AB magnitudesystem in the UVIT BaF2 (FUV) 1 http://www.iiap.res.in/Uvit 2 http://galex.stsci.edu/GR6 A33,page2of13 AnantaC.Pradhanetal.:AModelfortheUVstarcountsoftheGalaxy Table1.DetailsoftheGALEXfields. Field Longitude Latitude Surveytype Area Location Numberoftiles NUVexposure FUVexposure range(deg) range(deg) (deg2) time(s) time(s) 1 47.79 –43.56 AIS 0.785 GC 1 175 175 2 129.33 –43.15 AIS 0.785 GAC 1 258 258 3 47.01 –42.65 MIS 0.785 GC 1 1589 1589 4 146.57 –46.51 MIS 0.785 GAC 1 1657 1657 5 1–15 50–60 AIS 69.9 GC 89 64–400 64–272 6 160–175 50–60 AIS 69.9 GAC 89 90–442 96–220 7 13–29 35–41 MIS 22.77 GC 29 1541–2176 1541–2176 8 154–162 38–44 MIS 18.85 GAC 24 1597–4457 1512–3066 9 230–240 42–50 AIS 46.34 GAR 59 80–438 61–231 10 40-50 75–85 AIS 14.92 GHL 19 62–292 62–249 11 30–32 21–29 AIS 13.35 GLL 17 105–421 105–230 12 0–20 84–88 AIS 4.71 GP 6 107–383 107–271 13 49–51 17–23 AIS 7.07 – 9 130–199 130–199 14 48–53 29–33 AIS 11.0 – 14 79–184 79–184 15 46–53 37–43 AIS 20.42 – 28 96–384 95–265 16 46–54 46–54 AIS 17.28 – 22 94–345 73–226 17 46–54 57–63 AIS 11.78 – 15 152–342 152–229 18 46–54 67–73 AIS 7.07 – 9 90–340 90–169 19 44–54 77–83 AIS 4.71 – 6 75–259 75–151 Notes.TheareasofdifferentfieldsarechosendependingontheavailabilityofGALEX,WISE+2MASS,andSDSSoverlappingregions. 2.2.SelectionofGALEXfields entire sky in the J (1.24 μm), H (1.66 μm), and K (2.16 μm) s near-IR (NIR) bands with angular resolutions of 2.9(cid:3)(cid:3), 2.8(cid:3)(cid:3), Wehaveselected19GALEXfieldsforwhichboththedetectors and 2.9(cid:3)(cid:3), respectively (Skrutskie et al. 2006);while WISE has of GALEX were turned on. We retained sources which had a mappedtheskyintheW1(3.4μm),W2(4.6μm),W3(12μm), reliableNUV detection,however,FUVdetectionsareavailable andW4(22μm)mid-IRbands,withangularresolutionsof6.1(cid:3)(cid:3), for∼3.5%and∼6.8%oftheNUVdetectionsintheselectedAIS 6.4(cid:3)(cid:3), 6.5(cid:3)(cid:3), and 12.0(cid:3)(cid:3), respectively (Wright et al. 2010). The and MIS fields, respectively. The rest of the NUV sources do 5σ point source sensitivities of the four WISE bands are bet- nothaveaFUVdetectionbecausetheirFUVfluxesaretoolow terthan0.08,0.11,1,and6mJy(equivalentto16.6,15.6,11.3, to be detected.We includeonlyregionswithin a radiusof0.5◦ and 8.0 Vega magnitude)in unconfusedregionson the ecliptic from the centre of the tiles to eliminate edge artifacts and bad (Wrightetal.2010).TheexistingWISE+2MASScross-matched sourcesalongtheedgeaswellastoavoidoverlappingareasand catalogue available at Infrared Science Archive (IRSA3) has duplication of the sources. The coverageareas of the observed been used for convenience. This catalogue has been produced fieldsarecalculatedbysumminguptheareasofallthetilesina using a 3.0(cid:3)(cid:3) matchingradius, which was foundto be adequate field. consideringthepositionalaccuracyandresolution. The fields are selected in the footprints of the GALEX, We have made use of the Virtual Astronomical SDSS, WISE, and 2MASS surveys. The various fields are as Observatory (VAO4) for cross-matching GALEX sources follows: with WISE+2MASS sources. The GALEX sources were – FourGALEXtileswerechosenatthesouthernGalacticlat- uploadedintotheVAO,seekingtheirWISEand2MASScoun- itudes:twoeachinAISandMIS. terpartsusingamatchradiusof3.0(cid:3)(cid:3).Wefoundmostofthereal – Eightfieldswithlargeareacoverageoftheskywerechosen matched sources within this radius, with a very small fraction in several northern Galactic directions. The fields include: (<1%)havingmultiplematches,whichwereremovedfromthe two regions towards the Galactic centre (GC) (one each in final catalogue. We also estimated the possible contamination AISandMIS),tworegionstowardstheGalacticanti-centre by spurious matches (random coincidences) for the matched (GAC) (one each in AIS and MIS), and one regioneach in sourcesfollowingthemethodofBianchietal.(2011a).Forthis AIStowardstheGalacticanti-rotation(GAR),Galacticlow purpose, we used a match radius of 6.0(cid:3)(cid:3), which is equivalent latitude (GLL), Galactic high latitude (GHL), and Galactic to the resolution of WISE, to find the GALEX counterpartsof pole(GP)directions. WISEsources.Thespuriousmatcheswerefoundtobe∼10%of – SevenfieldsinAISwerechosenat10◦intervalsofbaround the total matched sources, and 75% of these spurious matched l∼50◦tostudythelatitudevariationofUVstarcounts. sourcesliebeyondadistanceof3.0(cid:3)(cid:3). Thecentrecoordinates,surveytypes,areacoverages,locationin theGalaxy,numberofGALEXtiles,andtherangeofexposure 2.4.SDSSdata times of NUV and FUV observations of each of the fields are giveninTable1. So far, SDSS has mapped over 35% of the full sky in five optical photometric bands (u,g,r,i,z) covering the wavelength rangefrom3000to11000Å(Aiharaetal.2011).TheGALEX 2.3.WISE+2MASSdata The AIS and MIS of GALEX overlap with the 2MASS and 3 http://irsa.ipac.caltech.edu/Missions/wise.html WISE footprints. The 2MASS observations have covered the 4 http://vao-web.ipac.caltech.edu/applications/VAOSCC A33,page3of13 A&A565,A33(2014) Fig.2. J (2MASS)−W1(WISE)versusNUVCMDfortheGALEXandWISE+2MASScross-matchedsourcesfortheAISfieldstowardsthe GCandtheGAC.Thematchedsourcesareclearlyseparatedintwogroupsindicatingisolationofstars(J−W1 < 1.2)fromtheextra-galactic sources(J−W1>1.2).Theverticaldashedlineshowsthelimitthatwechooseforselectingthepointsources(J−W1<1.2).QSOsareshown bybluecrossedsymbols(seethetext). GR6 has been cross-matched against SDSS DR7 and the pro- 2.6.Photometricerrorcutsandcompletenesslimits videdcross-matchedtableisxSDSSDR7.Severalworkshaveex- plainedthecross-matchingoftheGALEXcataloguewithSDSS, Figure3 showsthe distributionof UV-IRstars asa functionof astrophysical source classifications and related statistical anal- theGALEXUVmagnitudesforAISandMIS.StarswithNUV yses (Seibert et al. 2005; Budavári et al. 2009; Bianchi et al. andFUVmagnitudeerrorslessthan0.5,0.4,0.3,0.2,and0.15 2007, 2011a). We uploaded the GalexIDs of the objects into are displayed with magenta, green, red, blue, and cyan colour theGALEXCASjobs5SQL(StructuredQueryLanguage)inter- lines respectively, whereas the black line represents the stars facetodeterminetheirSDSScounterpartsina searchradiusof withoutanymagnitudeerrorcut.Thetypical5σmagnitudelim- 3.0(cid:3)(cid:3).Wehaveeliminatedthemultiplematches(<1%)fromthe itsoftheNUVandFUVbandsforAISandMIS(seeSect.2.1) GALEX+SDSSfinalcatalogue.Theestimatedspuriousmatches areshownbyverticaldashedlines.Asseenfromthehistograms, incaseofGALEX+SDSSarefoundtobe∼7%within3.0(cid:3)(cid:3) ra- aprogressivestringenterrorcuteliminatesthefainterstars.The dius. The SDSS star/galaxy classifications have been adopted completenesslimitsneedtobeestablishedaccordingtoagiven while performingthe match to separate outpoint sourcesfrom magnitudeerror.Ifweconsiderallstarswithoutaccountingfor thesourcelist. errors,thestarcountsgodeeperbuttheirvaluesarenotreliable becauseoftheuncertaintyonthemagnitudemeasurement.This The SDSS classified point sources (GALEX+SDSS) in- isparticularlytruefortheFUVfilterwheresomespuriousdetec- cludebothstars and quasi-stellarobjects(QSOs), outofwhich tionscanoccur.Finally,we retainedstarswithmagnitudeerror we selected QSOs using the SDSS colour cuts from Richards lessthan0.2inbothbandsasthiserrorcutgivesmagnitudelim- etal. (2002)andremovedthemfromtheGALEX+SDSSpoint itsalmostsimilartothetypical5σlimitsoftheGALEXbands sourcesandtermedthecleansampleas“GALEX+SDSSstars”. forAISandMISwhichareprovidedbyMorrisseyetal.(2007). We have applied magnitude error cuts (similar to the one 2.5.SelectionofstarsfromtheGALEXcatalogue showninFig.3forUV-IRstars)intheoriginalGALEXsource byIRcolourcutmethod cataloguethatincludesalltheGALEXdetections.Wehavealso appliedmagnitudeerrorcutsinthematchedGALEXsourcecat- Figure 2 shows J − W1 versus NUV colour–magnitude dia- alogue obtained after cross-matching with the WISE+2MASS gram(CMD)ofallGALEXsourcesthatarecross-matchedwith catalogue.WefindalossofGALEXsourcesinthematchedcat- WISE+2MASS sources for the regionsin the directionsof the alogue when compared with the original GALEX source cata- GC and the GAC, each covering 69.9 deg2 of the sky. The logue at a specific magnitudeerror cut. The completenesslim- QSO candidates are selected using the SDSS colour cuts from its for the original GALEX sources for NUV and FUV mag- Richardsetal.(2002)andarerepresentedbybluecrossedsym- nitude error cuts of 0.2 are 20.5/21.0mag (FUV/NUV) in AIS bols in the plot. We clearly see two groups of sources in the and22.5/22.5mag(FUV/NUV)inMIS.Thecompletenesslim- figure well separated by J − W1 colour. The stars verified by its at the same magnitude error cuts for the matched catalogue their SDSS classification as point sources in a cross-matched become20.0/20.5mag (FUV/NUV) in AIS and 22.5/22.0mag sample are identified to be bluer than J − W1 < 1.2 and the (FUV/NUV) in MIS, and these limits are the same for the extra-galacticobjects(e.g.,galaxies,QSOs,etc.)areredder,with UV-IR stars too. For a specific magnitude error cut, the FUV J−W1>1.2.SincethecontaminationbySDSS-identifiedQSOs and NUV completeness limits of the observed sources, which is estimated to be negligible in the J − W1 < 1.2 star counts arecross-matchedtothesurveysatlongerwavelengths,become (<0.1%ofthewholesample),wehaveusedtheJ−W1colourcut brighterthanthecompletenesslimitsoftheunmatchedGALEX procedureforall the fieldsto separate the stars from the extra- source catalogue because of the loss of faint sources in the galactic objects. Henceforth in the paper, we refer to GALEX former. andWISE+2MASScross-matchedsourceswith J −W1 < 1.2 In order to examine which objects are affected by the lim- (GALEX+WISE+2MASS)as“UV-IRstars”. itsofthecombinedsurveys(GALEX+WISE+2MASS),wesplit thestarsintotwoNUV−W1colourintervals:hot(NUV−W1< 5)andcool(NUV −W1 > 5)stars. We checkedthecomplete- 5 http://galex.stsci.edu/casjobs ness limitof the NUV band(AIS) for these two colourranges. A33,page4of13 AnantaC.Pradhanetal.:AModelfortheUVstarcountsoftheGalaxy Fig.3.DistributionoftheUV-IRstarsasafunctionoftheGALEXNUVandFUVmagnitudesforAISandMIS.Thestellarsourcesobtainedafter applyingvariousmagnitudeerrorcutsareshownbydifferentlinestylesincolours.Theverticaldashedlinesrepresenttherespective5σdetection limitsoftheGALEXbandsfortypicalexposuretimesasmentionedinSect.2. For hotstars, we foundthat the completenesslimit of GALEX function is normalised to fit Harris et al. (2006).Similarly, the NUV(AIS)isreducedby0.5mag(i.e.,theeffectivemagnitude BHBs are incorporated in the model by taking the evolution- limitgetsbrighter).TheGALEXcompletenesslimit(AIS)ofhot arytracksfromBaSTI(ABagofStellarTracksandIsochrones) starsisthereforelimitedbythedepthofWISE,andsimilarlyby models(Pietrinfernietal.2004).Ultimately,themodelproduces thedepthof2MASS.Forcoolstars, theNUV(AIS)complete- UV star countsby MonteCarlo simulationsusinga set ofstel- nesslimitisthesameintheGALEXcataloguealoneandinthe lar atmospheric models, observational photometric errors, and combinedcataloguewiththenear-IRsurveys. extinction. The modelincorporatesan extinction(A ) assuming an el- V lipsoidaldistributionofdiffuseabsorbingmatter,whichfollows 3. BesançonGalaxymodel an Einastoextinctionlaw andis depictedby anadjustable nor- malisation (extinction gradient) of 0.7 mag/kpc in the V band. The Besançonmodelis a populationsynthesismodelbased on WeproducedthemodelsimulationstowardsvariousGalacticdi- a scenario of Galactic evolution and constrained by dynam- rectionsassumingthe defaultextinctiongradient.However,the ics. In the model, five populations are taken into account: thin defaultvalueofdiffuseextinction(0.7mag/kpcintheV band), disc,thickdisc, stellarhalo,bar,andbulge(Robinetal.2012). whichmaynotbeappropriateatlowlatitudes,canbeadjustedby The previous versions of the model are extensively described addingafewabsorbingcloudswithagivenadhocdistanceand in Robinetal. (2003).We use the newestversionofthe model extinction from the Schlegel et al. (1998) maps. This has been (versionof April2013;Robinet al. 2012),whichhasbeen up- illustratedinSect.5.1.TheratiosbetweenUVbandtovisualex- gradedtoincludetheFUVandNUVpassbandsofGALEXand tinctionaretakentobe2.67and2.64fortheFUVandNUVband the upcomingUVIT/Astrosat,by applyingtheir filter transmis- ofGALEX,respectively,followingtheextinctionlawofCardelli sion curves to produce UV star counts in various Galactic di- etal.(1989). rections.Themodelusesasetofevolutionarytracks,astarfor- StarsinthesimulatedGALEXcataloguehaveaUVcolour, mationrateandanIMFasdefinedinHaywoodetal.(1997),to FUV, and NUV magnitudes,a temperaturerange from 1650K generatedifferentstellarpopulations.Thecoloursarecomputed to 65000 K, log g from –1 to 9, all luminosity classes and a from the Basel Stellar Library (BaSeL3.1) model atmospheres range of metallicities. In the simulations done for comparison (Westeraetal.2002).Inthisnewversionofthemodel,DAand with the GALEX observed star counts, the simulated stars are DBtypeWDsareincludedusingtheevolutionarytracksandat- mostly MS stars (∼77%) with a small contributionfrom giants mospheremodelsfromHolberg& Bergeron(2006).Thelumi- and subgiants (∼17%). The WDs are ∼6% of the sample and nosity functions are obtained assuming an initial-to-final-mass resideatthebluerendofFUV−NUV colour(seeSect.7). ratio (m = 9.1743m −3.6147) from Kalirai (2008). The dis- I f tribution in age is computed assuming a lifetime on the main sequence(MS)fromEggletonetal.(1989)andalifetimeonthe 4. ComparisonoftheGALEX+WISE+2MASS giantbranchof15%ofthetimeontheMS.TherepartitioninDA andGALEX+SDSSstars (WDwithhydrogenrichatmosphere)andDB(WDwithhelium richatmosphere)iscomputedassumingthatatTeff > 40000K Figure4 showsthe distributionofthe GALEX AISstar counts theyareallDA,andatTeff <20000K,50%areDBwithalinear (for field 5 in Table 1) as a function of NUV magnitude variationbetween20000Kand50000K.Thefinalluminosity for the model simulation (solid line), GALEX+SDSS stars A33,page5of13 A&A565,A33(2014) producedtoreducethestatisticalnoise.Appropriatephotometric errorswereappliedinthemodeltoproducerealisticsimulations andtheerrorinformationwasassumedfromtheobserveddata, whichisapolynomialfunctionofthemagnitude. We can simulate the catalogues using the “small field” op- tion, which assumes that the density does not vary across the field, or, we can simulate them using the “larger field” option with a given step in longitude and latitude, to account for the fieldswherethedensitycanvary.Weusedthe“smallfield”op- tion for the small fields (e.g., area <15 deg2) by providing the centre l and b coordinates of the fields along with their cover- agearea.Forthelargerfields(e.g.,area>15deg2),weusedthe “largerfield”optionwhereweprovidetherangeoflandbcoor- dinatesandastepsize(e.g.,from1.0◦ to2.5◦ forsmalltolarge fields,respectively)tocoverthefield.However,thegradientsin thefields(Table1)aresmallenoughthatconsideringeitherthe centreof the field orthe rangeofl/b doesnotmakeanydiffer- Fig.4. Distribution of the UV-IR stars (GALEX+WISE+2MASS: enceinthepredictedstarcounts. dashed-dotted line), GALEX+SDSS stars (dashed line), and model- simulated star counts (solid line) for the AIS field towards the GC, covering69.9deg2 ofthesky(field5inTable1).Thedottedlinerep- resentstheGALEX+WISEsourceswithno2MASScounterparts.The 5.1.ComparisonofobservedUVstarcountswiththemodel starcountsarebinnedin0.5magintervalinNUVmagnitude.Theerror invariousfields barsinthemodel-simulatedstarcountsareduetoPoissonnoise.The NUV5σdetectionlimit(NUVmagnitude=20.8)isshownbyasolid Initially, simulations were performed for four GALEX tiles verticalline.TheUV-IRstarcountsshowaturnoveratNUVmagnitude (fields1–4inTable1),eachcoveringanareaof0.785deg2atthe ∼20.5(demarcatedbyadashedverticalline). southern intermediate Galactic latitudes. We binned the model andtheUV-IRstarsin0.5magintervalsintheNUVband,forre- spectivetilesofAISandMIS,inthedirectionsoftheGCandthe (dashed line), UV-IR stars (GALEX+WISE+2MASS: dashed- GAC. As shownin Fig. 5,we foundthatthe modelstar counts dotted line) and GALEX+WISE star counts with no 2MASS (solid line) match the UV-IRstars (solid circles) as well as the detection (dotted line). The error bars shown in the model star GALEX+SDSSstars(opencircles)welluptothecompleteness counts are due to Poisson noise. The NUV 5σ detection limit limitsofAIS(20.5mag)andMIS(22.0mag)fortheregionsat (NUV magnitude =20.8; Morrissey et al. 2007) and the com- thesouthernintermediateGalacticlatitudes. pleteness limit (∼20.5 mag; see Sect. 2.5) for AIS are demar- cated by the solid and dashed vertical lines, respectively.Stars Inordertochecktheuniversalvalidityofthemodel,wepro- withNUVmagnitudeuptothecompletenesslimitarewellde- ducedsimulated cataloguesin variousdirectionsof the Galaxy tected by the GALEX, SDSS, WISE, and 2MASS surveys; a covering a large area of the sky. Figures 6a and b show the goodagreementinstarcountsamongthecross-matchedsurveys comparison of the model-predicted NUV star counts (solid andthemodelsimulationscanclearlybeseeninFig.4. line) with the UV-IR stars (solid circles) as well as with the It is also evidentfrom Fig. 4 that the GALEX+SDSS stars GALEX+SDSSstars(opencircles)forAISinthedirectionsof are slightly more than the UV-IR stars in the NUV band at the GC and the GAC, each covering 69.9 deg2 area of the sky the fainter magnitudes. This discrepancy could be caused by (fields 5–6 in Table 1). Similarly, Figs. 6c and d represent the 2MASSsincethetimegapbetweentheWISEand2MASSsur- comparison of star counts for MIS in the directions of the GC veys is ∼12 years, high proper motion stars may have moved andtheGAC,coveringanareaof22.77deg2and18.85deg2,re- outsidethecross-matchingradii.However,starswithpropermo- spectively(fields7–8inTable1).Thesefieldsarechosenatthe tionshighenoughtomoveby3.0(cid:3)(cid:3)in∼12yearsareveryrarein northernintermediateGalacticlatitudeoftheGalaxy.Theerror asurveyofafewsquaredegrees.Anotherpossibilityisthatthe barsshowninthemodel-predictedstarcountsareduetoPoisson 2MASS J band,which hasa 10σpointsource sensitivitylimit noise.Themaximumestimatedasymmetricerrorintheobserved ofabout15.8mag,doesnotpenetratedeeplyenoughtoprovide counts is ∼2%–10%, depending on the NUV magnitude bins counterparts for all WISE detections. Though GALEX+SDSS (i.e.,errorincreasestowardsthefaintermagnitudebins),which has a smaller sky area coverage and a fainter limit compared is not shown in the plots. The model shows a good agreement to GALEX+WISE+2MASS, both the selections yield a close with the observation (UV-IR stars and GALEX+SDSS stars) match of the star counts at the brighter end. It is also possi- downtoanNUVmagnitudeof∼20.5forAISand22.0forMIS blethattheGALEX+SDSSstarsarestillcontaminatedbyfaint (seeFigs.5and6). galaxiesandquasars.So,wepreferredtousethestarcountsde- Wealsoproducedthemodel-predictedstarcountsforoneof tGerAmLiEneXd+bSyDtSheSJst−arsW. 1colourcut(UV-IRstars)ratherthanthe cthoempinagssUbaVnIdTs/A(NstUroVsBat4,:w2h5ic0h5–is2s7h8o0wÅn,bλyeffad=as2h6ed1-2doÅt)teodfliunpe-. Star countsare enhancedin the UVITNUVB4 bandcompared to the GALEX NUV band because NUVB4 covers a smaller 5. Dataandmodelcomparison wavelengthrangeanditseffectivewavelengthislongerthanthe WemodeledthestellardensitydistributionoftheMilkyWayin effective wavelength of the NUV band. Most of the stars have UVusingtheBesançonmodelofstellarpopulationsynthesis(as flux peaksat longerwavelengths,such that NUV −NUVB4is describedinSect.3)fordifferentregionsofthesky.Foursimu- positive. Since the magnitudes are normalised to the AB sys- latedcataloguesforeachofthefieldschosenforourstudywere tem, the integral of the filter does not matter while computing A33,page6of13 AnantaC.Pradhanetal.:AModelfortheUVstarcountsoftheGalaxy (a) (b) (c) (d) Fig.5.ComparisonoftheUV-IRstars(solidcircles)withmodel-predictions(solidline)asafunctionofNUVmagnitudesfortheregionsatthe southernintermediateGalacticlatitudes.TheopencirclesrepresenttheGALEX+SDSSstars.TheplotsareforthefieldstowardstheGCandthe GACforindividualGALEXAISandMIStiles,eachcoveringanareaof0.785deg2 (fields1–4inTable1).Theerrorbarsshowninthemodel starcountsareduetoPoissonnoise,whiletheasymmetricerrorsintheobservedstarcountsarenotshownintheplot. magnitudes, though narrower filters will demand longer expo- FUV passband, the UVIT model simulated FUV star counts suretimestogettherequiredmagnitude. matchtheGALEXobservedFUVstarcountsreasonablywell. Overall,theBesançonmodelofstellarpopulationsynthesis Themodel-predictedstarcountsfortheregionsattheGHL, upgradedtotheUVpassbandssimulatesstarcounts,whichare theGAR, andthe GP(solidline:fields9–12inTable1)match consistentwiththeobservedGALEXstarcountsandcanbeused the UV-IR stars (solid circles) and the GALEX+SDSS stars efficientlyforthestudyofGalacticstructureparameters. (opencircles) wellexceptthe regionat theGLL (see Figs.7a– d). As seen in Fig. 7d, the model simulated NUV star counts (solidline)producedusingthestandarddiffuseextinctiondonot 5.2.Latitudevariationinstarcounts matchtheobservationsbeyondanNUVmagnitudefainterthan 18.5.This mismatchcouldbe becauseof the defaultextinction In order to study the latitude variation of UV star counts, we gradientbeingusedinthemodelnotbeingsufficientattheGLL. have chosen GALEX fields at 10◦ Galactic latitude intervals Wetookthelineofsightextinction(A =0.1mag)fortheGLL for l ∼ 50◦. We determined NUV star counts per square de- V fromtheSchlegeletal.(1998)mapsandthencorrectedtheex- gree in each field separately for the GALEX+SDSS stars and tinction by adding a cloud of A = 0.1 mag at a distance of the UV-IRstars. As showninFig. 9, thesolid circlesrepresent V 1 kpc (Sect. 3). The model-predicted star counts after correct- theUV-IRstarswhiletheopencirclesshowtheGALEX+SDSS ingtheextinction(dashedline)showagoodagreementwiththe stars.Thesolidlinerepresentsthemodelgeneratedstarcounts. UV-IRstars. The model errors due to Poisson noise are shown in the plot whilewedonotshowtheasymmetricerrorsontheUV-IRstar In Fig. 8, we have shown the distribution in FUV magni- countswhichariseduetothepropagationofphotometricerrors. tudes of the UV-IR stars (solid circles) and model-simulated ThestellardensitydecreasesfromlowertohigherGalactic lat- (solid histograms) star counts for AIS and MIS (fields 5–8 itudesin the case of bothobservedand modelstar counts.The in Table 1) towards the GC and the GAC. Despite the poor UV-IRstarcountswithNUVmagnitudebrighterthan20.5mag statistics, the model fit the observations well up to the com- match modelsimulationsat the intermediateand high Galactic pleteness limit of the data sets (see Sect. 2.5). We have also latitudes.However,aslightdeviationofmodelsimulatedcounts produced the model-predicted star counts for one of the FUV from observed counts is seen at low Galactic latitudes. This passbands (BaF2: 1370–1750 Å, λeff = 1504 Å) of the forth- could be because of the default extinction gradient used in the comingUVIT/Astrosat,whichisshownbyadashed-dottedline model which might be inappropriate at low latitudes because inFig.8.SincetheBaF2passbandrangeisclosetotheGALEX somecloudscanbepresentasdiscussedaboveforFig.7d. A33,page7of13 A&A565,A33(2014) (a) (b) (c) (d) Fig.6.Comparisonofthemodel-predictedstarcounts(solidline)withtheUV-IRstars(solidcircles)aswellaswiththeGALEX+SDSSstars(open circles)fortheGALEXfieldsatthenorthernintermediateGalacticlatitudes.TheGalacticcoordinateranges,surveytypes,andareacoveragesare mentionedineachpanel.Thedashed-dottedlineshowsthemodel-simulatedstarcountsfortheNUVB4bandofUVIT/Astrosat(2505–2780Å, λeff =2612Å).TheerrorbarsshowninthemodelcountsareduetoPoissonnoise. 5.3.ComparisonwiththeTRILEGALmodel more detailed account for the settling of the disc with age in theBesançonGalaxymodel(thedynamicalconstraintwhichis The predictions from the TRILEGAL model (Girardi et al. used,forcesthesub-componentsofthethindisctofollowatied 2005),whichisanotherstellarpopulationcode,havebeencom- age/verticalscaleheightrelationinagreementwiththeobserved pared with UV star counts by Bianchi et al. (2011a). It was age/velocitydispersionrelation). found that the TRILEGAL-predicted NUV star counts, which show an overallgoodmatch to observationsat brightermagni- tudes,arebetteratnorthernhighlatitudesandsouthernlowlati- 6. Distributionofthestars tudes.WeproducedNUVstarcountsusingthethreealternative IMFsthattheTRILEGALwebsite6 proposes.However,wesee WefindthatthemodelreproducestheobservedUVstarcounts in Fig. 10thatthe Besançonmodelproducesa betterfit to real as selected from the GALEX data. The star counts are domi- starcountsthanTRILEGALdoesintheGHLfieldclosetothe natedbyMSstars,WDs,andBHBs.Theverticaldistributionof pole as well as in the GAR field at the intermediate latitudes. differentstellarpopulationsdependsontheirstructuralparame- HereweuseaWDmodelsimilartoTRILEGAL,withsmalldif- ters.InFig.11a,weshowthecontributionofthethindisc(dotted ferences. The initial-to-final mass relation from Kalirai (2008) line),thickdisc(dashedline),halo(dashed-dottedline),andsum isusedintheBesançonmodel,whileTRILEGALalternatively of the three populations(solid line) predictedby the modelfor uses Marigo& Girardi(2007)or Weidemann(2000),the latter an AIS field towards the GC at the intermediate Galactic lati- givingabetterfittotheGALEXdata(seeFig.9inBianchietal. tude. The relatively bright stars are dominated by the thin disc 2011a). We also use different atmosphere models (Holberg & at NUV magnitudes brighter than 18.5 whereas thick disc and Bergeron2006),whileTRILEGALuseseitherKoester(2008)or halo stars become significant at NUV magnitudes fainter than TLUSTYmodels(Hubeny&Lanz1995).Bianchietal.(2011a) ∼18.5and∼19.5,respectively.Thisis verysimilar to the com- pointedoutthatthedifferencebetweenthesetwomodelsisnot parison made by Bianchiet al. (2011a)for hot star candidates. largerthan0.05maginFUV−NUVcolourformostoftheWDs. ConsideringthestarswithNUVmagnitudesbrighterthan20.5, Finally, TRILEGAL does notconsider DB WDs because it in- we found that thick disc stars are the most dominant popula- cludesonlyWDshotterthan18000K,althoughwehavetaken tionand∼54%–∼60%ofthetotalpopulation(dependingonthe themintoaccount.However,thedifferencebetweenTRILEGAL Galacticdirection). and the Besançon model predictions is mainly because of the We have shown the vertical distribution of the model sim- ulated stars in Fig. 11b. It is evident that thin disc star counts 6 http://stev.oapd.inaf.it/cgi-bin/trilegal (dotted line) dominate up to a distance of 1.5 kpc over the A33,page8of13 AnantaC.Pradhanetal.:AModelfortheUVstarcountsoftheGalaxy (a) (b) (c) (d) Fig.7.Comparisonofmodel-predictions(solidline)withtheUV-IRstars(solidcircles)andGALEX+SDSSstars(opencircles)asafunctionof theGALEXNUVmagnitudefortheregionsattheGHL(7a),theGP(7b),theGAR(7c),andtheGLL(7d).Inplot7d,thesolidlinerepresentsthe modelNUVstarcountsproducedassumingthestandarddiffuseextinction(asinotherfields)whereasthedashedlineisthesameaftercorrecting theextinctionusingthevaluefromtheSchlegeletal.(1998)maps(seeSect.5).ThemodelerrorbarsshownintheplotsareduetoPoissonnoise. Galacticplanewhereasthickdiscstarcounts(dashedline)dom- (long-dashed line) populations are also shown along with the inate at distances between 1.5 and 4.0 kpc beyond which halo UVIT FUV − NUV (BaF2 – NUVB4) colour (dashed line) in stars (dashed-dottedline) dominatethe total stellar population. the plot. Looking at the FUV − NUV model predictions, the A similar trendhas beenobservedbybothDu et al. (2003)for stars can be classified into two groups:one groupof stars with BATC (Beijing-Arizona-Taiwan-Connecticut)multicolour pho- FUV −NUV > 2.5are the red coolstars, andthe other group tometricsurveystarcountsandPhlepsetal.(2000)forCADIS of stars with FUV − NUV < 2.5 are blue hot stars. The blue (CalarAltoDeepImagingSurvey)deepstar countsforregions stars exhibit a bimodal distribution indicating the existence of atintermediateGalacticlatitudes. two populations;the peak at FUV − NUV ∼ −0.5 are the hot WDsofthedisc,andthepeakatFUV −NUV ∼ 2.0areBHBs of the Galactic halo. In the Besançon model, the temperature 7. Bluehotstars rangeofWDsisfrom10000Kto27000KandthatofBHBsis TheFUV−NUV colourisanimportantindicatorofthespectral from5000Kto20000K.Hotterstarswithtemperaturegreater than27000Karerarelyfoundinsignificantnumbersinthedata type of the stars. Particularly, UV colour can be used to iden- consideredhere. tify hot BHBs and WDs (Kinman et al. 2007; Bianchi et al. 2011a), which emit most of their light in UV because of their ThecolourdistributionsinFig.12towardsboththeGCand hightemperatures.TheBHBsarecomparativelymoreluminous the GACshowsome differencesbetweenthe modeland obser- in UV than the other population II stars. Similarly, the WDs, vations.Especially,wenoticethattheverybluepeakatFUV − whicharetheendproductofthestellarevolutionoftheinterme- NUV < 0,duetohotWDs,istoohighinthemodel.Moreover, diate and low mass stars, provide important information about thereisalackofstarsintheGCfieldat0< FUV−NUV <1.5. theGalacticdiscstarformationhistory.Comparingtheobserved In the colour range where the BHBs dominate, the number of FUV−NUVcolourofstarswiththemodel,wewereabletosep- predictedstarsiswellinagreementwiththeobservationsinboth arate outthe halo BHBs anddisc WDs fromthe whole sample fields, indicating that the halo BHB density is well simulated. ofstars. There is a dearth of model-simulatedstars in the colour range, Figure 12 shows the comparisonof GALEX FUV − NUV 2< FUV−NUV <3.5,whichisnotunderstoodyetandwillbe coloursfortheUV-IRstars(solidcircles) andmodelsimulated investigatedinafurtherstudy.AtFUV −NUV > 4,themodel starcounts(solid-linedhistogram)fortheAISfieldstowardsthe lacks stars but this is the case more towards the GAC than to- GC and the GAC. We have consideredstars with NUV magni- wardstheGC.Thiscolourdomainismostlydominatedbythick tude < 20.5 and FUV magnitude < 20.0 for the GALEX AIS disc MS stars. We guess that it is because of the scale length survey.TheFUV−NUV coloursofWD(dottedline)andBHB whichwillbeinvestigatedinaforthcomingpaper. A33,page9of13 A&A565,A33(2014) Fig.8.DistributioninFUVmagnitudesoftheUV-IRstars(solidcircles)andmodel-predicted(solidline)starcountsfortheAISandMISfields towardstheGCand theGAC (fields5–8 inTable1).Thedashed-dotted linerepresentsFUVstar countsfor theBaF2band ofUVIT/Astrosat (1350–1750Å,λeff = 1504Å).TheFUVmagnitudesarebinnedinintervalsof0.5.ThemodelerrorbarsshownintheplotsareduetoPoisson noise. Both photometry and spectroscopy can be used to identify WDsandBHBs.Severallargeareaskysurveyssuchas2MASS, SDSS, and GALEX have been used to distinguish them with appropriate colour selections and it is worth mentioning a few of the works. Kleinman et al. (2013) produced the latest cat- alogue of spectroscopically confirmed DA- and DB-type WDs fromSDSSDataRelease7.UsingthedatafromGALEXFUV andNUVimaging,Bianchietal.(2011b)presentedacatalogue of hotstar candidates,particularlyWDs. Similarly,the firstse- lectionofBHBs fromSDSS colourswasmadebyYannyet al. (2000) and then followed by many others (Sirko et al. 2004; Bell et al. 2010; Deason et al. 2011; Vickers et al. 2012). We haveidentifiedWDandBHBcandidatesusingsuitableGALEX FUV−NUVcolours.ItwasfoundfromthemodelFUV−NUV colour (Fig. 11) that BHB and WD star candidates occupy the colourrange,1.5< FUV−NUV <2.5andFUV−NUV <0.5, respectively. In the mentioned colour range, we obtain a clean Fig.9. Latitude variation of NUV star counts (per deg2) for both sampleof WD candidates,whereasinthe sample ofBHB can- the observation and model simulation at l ∼ 50◦. The UV-IR stars, didates,a contaminationofnon-BHBcandidates,suchas WDs GALEX+SDSS stars, and model-predicted star counts are repre- and MS stars, constitute about 7%. These colour ranges have sented by solid circles, open circles, and solid line, respectively. The been used for the separation of WD and BHB candidatesfrom UV-IRstarsandtheGALEX+SDSSstarsshownintheplotareforNUV otherpopulationsintheobservedsample. magnitudeerrors<0.2.Theerrorbarsdisplayedinthemodelstarcounts are due to Poisson noise. The asymmetric errors in the UV-IR star In order to substantiate our identification of the WD and counts which arise because of the propagation of photometric errors BHB star candidates using GALEX FUV − NUV colour, we arenotshown. comparedthemwiththeirknown2MASScolours.TheE(B−V) values for the stars were measured from Schlegel et al. (1998) andconvertedto NUV, J and H extinctionusingCardelliet al. the area used by Kinman et al. (2007),which contains 66% of (1989) extinction law: A(NUV) = 8.90E(B − V), A(J) = theBHBcandidatesselectedonthebasisofFUV−NUVcolour. 0.874E(B−V),andA(H)=0.589E(B−V).Figure13ashowsthe Similarly,Fig.13bshowstheH−KversusJ−Hcolour–colour (J−K) versus(NUV−J) colour–colourdiagramfortheBHB diagramforthe WD candidates.Thedashedrectangleencloses o o candidates.The sourcesatdifferentlatitude intervalsarerepre- theareainthecolour–colourdiagramchosenfromHoardetal. sentedbydifferentsymbols.Thedashedparallelogramencloses (2007) that contains a majority of the WD candidates of our A33,page10of13

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In this paper, we have used the GALEX GR6 data which is . sources and termed the clean sample as “GALEX+SDSS stars”. 2.5. If we consider all stars without accounting for .. A similar trend has been observed by both Du et al. Kalirai, J. S., Hansen, B. M. S., & Kelson, D. D. 2008, ApJ, 676, 594
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A&A565,A33(2014) Astronomy DOI:10.1051/0004-6361/201321757 & (cid:2)c ESO2014 Astrophysics A stellar population synthesis model for the study of ultraviolet star counts of the Galaxy AnantaC.Pradhan1,DevendraK.Ojha1,AnnieC.Robin2,SwarnaK.Ghosh1,3,andJohnJ.Vickers(cid:2),4 1 TataInstituteofFundamentalResearch,HomiBhabhaRoad,400005Mumbai,India e-mail:[email protected] 2 InstitutUtinam,CNRSUMR6213,OSUTHETA,UniversitédeFranche-Comté,41bisavenuedel’Observatoire25000Besançon, France 3 NationalCentreforRadioAstrophysics,TataInstituteofFundamentalResearch,411007Pune,India 4 Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr, 12-14, 69120 Heidelberg, Germany Received23April2013/Accepted11March2014 ABSTRACT Context.GalaxyEvolutionExplorer(GALEX),thefirstallskyimagingultraviolet(UV)satellite,hasimagedalargepartofthesky providinganexcellentopportunityforstudyingUVstarcounts.Combiningphotometryfromthedifferentwavelengthsintheinfrared (fromWide-fieldInfraredSurvey(WISE)andTwoMicronAllSkySurvey(2MASS))toUVallowsustoextractarealstarcatalogue fromtheGALEXsourcecatalogue. Aims.TheaimofourstudyistoinvestigateindetailtheobservedUVstarcountsobtainedbyGALEXvis-à-visthemodelsimulated cataloguesproducedbytheBesançonmodelofstellarpopulationsynthesisinvariousGalacticdirections,andtoexplorethepotential for studying the structure of our Galaxy from images in multiple near-UV (NUV) and far-UV (FUV) filters of the forthcoming UltravioletImagingTelescope(UVIT)tobeflownonboardAstrosat. Methods.Wehave upgraded the Besançon model of stellar population synthesis to include the UV bands of GALEX and UVIT. Depending ontheavailabilityofcontiguous GALEX,SloanDigitalSkySurvey(SDSS),WISE,and2MASSoverlapping regions, wehavechosenasetof19GALEXfieldswhichspreadoverarangeofGalacticdirections.Weselectedasampleofobjectsfrom theGALEXdatabaseusingtheCASjobsinterfaceandthencross-matchedthemwiththeWISE+2MASSandSDSScatalogues.The UVstarsintheGALEXcatalogueareidentifiedbychoosingasuitableinfrared(IR)colour,J−W1(W1isaWISEbandat3.4μm), whichcorrespondstoatemperaturerangefrom1650Kto65000K.TheIRcolourcutmethod,whichisusedforthefirsttimefor separationofstars,isdiscussedincomparisonwiththeGALEX+SDSSstarcountsmethod. Results.WepresenttheresultsoftheUVstarcountsanalysiscarriedoutusingthedatafromGALEX.WefindthattheBesançon modelsimulationsrepresenttheobservedstarcountsofboththeGALEXAll-skyImagingSurveyandMediumImagingSurveywell withintheerrorbarsinvariousGalacticdirections.BasedontheanalysisofthemodelFUV−NUV colour,weseparatedoutwhite dwarfsofthediscandbluehorizontalbranchstarsofthehalofromtheobservedsamplebyselectingasuitableFUV−NUVcolour. Conclusions.TheBesançonmodelisnowreadyforfurthercomparisonsintheUVdomainandwillbeusedforprospectivestudies fortheUVITinstrumenttobeflownonboardAstrosat. Keywords.stars:general–ultraviolet:stars–Galaxy:disk–Galaxy:stellarcontent–Galaxy:halo 1. Introduction in predicting the different structural parameters of the Galaxy, such as stellar densities, scale length,scale height,etc. Among The Milky Way is the best studied Galaxy in the universe; its themodelsdevelopedtounderstandtheGalacticstructureusing structureandevolutionhavebeenstudiedusingavarietyoftech- starcountingmethods,onecancite:Bahcall&Soneira(1980), niques. In the early 20th century, Kapteyn (1922) first studied Gilmore & Reid (1983), Robin & Crézé (1986), Robin et al. the geometrical structure of the Galaxy using the star counts (2003), Girardi et al. (2005) and Juric´ et al. (2008). However, methodwherebyhecountedstarsonthephotographicplatesin theaboveGalaxymodelsarepredominantlybasedonthevisible selectedareasofthesky.Sincethenthestarcountsmethodhas and infrared (IR) photometric surveys. Very few attempts had beenusedasoneofthepreferredmethodstoconstrainthestruc- been made to study the star counts in ultraviolet (UV) prior to turalparametersoftheGalaxyeffectively.Severalreviewshave GalaxyEvolutionExplorer(GALEX) becauseof lack of avail- discussedtheconnectionofstarcountstotheGalacticstructure ability of UV photometric surveys (Brosch 1991; Cohen et al. (Bahcall 1986; Freeman 1987; Gilmore et al. 1988; Majewski 1994).TheadventofGALEX,whichprovidedawideskycov- 1993; Helmi 2008; Ivezic´ et al. 2012). The advent of instru- eragein UV, nowallowsfora newanalysisofthe UVsky(Xu mentswithbetterresolutionandgreatersensitivityhaveenabled etal.2005;Bianchietal.2011a,b,2014, amongothers).Anat- ustoobtainphotometricobservationscoveringlargepartsofthe tempthasalsobeenmadetopredictthestarcountsintheX-ray skyinseveralwavelengthbands.Thepopulationsynthesismod- band(Guilloutetal.1996)byextendingtheBesançonmodelof els of the Milky Way are well supportedby these observations stellarpopulationsynthesis(Robin&Crézé1986)totheROSAT (cid:2) Member of the International Max-Planck Research School for positionsensitiveproportionalcounterenergybands. Astronomy&CosmicPhysicsattheUniversityofHeidelberg. ArticlepublishedbyEDPSciences A33,page1of13 A&A565,A33(2014) Fig.1. Effective area versus wavelength plots for the GALEX FUV and NUV bands are shown in relation to the FUV and NUV filters of UVIT/Astrosatforimagingmode.TheleftpanelshowsthefiveFUVfiltersofUVIT/AstrosatindifferentcoloursalongwiththeGALEXFUV filter(dashedline)andtherightpanelshowsthesamefortheNUVfilters. Indeed, the UV surveys, among others, could help in trac- andNUVB4(NUV)wavebands,foranexposuretimeof200s, ingthespiralstructureswhichmainlycontainveryyoungstars. are 20.0and 21.2 mag, respectively(Astrosat Handbook2013; The UV surveysalso help in constraining the shape of the ini- priv.comm.). tialmassfunction(IMF)towardsthehigh-massstarendaswell It is worth mentioning here that throughout the paper we aselucidatingthe recentstar formationhistory.Moreover,they haveusedABsystemfortheGALEX,UVIT,andSloanDigital also trace very blue populations such as white dwarfs (WDs) SkySurvey(SDSS)datasets,whereastheTwoMicronAllSky and blue horizontalbranch stars (BHBs) deep in the halo pop- Survey (2MASS) and Wide-field Infrared Survey (WISE) data ulation, which in turn trace the streams and relics of ancient setsareintheJohnsonsystem(seeSect.2). accretion in the Milky Way halo. The GALEX has covered a WegivedetailsoftheobservationsandselectionofUVstars large partof the sky, which providesan opportunityto explore in Sect. 2. We describe the Besançon Galaxy model in Sect. 3 and characterise these hot sources in the far-UV (FUV: 1344– 1λ7eff86=Å2,3λ1e5ff.7=Å1)5w3a8v.6ebÅan)dasnwdintheabr-eUtteVr r(eNsUolVu:tio1n77a1n–d28g3re1atÅer, aGnAdLdEisXcu+sSsDthSeScostmarpacroiusonntsoifnthSeecGt.A4L.EWXe+pWreIsSeEn+t2thMeAcoSmSpaanrd- ison of the modelwith the observationsin Sect. 5, and discuss sensitivity than the previous surveys. A vivid description of thedistributionofthemodelstarcountsinSect.6.Wemention the source selection, FUV and NUV magnitudeerror cuts, and theidentificationofWDsandBHBsusingFUV −NUV colour the statistical analysis of the GALEX catalogue is providedby inSect.7.Finally,wesummariseourconclusionsinSect.8. Bianchietal.(2007),Bianchi(2009),andBianchietal.(2011a, 2014).DetectionofWDsandBHBsisoneofthemainachieve- mentsofGALEXasthesesourcesareelusiveintheotherwave- lengthbandsof the electromagneticspectrumdue to their high temperature.ThepopulationofWDsandBHBsisintegraltothe 2. Observationsandcross-correlationofGALEX studyofstellarevolutionandstructureoftheMilkyWayasthey sources belongtodifferentstellarpopulationsoftheGalaxy. We have upgraded the Besançon model of stellar popula- 2.1.GALEXdata tion synthesis to include the UV bands of GALEX and the upcoming Ultraviolet Imaging Telescope (UVIT1) (which will TheGALEXwasanorbitingspacetelescopelaunchedinApril, be flown onboard Astrosat) to predict star counts in different 2003, which was terminated in mid-February,2012. The satel- parts of the sky (Todmal et al. 2010). The UVIT will image lite and on-orbit performance are described in Martin et al. the sky in the FUV (1300–1800 Å) and NUV (2000–3000 Å) (2005) and Morrissey et al. (2005, 2007). It observed the sky channels, each having five filters, at a high resolution of 1.8(cid:3)(cid:3) in two UV bands, FUV, and NUV, simultaneously, with a spa- (Postma et al. 2011; Kumar et al. 2012a,b). Better positional tial resolutionof 4.2(cid:3)(cid:3) and 5.3(cid:3)(cid:3), respectively.The field of view accuracy of UVIT as compared to GALEX will enable more is1.25◦ indiameterandthe imagesaresampledwith1.5(cid:3)(cid:3) pix- reliable cross correlation with other catalogues, which will be els. The typical AB magnitude limits (5σ depth) met by All- of greatutility in inferringthe Galactic structure using the star sky Imaging Survey (AIS) for an exposure time of 100 s and counts technique. The transmission curves (effective area ver- MediumImagingSurvey(MIS)foranexposuretimeof1500s suswavelength)forthe FUV and NUV bandsof GALEX with are 19.9/20.8(FUV/NUV) and 22.6/22.7(FUV/NUV), respec- each of the five FUV (left panel)and NUV (rightpanel)filters tively(Morrisseyetal.2007).TheAIShasthelargestskycover- of the upcoming UVIT/Astrosat are shown in Fig. 1. We have agewhencomparedtotheotherGALEXsurveys.TheGALEX included the effective area curves of both the GALEX and all observationshavecovereda largepartofthesky(∼75%),with the UVIT/Astrosat bands in the modelto simulate the UV star the exception of the Galactic plane and some regions of the counts in these bands. Apart from the GALEX bands, we will Magellanic Clouds due to safety concerns of the detectors. discuss the model simulated star counts of the BaF2 (FUV: In this paper, we have used the GALEX GR6 data which is 1370–1750Å,λeff =1504Å)andNUVB4(NUV:2505–2780Å, available in Multi-mission ArchiveatSpace TelescopeScience λeff =2612Å)bandsofUVIT/Astrosat.Theexpectedsensitivity Institute(MAST2). limits (5σ)in AB magnitudesystem in the UVIT BaF2 (FUV) 1 http://www.iiap.res.in/Uvit 2 http://galex.stsci.edu/GR6 A33,page2of13 AnantaC.Pradhanetal.:AModelfortheUVstarcountsoftheGalaxy Table1.DetailsoftheGALEXfields. Field Longitude Latitude Surveytype Area Location Numberoftiles NUVexposure FUVexposure range(deg) range(deg) (deg2) time(s) time(s) 1 47.79 –43.56 AIS 0.785 GC 1 175 175 2 129.33 –43.15 AIS 0.785 GAC 1 258 258 3 47.01 –42.65 MIS 0.785 GC 1 1589 1589 4 146.57 –46.51 MIS 0.785 GAC 1 1657 1657 5 1–15 50–60 AIS 69.9 GC 89 64–400 64–272 6 160–175 50–60 AIS 69.9 GAC 89 90–442 96–220 7 13–29 35–41 MIS 22.77 GC 29 1541–2176 1541–2176 8 154–162 38–44 MIS 18.85 GAC 24 1597–4457 1512–3066 9 230–240 42–50 AIS 46.34 GAR 59 80–438 61–231 10 40-50 75–85 AIS 14.92 GHL 19 62–292 62–249 11 30–32 21–29 AIS 13.35 GLL 17 105–421 105–230 12 0–20 84–88 AIS 4.71 GP 6 107–383 107–271 13 49–51 17–23 AIS 7.07 – 9 130–199 130–199 14 48–53 29–33 AIS 11.0 – 14 79–184 79–184 15 46–53 37–43 AIS 20.42 – 28 96–384 95–265 16 46–54 46–54 AIS 17.28 – 22 94–345 73–226 17 46–54 57–63 AIS 11.78 – 15 152–342 152–229 18 46–54 67–73 AIS 7.07 – 9 90–340 90–169 19 44–54 77–83 AIS 4.71 – 6 75–259 75–151 Notes.TheareasofdifferentfieldsarechosendependingontheavailabilityofGALEX,WISE+2MASS,andSDSSoverlappingregions. 2.2.SelectionofGALEXfields entire sky in the J (1.24 μm), H (1.66 μm), and K (2.16 μm) s near-IR (NIR) bands with angular resolutions of 2.9(cid:3)(cid:3), 2.8(cid:3)(cid:3), Wehaveselected19GALEXfieldsforwhichboththedetectors and 2.9(cid:3)(cid:3), respectively (Skrutskie et al. 2006);while WISE has of GALEX were turned on. We retained sources which had a mappedtheskyintheW1(3.4μm),W2(4.6μm),W3(12μm), reliableNUV detection,however,FUVdetectionsareavailable andW4(22μm)mid-IRbands,withangularresolutionsof6.1(cid:3)(cid:3), for∼3.5%and∼6.8%oftheNUVdetectionsintheselectedAIS 6.4(cid:3)(cid:3), 6.5(cid:3)(cid:3), and 12.0(cid:3)(cid:3), respectively (Wright et al. 2010). The and MIS fields, respectively. The rest of the NUV sources do 5σ point source sensitivities of the four WISE bands are bet- nothaveaFUVdetectionbecausetheirFUVfluxesaretoolow terthan0.08,0.11,1,and6mJy(equivalentto16.6,15.6,11.3, to be detected.We includeonlyregionswithin a radiusof0.5◦ and 8.0 Vega magnitude)in unconfusedregionson the ecliptic from the centre of the tiles to eliminate edge artifacts and bad (Wrightetal.2010).TheexistingWISE+2MASScross-matched sourcesalongtheedgeaswellastoavoidoverlappingareasand catalogue available at Infrared Science Archive (IRSA3) has duplication of the sources. The coverageareas of the observed been used for convenience. This catalogue has been produced fieldsarecalculatedbysumminguptheareasofallthetilesina using a 3.0(cid:3)(cid:3) matchingradius, which was foundto be adequate field. consideringthepositionalaccuracyandresolution. The fields are selected in the footprints of the GALEX, We have made use of the Virtual Astronomical SDSS, WISE, and 2MASS surveys. The various fields are as Observatory (VAO4) for cross-matching GALEX sources follows: with WISE+2MASS sources. The GALEX sources were – FourGALEXtileswerechosenatthesouthernGalacticlat- uploadedintotheVAO,seekingtheirWISEand2MASScoun- itudes:twoeachinAISandMIS. terpartsusingamatchradiusof3.0(cid:3)(cid:3).Wefoundmostofthereal – Eightfieldswithlargeareacoverageoftheskywerechosen matched sources within this radius, with a very small fraction in several northern Galactic directions. The fields include: (<1%)havingmultiplematches,whichwereremovedfromthe two regions towards the Galactic centre (GC) (one each in final catalogue. We also estimated the possible contamination AISandMIS),tworegionstowardstheGalacticanti-centre by spurious matches (random coincidences) for the matched (GAC) (one each in AIS and MIS), and one regioneach in sourcesfollowingthemethodofBianchietal.(2011a).Forthis AIStowardstheGalacticanti-rotation(GAR),Galacticlow purpose, we used a match radius of 6.0(cid:3)(cid:3), which is equivalent latitude (GLL), Galactic high latitude (GHL), and Galactic to the resolution of WISE, to find the GALEX counterpartsof pole(GP)directions. WISEsources.Thespuriousmatcheswerefoundtobe∼10%of – SevenfieldsinAISwerechosenat10◦intervalsofbaround the total matched sources, and 75% of these spurious matched l∼50◦tostudythelatitudevariationofUVstarcounts. sourcesliebeyondadistanceof3.0(cid:3)(cid:3). Thecentrecoordinates,surveytypes,areacoverages,locationin theGalaxy,numberofGALEXtiles,andtherangeofexposure 2.4.SDSSdata times of NUV and FUV observations of each of the fields are giveninTable1. So far, SDSS has mapped over 35% of the full sky in five optical photometric bands (u,g,r,i,z) covering the wavelength rangefrom3000to11000Å(Aiharaetal.2011).TheGALEX 2.3.WISE+2MASSdata The AIS and MIS of GALEX overlap with the 2MASS and 3 http://irsa.ipac.caltech.edu/Missions/wise.html WISE footprints. The 2MASS observations have covered the 4 http://vao-web.ipac.caltech.edu/applications/VAOSCC A33,page3of13 A&A565,A33(2014) Fig.2. J (2MASS)−W1(WISE)versusNUVCMDfortheGALEXandWISE+2MASScross-matchedsourcesfortheAISfieldstowardsthe GCandtheGAC.Thematchedsourcesareclearlyseparatedintwogroupsindicatingisolationofstars(J−W1 < 1.2)fromtheextra-galactic sources(J−W1>1.2).Theverticaldashedlineshowsthelimitthatwechooseforselectingthepointsources(J−W1<1.2).QSOsareshown bybluecrossedsymbols(seethetext). GR6 has been cross-matched against SDSS DR7 and the pro- 2.6.Photometricerrorcutsandcompletenesslimits videdcross-matchedtableisxSDSSDR7.Severalworkshaveex- plainedthecross-matchingoftheGALEXcataloguewithSDSS, Figure3 showsthe distributionof UV-IRstars asa functionof astrophysical source classifications and related statistical anal- theGALEXUVmagnitudesforAISandMIS.StarswithNUV yses (Seibert et al. 2005; Budavári et al. 2009; Bianchi et al. andFUVmagnitudeerrorslessthan0.5,0.4,0.3,0.2,and0.15 2007, 2011a). We uploaded the GalexIDs of the objects into are displayed with magenta, green, red, blue, and cyan colour theGALEXCASjobs5SQL(StructuredQueryLanguage)inter- lines respectively, whereas the black line represents the stars facetodeterminetheirSDSScounterpartsina searchradiusof withoutanymagnitudeerrorcut.Thetypical5σmagnitudelim- 3.0(cid:3)(cid:3).Wehaveeliminatedthemultiplematches(<1%)fromthe itsoftheNUVandFUVbandsforAISandMIS(seeSect.2.1) GALEX+SDSSfinalcatalogue.Theestimatedspuriousmatches areshownbyverticaldashedlines.Asseenfromthehistograms, incaseofGALEX+SDSSarefoundtobe∼7%within3.0(cid:3)(cid:3) ra- aprogressivestringenterrorcuteliminatesthefainterstars.The dius. The SDSS star/galaxy classifications have been adopted completenesslimitsneedtobeestablishedaccordingtoagiven while performingthe match to separate outpoint sourcesfrom magnitudeerror.Ifweconsiderallstarswithoutaccountingfor thesourcelist. errors,thestarcountsgodeeperbuttheirvaluesarenotreliable becauseoftheuncertaintyonthemagnitudemeasurement.This The SDSS classified point sources (GALEX+SDSS) in- isparticularlytruefortheFUVfilterwheresomespuriousdetec- cludebothstars and quasi-stellarobjects(QSOs), outofwhich tionscanoccur.Finally,we retainedstarswithmagnitudeerror we selected QSOs using the SDSS colour cuts from Richards lessthan0.2inbothbandsasthiserrorcutgivesmagnitudelim- etal. (2002)andremovedthemfromtheGALEX+SDSSpoint itsalmostsimilartothetypical5σlimitsoftheGALEXbands sourcesandtermedthecleansampleas“GALEX+SDSSstars”. forAISandMISwhichareprovidedbyMorrisseyetal.(2007). We have applied magnitude error cuts (similar to the one 2.5.SelectionofstarsfromtheGALEXcatalogue showninFig.3forUV-IRstars)intheoriginalGALEXsource byIRcolourcutmethod cataloguethatincludesalltheGALEXdetections.Wehavealso appliedmagnitudeerrorcutsinthematchedGALEXsourcecat- Figure 2 shows J − W1 versus NUV colour–magnitude dia- alogue obtained after cross-matching with the WISE+2MASS gram(CMD)ofallGALEXsourcesthatarecross-matchedwith catalogue.WefindalossofGALEXsourcesinthematchedcat- WISE+2MASS sources for the regionsin the directionsof the alogue when compared with the original GALEX source cata- GC and the GAC, each covering 69.9 deg2 of the sky. The logue at a specific magnitudeerror cut. The completenesslim- QSO candidates are selected using the SDSS colour cuts from its for the original GALEX sources for NUV and FUV mag- Richardsetal.(2002)andarerepresentedbybluecrossedsym- nitude error cuts of 0.2 are 20.5/21.0mag (FUV/NUV) in AIS bols in the plot. We clearly see two groups of sources in the and22.5/22.5mag(FUV/NUV)inMIS.Thecompletenesslim- figure well separated by J − W1 colour. The stars verified by its at the same magnitude error cuts for the matched catalogue their SDSS classification as point sources in a cross-matched become20.0/20.5mag (FUV/NUV) in AIS and 22.5/22.0mag sample are identified to be bluer than J − W1 < 1.2 and the (FUV/NUV) in MIS, and these limits are the same for the extra-galacticobjects(e.g.,galaxies,QSOs,etc.)areredder,with UV-IR stars too. For a specific magnitude error cut, the FUV J−W1>1.2.SincethecontaminationbySDSS-identifiedQSOs and NUV completeness limits of the observed sources, which is estimated to be negligible in the J − W1 < 1.2 star counts arecross-matchedtothesurveysatlongerwavelengths,become (<0.1%ofthewholesample),wehaveusedtheJ−W1colourcut brighterthanthecompletenesslimitsoftheunmatchedGALEX procedureforall the fieldsto separate the stars from the extra- source catalogue because of the loss of faint sources in the galactic objects. Henceforth in the paper, we refer to GALEX former. andWISE+2MASScross-matchedsourceswith J −W1 < 1.2 In order to examine which objects are affected by the lim- (GALEX+WISE+2MASS)as“UV-IRstars”. itsofthecombinedsurveys(GALEX+WISE+2MASS),wesplit thestarsintotwoNUV−W1colourintervals:hot(NUV−W1< 5)andcool(NUV −W1 > 5)stars. We checkedthecomplete- 5 http://galex.stsci.edu/casjobs ness limitof the NUV band(AIS) for these two colourranges. A33,page4of13 AnantaC.Pradhanetal.:AModelfortheUVstarcountsoftheGalaxy Fig.3.DistributionoftheUV-IRstarsasafunctionoftheGALEXNUVandFUVmagnitudesforAISandMIS.Thestellarsourcesobtainedafter applyingvariousmagnitudeerrorcutsareshownbydifferentlinestylesincolours.Theverticaldashedlinesrepresenttherespective5σdetection limitsoftheGALEXbandsfortypicalexposuretimesasmentionedinSect.2. For hotstars, we foundthat the completenesslimit of GALEX function is normalised to fit Harris et al. (2006).Similarly, the NUV(AIS)isreducedby0.5mag(i.e.,theeffectivemagnitude BHBs are incorporated in the model by taking the evolution- limitgetsbrighter).TheGALEXcompletenesslimit(AIS)ofhot arytracksfromBaSTI(ABagofStellarTracksandIsochrones) starsisthereforelimitedbythedepthofWISE,andsimilarlyby models(Pietrinfernietal.2004).Ultimately,themodelproduces thedepthof2MASS.Forcoolstars, theNUV(AIS)complete- UV star countsby MonteCarlo simulationsusinga set ofstel- nesslimitisthesameintheGALEXcataloguealoneandinthe lar atmospheric models, observational photometric errors, and combinedcataloguewiththenear-IRsurveys. extinction. The modelincorporatesan extinction(A ) assuming an el- V lipsoidaldistributionofdiffuseabsorbingmatter,whichfollows 3. BesançonGalaxymodel an Einastoextinctionlaw andis depictedby anadjustable nor- malisation (extinction gradient) of 0.7 mag/kpc in the V band. The Besançonmodelis a populationsynthesismodelbased on WeproducedthemodelsimulationstowardsvariousGalacticdi- a scenario of Galactic evolution and constrained by dynam- rectionsassumingthe defaultextinctiongradient.However,the ics. In the model, five populations are taken into account: thin defaultvalueofdiffuseextinction(0.7mag/kpcintheV band), disc,thickdisc, stellarhalo,bar,andbulge(Robinetal.2012). whichmaynotbeappropriateatlowlatitudes,canbeadjustedby The previous versions of the model are extensively described addingafewabsorbingcloudswithagivenadhocdistanceand in Robinetal. (2003).We use the newestversionofthe model extinction from the Schlegel et al. (1998) maps. This has been (versionof April2013;Robinet al. 2012),whichhasbeen up- illustratedinSect.5.1.TheratiosbetweenUVbandtovisualex- gradedtoincludetheFUVandNUVpassbandsofGALEXand tinctionaretakentobe2.67and2.64fortheFUVandNUVband the upcomingUVIT/Astrosat,by applyingtheir filter transmis- ofGALEX,respectively,followingtheextinctionlawofCardelli sion curves to produce UV star counts in various Galactic di- etal.(1989). rections.Themodelusesasetofevolutionarytracks,astarfor- StarsinthesimulatedGALEXcataloguehaveaUVcolour, mationrateandanIMFasdefinedinHaywoodetal.(1997),to FUV, and NUV magnitudes,a temperaturerange from 1650K generatedifferentstellarpopulations.Thecoloursarecomputed to 65000 K, log g from –1 to 9, all luminosity classes and a from the Basel Stellar Library (BaSeL3.1) model atmospheres range of metallicities. In the simulations done for comparison (Westeraetal.2002).Inthisnewversionofthemodel,DAand with the GALEX observed star counts, the simulated stars are DBtypeWDsareincludedusingtheevolutionarytracksandat- mostly MS stars (∼77%) with a small contributionfrom giants mospheremodelsfromHolberg& Bergeron(2006).Thelumi- and subgiants (∼17%). The WDs are ∼6% of the sample and nosity functions are obtained assuming an initial-to-final-mass resideatthebluerendofFUV−NUV colour(seeSect.7). ratio (m = 9.1743m −3.6147) from Kalirai (2008). The dis- I f tribution in age is computed assuming a lifetime on the main sequence(MS)fromEggletonetal.(1989)andalifetimeonthe 4. ComparisonoftheGALEX+WISE+2MASS giantbranchof15%ofthetimeontheMS.TherepartitioninDA andGALEX+SDSSstars (WDwithhydrogenrichatmosphere)andDB(WDwithhelium richatmosphere)iscomputedassumingthatatTeff > 40000K Figure4 showsthe distributionofthe GALEX AISstar counts theyareallDA,andatTeff <20000K,50%areDBwithalinear (for field 5 in Table 1) as a function of NUV magnitude variationbetween20000Kand50000K.Thefinalluminosity for the model simulation (solid line), GALEX+SDSS stars A33,page5of13 A&A565,A33(2014) producedtoreducethestatisticalnoise.Appropriatephotometric errorswereappliedinthemodeltoproducerealisticsimulations andtheerrorinformationwasassumedfromtheobserveddata, whichisapolynomialfunctionofthemagnitude. We can simulate the catalogues using the “small field” op- tion, which assumes that the density does not vary across the field, or, we can simulate them using the “larger field” option with a given step in longitude and latitude, to account for the fieldswherethedensitycanvary.Weusedthe“smallfield”op- tion for the small fields (e.g., area <15 deg2) by providing the centre l and b coordinates of the fields along with their cover- agearea.Forthelargerfields(e.g.,area>15deg2),weusedthe “largerfield”optionwhereweprovidetherangeoflandbcoor- dinatesandastepsize(e.g.,from1.0◦ to2.5◦ forsmalltolarge fields,respectively)tocoverthefield.However,thegradientsin thefields(Table1)aresmallenoughthatconsideringeitherthe centreof the field orthe rangeofl/b doesnotmakeanydiffer- Fig.4. Distribution of the UV-IR stars (GALEX+WISE+2MASS: enceinthepredictedstarcounts. dashed-dotted line), GALEX+SDSS stars (dashed line), and model- simulated star counts (solid line) for the AIS field towards the GC, covering69.9deg2 ofthesky(field5inTable1).Thedottedlinerep- resentstheGALEX+WISEsourceswithno2MASScounterparts.The 5.1.ComparisonofobservedUVstarcountswiththemodel starcountsarebinnedin0.5magintervalinNUVmagnitude.Theerror invariousfields barsinthemodel-simulatedstarcountsareduetoPoissonnoise.The NUV5σdetectionlimit(NUVmagnitude=20.8)isshownbyasolid Initially, simulations were performed for four GALEX tiles verticalline.TheUV-IRstarcountsshowaturnoveratNUVmagnitude (fields1–4inTable1),eachcoveringanareaof0.785deg2atthe ∼20.5(demarcatedbyadashedverticalline). southern intermediate Galactic latitudes. We binned the model andtheUV-IRstarsin0.5magintervalsintheNUVband,forre- spectivetilesofAISandMIS,inthedirectionsoftheGCandthe (dashed line), UV-IR stars (GALEX+WISE+2MASS: dashed- GAC. As shownin Fig. 5,we foundthatthe modelstar counts dotted line) and GALEX+WISE star counts with no 2MASS (solid line) match the UV-IRstars (solid circles) as well as the detection (dotted line). The error bars shown in the model star GALEX+SDSSstars(opencircles)welluptothecompleteness counts are due to Poisson noise. The NUV 5σ detection limit limitsofAIS(20.5mag)andMIS(22.0mag)fortheregionsat (NUV magnitude =20.8; Morrissey et al. 2007) and the com- thesouthernintermediateGalacticlatitudes. pleteness limit (∼20.5 mag; see Sect. 2.5) for AIS are demar- cated by the solid and dashed vertical lines, respectively.Stars Inordertochecktheuniversalvalidityofthemodel,wepro- withNUVmagnitudeuptothecompletenesslimitarewellde- ducedsimulated cataloguesin variousdirectionsof the Galaxy tected by the GALEX, SDSS, WISE, and 2MASS surveys; a covering a large area of the sky. Figures 6a and b show the goodagreementinstarcountsamongthecross-matchedsurveys comparison of the model-predicted NUV star counts (solid andthemodelsimulationscanclearlybeseeninFig.4. line) with the UV-IR stars (solid circles) as well as with the It is also evidentfrom Fig. 4 that the GALEX+SDSS stars GALEX+SDSSstars(opencircles)forAISinthedirectionsof are slightly more than the UV-IR stars in the NUV band at the GC and the GAC, each covering 69.9 deg2 area of the sky the fainter magnitudes. This discrepancy could be caused by (fields 5–6 in Table 1). Similarly, Figs. 6c and d represent the 2MASSsincethetimegapbetweentheWISEand2MASSsur- comparison of star counts for MIS in the directions of the GC veys is ∼12 years, high proper motion stars may have moved andtheGAC,coveringanareaof22.77deg2and18.85deg2,re- outsidethecross-matchingradii.However,starswithpropermo- spectively(fields7–8inTable1).Thesefieldsarechosenatthe tionshighenoughtomoveby3.0(cid:3)(cid:3)in∼12yearsareveryrarein northernintermediateGalacticlatitudeoftheGalaxy.Theerror asurveyofafewsquaredegrees.Anotherpossibilityisthatthe barsshowninthemodel-predictedstarcountsareduetoPoisson 2MASS J band,which hasa 10σpointsource sensitivitylimit noise.Themaximumestimatedasymmetricerrorintheobserved ofabout15.8mag,doesnotpenetratedeeplyenoughtoprovide counts is ∼2%–10%, depending on the NUV magnitude bins counterparts for all WISE detections. Though GALEX+SDSS (i.e.,errorincreasestowardsthefaintermagnitudebins),which has a smaller sky area coverage and a fainter limit compared is not shown in the plots. The model shows a good agreement to GALEX+WISE+2MASS, both the selections yield a close with the observation (UV-IR stars and GALEX+SDSS stars) match of the star counts at the brighter end. It is also possi- downtoanNUVmagnitudeof∼20.5forAISand22.0forMIS blethattheGALEX+SDSSstarsarestillcontaminatedbyfaint (seeFigs.5and6). galaxiesandquasars.So,wepreferredtousethestarcountsde- Wealsoproducedthemodel-predictedstarcountsforoneof tGerAmLiEneXd+bSyDtSheSJst−arsW. 1colourcut(UV-IRstars)ratherthanthe cthoempinagssUbaVnIdTs/A(NstUroVsBat4,:w2h5ic0h5–is2s7h8o0wÅn,bλyeffad=as2h6ed1-2doÅt)teodfliunpe-. Star countsare enhancedin the UVITNUVB4 bandcompared to the GALEX NUV band because NUVB4 covers a smaller 5. Dataandmodelcomparison wavelengthrangeanditseffectivewavelengthislongerthanthe WemodeledthestellardensitydistributionoftheMilkyWayin effective wavelength of the NUV band. Most of the stars have UVusingtheBesançonmodelofstellarpopulationsynthesis(as flux peaksat longerwavelengths,such that NUV −NUVB4is describedinSect.3)fordifferentregionsofthesky.Foursimu- positive. Since the magnitudes are normalised to the AB sys- latedcataloguesforeachofthefieldschosenforourstudywere tem, the integral of the filter does not matter while computing A33,page6of13 AnantaC.Pradhanetal.:AModelfortheUVstarcountsoftheGalaxy (a) (b) (c) (d) Fig.5.ComparisonoftheUV-IRstars(solidcircles)withmodel-predictions(solidline)asafunctionofNUVmagnitudesfortheregionsatthe southernintermediateGalacticlatitudes.TheopencirclesrepresenttheGALEX+SDSSstars.TheplotsareforthefieldstowardstheGCandthe GACforindividualGALEXAISandMIStiles,eachcoveringanareaof0.785deg2 (fields1–4inTable1).Theerrorbarsshowninthemodel starcountsareduetoPoissonnoise,whiletheasymmetricerrorsintheobservedstarcountsarenotshownintheplot. magnitudes, though narrower filters will demand longer expo- FUV passband, the UVIT model simulated FUV star counts suretimestogettherequiredmagnitude. matchtheGALEXobservedFUVstarcountsreasonablywell. Overall,theBesançonmodelofstellarpopulationsynthesis Themodel-predictedstarcountsfortheregionsattheGHL, upgradedtotheUVpassbandssimulatesstarcounts,whichare theGAR, andthe GP(solidline:fields9–12inTable1)match consistentwiththeobservedGALEXstarcountsandcanbeused the UV-IR stars (solid circles) and the GALEX+SDSS stars efficientlyforthestudyofGalacticstructureparameters. (opencircles) wellexceptthe regionat theGLL (see Figs.7a– d). As seen in Fig. 7d, the model simulated NUV star counts (solidline)producedusingthestandarddiffuseextinctiondonot 5.2.Latitudevariationinstarcounts matchtheobservationsbeyondanNUVmagnitudefainterthan 18.5.This mismatchcouldbe becauseof the defaultextinction In order to study the latitude variation of UV star counts, we gradientbeingusedinthemodelnotbeingsufficientattheGLL. have chosen GALEX fields at 10◦ Galactic latitude intervals Wetookthelineofsightextinction(A =0.1mag)fortheGLL for l ∼ 50◦. We determined NUV star counts per square de- V fromtheSchlegeletal.(1998)mapsandthencorrectedtheex- gree in each field separately for the GALEX+SDSS stars and tinction by adding a cloud of A = 0.1 mag at a distance of the UV-IRstars. As showninFig. 9, thesolid circlesrepresent V 1 kpc (Sect. 3). The model-predicted star counts after correct- theUV-IRstarswhiletheopencirclesshowtheGALEX+SDSS ingtheextinction(dashedline)showagoodagreementwiththe stars.Thesolidlinerepresentsthemodelgeneratedstarcounts. UV-IRstars. The model errors due to Poisson noise are shown in the plot whilewedonotshowtheasymmetricerrorsontheUV-IRstar In Fig. 8, we have shown the distribution in FUV magni- countswhichariseduetothepropagationofphotometricerrors. tudes of the UV-IR stars (solid circles) and model-simulated ThestellardensitydecreasesfromlowertohigherGalactic lat- (solid histograms) star counts for AIS and MIS (fields 5–8 itudesin the case of bothobservedand modelstar counts.The in Table 1) towards the GC and the GAC. Despite the poor UV-IRstarcountswithNUVmagnitudebrighterthan20.5mag statistics, the model fit the observations well up to the com- match modelsimulationsat the intermediateand high Galactic pleteness limit of the data sets (see Sect. 2.5). We have also latitudes.However,aslightdeviationofmodelsimulatedcounts produced the model-predicted star counts for one of the FUV from observed counts is seen at low Galactic latitudes. This passbands (BaF2: 1370–1750 Å, λeff = 1504 Å) of the forth- could be because of the default extinction gradient used in the comingUVIT/Astrosat,whichisshownbyadashed-dottedline model which might be inappropriate at low latitudes because inFig.8.SincetheBaF2passbandrangeisclosetotheGALEX somecloudscanbepresentasdiscussedaboveforFig.7d. A33,page7of13 A&A565,A33(2014) (a) (b) (c) (d) Fig.6.Comparisonofthemodel-predictedstarcounts(solidline)withtheUV-IRstars(solidcircles)aswellaswiththeGALEX+SDSSstars(open circles)fortheGALEXfieldsatthenorthernintermediateGalacticlatitudes.TheGalacticcoordinateranges,surveytypes,andareacoveragesare mentionedineachpanel.Thedashed-dottedlineshowsthemodel-simulatedstarcountsfortheNUVB4bandofUVIT/Astrosat(2505–2780Å, λeff =2612Å).TheerrorbarsshowninthemodelcountsareduetoPoissonnoise. 5.3.ComparisonwiththeTRILEGALmodel more detailed account for the settling of the disc with age in theBesançonGalaxymodel(thedynamicalconstraintwhichis The predictions from the TRILEGAL model (Girardi et al. used,forcesthesub-componentsofthethindisctofollowatied 2005),whichisanotherstellarpopulationcode,havebeencom- age/verticalscaleheightrelationinagreementwiththeobserved pared with UV star counts by Bianchi et al. (2011a). It was age/velocitydispersionrelation). found that the TRILEGAL-predicted NUV star counts, which show an overallgoodmatch to observationsat brightermagni- tudes,arebetteratnorthernhighlatitudesandsouthernlowlati- 6. Distributionofthestars tudes.WeproducedNUVstarcountsusingthethreealternative IMFsthattheTRILEGALwebsite6 proposes.However,wesee WefindthatthemodelreproducestheobservedUVstarcounts in Fig. 10thatthe Besançonmodelproducesa betterfit to real as selected from the GALEX data. The star counts are domi- starcountsthanTRILEGALdoesintheGHLfieldclosetothe natedbyMSstars,WDs,andBHBs.Theverticaldistributionof pole as well as in the GAR field at the intermediate latitudes. differentstellarpopulationsdependsontheirstructuralparame- HereweuseaWDmodelsimilartoTRILEGAL,withsmalldif- ters.InFig.11a,weshowthecontributionofthethindisc(dotted ferences. The initial-to-final mass relation from Kalirai (2008) line),thickdisc(dashedline),halo(dashed-dottedline),andsum isusedintheBesançonmodel,whileTRILEGALalternatively of the three populations(solid line) predictedby the modelfor uses Marigo& Girardi(2007)or Weidemann(2000),the latter an AIS field towards the GC at the intermediate Galactic lati- givingabetterfittotheGALEXdata(seeFig.9inBianchietal. tude. The relatively bright stars are dominated by the thin disc 2011a). We also use different atmosphere models (Holberg & at NUV magnitudes brighter than 18.5 whereas thick disc and Bergeron2006),whileTRILEGALuseseitherKoester(2008)or halo stars become significant at NUV magnitudes fainter than TLUSTYmodels(Hubeny&Lanz1995).Bianchietal.(2011a) ∼18.5and∼19.5,respectively.Thisis verysimilar to the com- pointedoutthatthedifferencebetweenthesetwomodelsisnot parison made by Bianchiet al. (2011a)for hot star candidates. largerthan0.05maginFUV−NUVcolourformostoftheWDs. ConsideringthestarswithNUVmagnitudesbrighterthan20.5, Finally, TRILEGAL does notconsider DB WDs because it in- we found that thick disc stars are the most dominant popula- cludesonlyWDshotterthan18000K,althoughwehavetaken tionand∼54%–∼60%ofthetotalpopulation(dependingonthe themintoaccount.However,thedifferencebetweenTRILEGAL Galacticdirection). and the Besançon model predictions is mainly because of the We have shown the vertical distribution of the model sim- ulated stars in Fig. 11b. It is evident that thin disc star counts 6 http://stev.oapd.inaf.it/cgi-bin/trilegal (dotted line) dominate up to a distance of 1.5 kpc over the A33,page8of13 AnantaC.Pradhanetal.:AModelfortheUVstarcountsoftheGalaxy (a) (b) (c) (d) Fig.7.Comparisonofmodel-predictions(solidline)withtheUV-IRstars(solidcircles)andGALEX+SDSSstars(opencircles)asafunctionof theGALEXNUVmagnitudefortheregionsattheGHL(7a),theGP(7b),theGAR(7c),andtheGLL(7d).Inplot7d,thesolidlinerepresentsthe modelNUVstarcountsproducedassumingthestandarddiffuseextinction(asinotherfields)whereasthedashedlineisthesameaftercorrecting theextinctionusingthevaluefromtheSchlegeletal.(1998)maps(seeSect.5).ThemodelerrorbarsshownintheplotsareduetoPoissonnoise. Galacticplanewhereasthickdiscstarcounts(dashedline)dom- (long-dashed line) populations are also shown along with the inate at distances between 1.5 and 4.0 kpc beyond which halo UVIT FUV − NUV (BaF2 – NUVB4) colour (dashed line) in stars (dashed-dottedline) dominatethe total stellar population. the plot. Looking at the FUV − NUV model predictions, the A similar trendhas beenobservedbybothDu et al. (2003)for stars can be classified into two groups:one groupof stars with BATC (Beijing-Arizona-Taiwan-Connecticut)multicolour pho- FUV −NUV > 2.5are the red coolstars, andthe other group tometricsurveystarcountsandPhlepsetal.(2000)forCADIS of stars with FUV − NUV < 2.5 are blue hot stars. The blue (CalarAltoDeepImagingSurvey)deepstar countsforregions stars exhibit a bimodal distribution indicating the existence of atintermediateGalacticlatitudes. two populations;the peak at FUV − NUV ∼ −0.5 are the hot WDsofthedisc,andthepeakatFUV −NUV ∼ 2.0areBHBs of the Galactic halo. In the Besançon model, the temperature 7. Bluehotstars rangeofWDsisfrom10000Kto27000KandthatofBHBsis TheFUV−NUV colourisanimportantindicatorofthespectral from5000Kto20000K.Hotterstarswithtemperaturegreater than27000Karerarelyfoundinsignificantnumbersinthedata type of the stars. Particularly, UV colour can be used to iden- consideredhere. tify hot BHBs and WDs (Kinman et al. 2007; Bianchi et al. 2011a), which emit most of their light in UV because of their ThecolourdistributionsinFig.12towardsboththeGCand hightemperatures.TheBHBsarecomparativelymoreluminous the GACshowsome differencesbetweenthe modeland obser- in UV than the other population II stars. Similarly, the WDs, vations.Especially,wenoticethattheverybluepeakatFUV − whicharetheendproductofthestellarevolutionoftheinterme- NUV < 0,duetohotWDs,istoohighinthemodel.Moreover, diate and low mass stars, provide important information about thereisalackofstarsintheGCfieldat0< FUV−NUV <1.5. theGalacticdiscstarformationhistory.Comparingtheobserved In the colour range where the BHBs dominate, the number of FUV−NUVcolourofstarswiththemodel,wewereabletosep- predictedstarsiswellinagreementwiththeobservationsinboth arate outthe halo BHBs anddisc WDs fromthe whole sample fields, indicating that the halo BHB density is well simulated. ofstars. There is a dearth of model-simulatedstars in the colour range, Figure 12 shows the comparisonof GALEX FUV − NUV 2< FUV−NUV <3.5,whichisnotunderstoodyetandwillbe coloursfortheUV-IRstars(solidcircles) andmodelsimulated investigatedinafurtherstudy.AtFUV −NUV > 4,themodel starcounts(solid-linedhistogram)fortheAISfieldstowardsthe lacks stars but this is the case more towards the GAC than to- GC and the GAC. We have consideredstars with NUV magni- wardstheGC.Thiscolourdomainismostlydominatedbythick tude < 20.5 and FUV magnitude < 20.0 for the GALEX AIS disc MS stars. We guess that it is because of the scale length survey.TheFUV−NUV coloursofWD(dottedline)andBHB whichwillbeinvestigatedinaforthcomingpaper. A33,page9of13 A&A565,A33(2014) Fig.8.DistributioninFUVmagnitudesoftheUV-IRstars(solidcircles)andmodel-predicted(solidline)starcountsfortheAISandMISfields towardstheGCand theGAC (fields5–8 inTable1).Thedashed-dotted linerepresentsFUVstar countsfor theBaF2band ofUVIT/Astrosat (1350–1750Å,λeff = 1504Å).TheFUVmagnitudesarebinnedinintervalsof0.5.ThemodelerrorbarsshownintheplotsareduetoPoisson noise. Both photometry and spectroscopy can be used to identify WDsandBHBs.Severallargeareaskysurveyssuchas2MASS, SDSS, and GALEX have been used to distinguish them with appropriate colour selections and it is worth mentioning a few of the works. Kleinman et al. (2013) produced the latest cat- alogue of spectroscopically confirmed DA- and DB-type WDs fromSDSSDataRelease7.UsingthedatafromGALEXFUV andNUVimaging,Bianchietal.(2011b)presentedacatalogue of hotstar candidates,particularlyWDs. Similarly,the firstse- lectionofBHBs fromSDSS colourswasmadebyYannyet al. (2000) and then followed by many others (Sirko et al. 2004; Bell et al. 2010; Deason et al. 2011; Vickers et al. 2012). We haveidentifiedWDandBHBcandidatesusingsuitableGALEX FUV−NUVcolours.ItwasfoundfromthemodelFUV−NUV colour (Fig. 11) that BHB and WD star candidates occupy the colourrange,1.5< FUV−NUV <2.5andFUV−NUV <0.5, respectively. In the mentioned colour range, we obtain a clean Fig.9. Latitude variation of NUV star counts (per deg2) for both sampleof WD candidates,whereasinthe sample ofBHB can- the observation and model simulation at l ∼ 50◦. The UV-IR stars, didates,a contaminationofnon-BHBcandidates,suchas WDs GALEX+SDSS stars, and model-predicted star counts are repre- and MS stars, constitute about 7%. These colour ranges have sented by solid circles, open circles, and solid line, respectively. The been used for the separation of WD and BHB candidatesfrom UV-IRstarsandtheGALEX+SDSSstarsshownintheplotareforNUV otherpopulationsintheobservedsample. magnitudeerrors<0.2.Theerrorbarsdisplayedinthemodelstarcounts are due to Poisson noise. The asymmetric errors in the UV-IR star In order to substantiate our identification of the WD and counts which arise because of the propagation of photometric errors BHB star candidates using GALEX FUV − NUV colour, we arenotshown. comparedthemwiththeirknown2MASScolours.TheE(B−V) values for the stars were measured from Schlegel et al. (1998) andconvertedto NUV, J and H extinctionusingCardelliet al. the area used by Kinman et al. (2007),which contains 66% of (1989) extinction law: A(NUV) = 8.90E(B − V), A(J) = theBHBcandidatesselectedonthebasisofFUV−NUVcolour. 0.874E(B−V),andA(H)=0.589E(B−V).Figure13ashowsthe Similarly,Fig.13bshowstheH−KversusJ−Hcolour–colour (J−K) versus(NUV−J) colour–colourdiagramfortheBHB diagramforthe WD candidates.Thedashedrectangleencloses o o candidates.The sourcesatdifferentlatitude intervalsarerepre- theareainthecolour–colourdiagramchosenfromHoardetal. sentedbydifferentsymbols.Thedashedparallelogramencloses (2007) that contains a majority of the WD candidates of our A33,page10of13

Description:
In this paper, we have used the GALEX GR6 data which is . sources and termed the clean sample as “GALEX+SDSS stars”. 2.5. If we consider all stars without accounting for .. A similar trend has been observed by both Du et al. Kalirai, J. S., Hansen, B. M. S., & Kelson, D. D. 2008, ApJ, 676, 594
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