The AAT/WFI survey of the Monoceros Ring and Canis Major Dwarf galaxy: I. from l = (193 - 276)o

0.52 MB·English

by  Blair C. Conn

Mon.Not.R.Astron.Soc.000,1–22(2006) Printed5February2008 (MNLaTEXstylefilev2.2)
The AAT/WFI survey of the Monoceros Ring and Canis Major
◦
Dwarf galaxy: I. from l = (193 - 276)
Blair C. Conn1,2, Richard R. Lane1, Geraint F. Lewis1, Rodrigo Gil-Merino1, Mike J. Irwin3,
4 4 5 6
Rodrigo A. Ibata , Nicolas F. Martin , Michele Bellazzini , Robert Sharp ,
7
0 Artem V. Tuntsov1 & Annette M. N. Ferguson7
0
1InstituteofAstronomy,SchoolofPhysics,A29,UniversityofSydney,NSW2006,Australia
2
2EuropeanSouthernObservatory,AlonsodeCordova3107,Vitacura,Santiago,Chile.Email:[email protected]

n 3InstituteofAstronomy,MadingleyRoad,Cambridge,CB30HA,U.K.
a 4ObservatoiredeStrasbourg,11,ruedel’Universite´,F-67000,Strasbourg,France
J 5INAF-OsservatorioAstronomicodiBologna,ViaRanzani1,40127,Bologna,Italy
3 6Anglo-AustralianObservatory,Epping,NSW,1710,Australia
2 7InstituteforAstronomy,UniversityofEdinburgh,RoyalObservatory,BlackfordHill,Edinburgh,EH93HJ,U.K.

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v 5February2008 (Version2.6)
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ABSTRACT
1
We present the results of an AAT wide-field camera survey of the stars in the Monoceros
0 Ring and purportedCanis Major overdensityin the Galactic longitudesof l = (193 - 276)◦.
7
Currentnumericalsimulationssuggestthatbothofthesestructuresaretheresultofa single
0
/ on-goingaccretion event,althoughan alternativesolution is thatthe warpedand flareddisc
h ofthe Galaxy canexplainthe originofboth ofthese structures.Our resultsshow that, with
p
regardstheMonocerosRing,thewarpedandflareddiscisunabletoreproducethelocations
-
o and strengths of the detections observed around the Galaxy. This supports a non-Galactic
r originforthisstructure.We report8newdetectionsand2tentativedetectionsoftheMono-
t
s cerosRinginthissurvey.TheexactnatureoftheCanisMajoroverdensityisstillunresolved
a althoughthissurveyprovidesevidencethatinvokingtheGalacticwarpisnotasufficientso-
:
v lutionwhencomparedwith observation.Severalfieldsinthissurveyarehighlyinconsistent
i withthecurrentGalacticdiscmodelsthatincludeawarpandflare,tosuchanextentthatex-
X
plainingtheiroriginswiththesestructuresisproblematic.WealsoreportthattheBluePlume
r starspreviouslyinvokedtosupportthedwarfgalaxyhypothesisisunfounded,andassociating
a
themwithanouterspiralarmisequallyproblematic.StandardGalacticmodelsareunableto
accommodatealltheobservationsofthesenewstructures,leadingawayfromawarped/flared
disc explanationfortheir originsandmoretowarda non-Galacticsource.Additionally,evi-
dence is presented in favour of a detection of the Canis Major dwarf stream away from the
CanisMajorregion.AstheouterreachesoftheGalacticdisccontinuetobeprobed,thefas-
cinatingstructuresthat are the MonocerosRing and Canis Major overdensitywill no doubt
continuetoinformusoftheuniquestructureandformationoftheMilkyWay.
Keywords: Galaxy:formation–Galaxy:structure–galaxies:interactions
1 INTRODUCTION graduallyaccreted over time,anddo theyresolveprevious issues
suchasthemissingsatelliteproblem?
Recentdeepoptical/IRsurveys,suchastheSloanDigitalSkySur- While it may be a little premature to answer these ques-
vey(SDSS)(Adelman-McCarthyetal.2006)andtheTwo-Micron tions with the current knowledge of Local Group environment,
All Sky Survey (2MASS) (Skrutskieetal. 2006) are revealing there is still little direct evidence to support the idea of a
increasingly complex structures in the Halo of the Milky Way. merger history for the Milky Way. The Sagittarius dwarf galaxy
Among these, several new dwarf galaxies and tidal streams have (Ibata,Gilmore,&Irwin1994)hasshownthatsuchaccretiondoes
beenuncovered,butdotheseresultsfavourthecurrentgalaxyfor- occurandisinfacton-goingbuttherearestilltoofewnearbytidal
mation paradigm? Can we consider these new structures as sup- stream remnants to confirm it as the primary Galaxy formation
portingtheΛColdDarkMattertheories(Abadietal.2003a,b)of mechanism. However, analyses of these large surveys are uncov-
galaxyformation?Arethesethelow-masssatelliteswhichwillbe eringnewdwarfgalaxiesandtidalstreamsintheinnerhaloofthe
2 BlairConn et al.
MilkyWay.Forexample,thediscoveryoftidaldebriscovering60◦ tative detections in the fields (l,b) = (61,±15)◦ and (75,+15)◦.
oftheskyonly20 kpcfromtheSun,alongwithanewdwarfgalaxy More recently, Belokurovetal. (2006a), while tracing the Sagit-
andothertidaldebrisispresentedbyGrillmair&Johnson(2006); tarius stream with SDSS, notes the presence of the MRi in two
Grillmair&Dionatos(2006a,b);Grillmair(2006a).Willmanetal. bandsatlatitudesofb=(20-30)◦.Additionally,Grillmair(2006b)
(2005a,b)haveuncoveredanewdwarfgalaxyinUrsaMajoralong discussessubstructureintheMRi,againrevealedintheSDSScat-
with the tiny Willman I object, which is on the border between alogue.Consistently,theMRiisfoundonbothsidesoftheplaneof
globular clusters and dwarf galaxies. Even closer satellite galax- theGalaxyatgalactocentricdistancesof∼17kpc.
ieshavebeenfoundwithBelokurovetal.(2006c)andZuckeretal. Revealing an overdensity in the 2MASS data, Martinetal.
(2006)presentingthediscoveryofanothertwodwarfgalaxiesus- (2004a) fulfilled a prediction by Newbergetal. (2002) that a po-
ingtheSDSSdatabase.ThesetwoinBoo¨tesandCanesVenaticiare tential progenitor of the MRi could lie in the nearby Canis Ma-
located heliocentrically at ∼60 and 220kpcrespectively, and are, jor constellation. Nestled under the Galactic disc, this overden-
withtheUrsaMajordwarfgalaxy,pointingtoapossibleresolution sity, dubbed the Canis Major dwarf galaxy (CMa), can be found
ofthemissingGalacticsatellitesproblemoutlinedinKlypinetal. at (l,b) = (240,−9)◦ and ∼7 kpc from the Sun. Its close prox-
(1999). Further structure is also highlighted in the discussion by imityto the Galactic disc led Momanyetal. (2004) to argue that
Belokurovetal.(2006a)onthepresenceoftidalarmsinthenearby the overdensity was simply a consequence of the Galactic warp.
globularclusterNGC5466. In response to this interpretation, Martinetal. (2005a) presented
Ournearestspiralneighbour,M31,alsoshowsacomplicated results from a 2-degree Field (2dF) spectrographic radial veloc-
formation history as Fergusonetal. (2002) have revealed in the ity survey taken at the Anglo-Australian Telescope. The initial
Isaac Newton Telescope WideFieldCameraSurvey (INT/WFC). results were complicated through difficulties in removing the in-
M31waslongthoughttohavebeenarelativelyquietspiralgalaxy strumental signature from the data, but after resolving these is-
with a well defined edge, but a faint diffuse outer disc, riddled suesMartinetal.(2005a)maintainedaninterestingpopulationof
with substructure, is now visible. Within this substructure, a gi- starswithavelocityanomaloustotheGalacticdisc.Additionally,
ant stellar stream (Ibataetal. 2001c), a new class of stellar clus- Connetal. (2005b) showed that in the background of CMa, the
ter(Huxoretal.2005)and thenew dwarf galaxy, Andromeda IX MRi was present at a distance of ∼13.5 kpc, with a velocity of
(Lewisetal. 2004; Zuckeretal. 2004), have been discovered. As ∼133 kms−1 and a dispersion of 23 kms−1. Finally, using ra-
weprobeourownGalaxytosimilardepths,willweuncoversimi- dialvelocitydataoftheCarinadwarfandAndromedagalaxies,the
larstructure?TheresultscomingfromtheSDSSand2MASSsur- MRiwasrevealedintheforegroundoftheseobjects(Martinetal.
veys are suggestive of this. The Milky Way however, does seem 2006a).InfrontoftheCarinadwarf,ithaspropertiesof<V >=
r
tobealesscomplexsystem.AlthoughtheMilkyWayshowsevi- 145±5 kms−1 withavelocitydispersionofonly17±5 kms−1.
denceoftidaldebrisintheHalo,withintheDiscoftheGalaxya InfrontoftheAndromedagalaxy,withstarstakenfromthe“One
majorchangeinrecentyearshasbeenthediscoveryoftheMono- Ring”field(l,b)=(123,−19)◦,ithaspropertiesof<V >=−75±4
r
cerosRingbyNewbergetal.(2002)anditspurportedprogenitor, kms−1andadispersionof26±3 kms−1.Slowly,bothavelocity
theCanisMajordwarfgalaxy(Martinetal.2004a).Thealternative andspatialdistributionoftheMRiisbeingrevealed.
sourceoftheexcessstarsinCanisMajoristheArgoStarSystem The three main sources of evidence for the CMa dwarf
asdiscussedinRocha-Pintoetal.(2006). are (in order of significance): the overabundance of Red Clump
The focus of the survey presented here has been to extend and RGB stars in this region as seen in the 2MASS catalogue
the INT/WFCsurvey of theMonoceros Ring (Connetal. 2005a) (Bellazzinietal.2006);theadditionalvelocitycomponentasseen
around the Galacticplane, aswell assurveying around theCanis in the 2dF survey of the CMa region and the presence of a
Major region to provide insight into the possibility of locating a strong Blue Plume population as can be seen in Figure 1 of
dwarfgalaxythere.Thepresentsurveyhasalsoattemptedtoinves- Mart´ınez-Delgadoetal.(2005a).Recently,theoriginsoftheBlue
tigatetheTriangulum-Andromedaeoverdensity(Rocha-Pintoetal. PlumepopulationhasbeenbroughtintoquestionbyCarraroetal.
2004) and the Virgooverdensity (Juric´etal. 2005). Theseresults (2005)andMoitinhoetal.(2006),whodonotassociatethesestars
willbepresentedelsewhere. withtheforeground overdensity, butrather part ofamoredistant
Thelayoutofthispaperisasfollows:§2summarisesthedis- populationofstars.Thisdistantpopulationisclaimedtobetheex-
coveryoftheMonocerosRingandtheCanisMajordwarfgalaxy; tensionoftheNorma-CygnusspiralarmintotheCMaregion;and
§3describestheobservational procedureanddatareduction.Sec- sothesestarsnowrequiregreaterscrutinyinlightofthisnewin-
tion 4 outlines the method employed to analyse the data, the use terpretation.Momanyetal.(2006)hasalsoproducedamorecom-
of a synthetic Galaxy model for comparison and the procedures pletestudyoftheirwarpscenario,inwhichtheyconfrontthefirst
fordeterminingdistancesandcompleteness.Section5presentsthe andsecondsourcesofevidenceforCMa.Theyarguethatnotonly
dataandthediscussionandconclusionsofthisstudyarefoundin cantheCMaoverdensitybeexplainedbythewarp,butthattheve-
§6and§7. locitydispersionaspresentedbyMartinetal.(2005a)isexpected
bycurrentGalaxymodelsandshowsnothingnew.Instudyingthe
outerdisctheysuggest theMRiisnotatidalarmfromanaccre-
tioneventbutrathertheextensionoftheflareofthediscintothose
2 THEMONOCEROSRINGANDTHECANISMAJOR
latitudes.Inshort,theyclaimallofthenew structuresinthedisc
DWARFGALAXY
oftheMilkyWayaresimplyexplainedintermsofknownGalac-
Discovering the Monoceros Ring (MRi) via an overdensity of ticstructure.Lo´pez-Corredoira(2006)concludesonthe“Galactic
colourselectedF-starsintheSDSSdataset,Newbergetal.(2002) warpversusdwarfgalaxy”debatewiththestatementthatthewarp
describeditsoriginalextentfroml=(170-220)◦.Subsequentsur- can be formed by such a wide variety of causes that neither ra-
veysin2MASS(Rocha-Pintoetal.2003;Martinetal.2004a)and dialvelocityorphotographicsurveyscandisentangletheirorigins.
the INT/WFC (Ibataetal. 2003; Connetal. 2005a) extended the Thisismaybethecase,althoughasystematicsurveyoftheages,
MRi detections back towards the Galactic centre with some ten- metallicities, distances and abnormal velocity profiles in this re-
TheAAT/WFIsurveyof theMonocerosRingandCanis MajorDwarfgalaxy 3
gionshouldprovidestrongindicationsifadwarfgalaxyresidesin
the plane or not. Thewarp isan important part of thepuzzle but
shouldnotstopprogressinresolvingthisissue.
Although the results from this paper are unable to answer
questionsregardingthevelocityprofileoftheseouterdiscobjects,
itwillattempttounderstandthoseoftheCMaandMRioverden-
sities with regard to their distance and position in the Galaxy. In
ourdiscussion,wecommentonthelikelihoodoftheproposalsput
forward byMomanyetal. (2006) in explaining theMRi interms
oftheouterdiscandwhetherthewarpsatisfactorilydescribesthe
CMaoverdensity.
3 OBSERVATIONSANDREDUCTION
ThedatawereobtainedusingtheAnglo-AustralianTelescopeWide
Figure1.Thelocationofthesurveyfieldsintheregion(l,b)=(193-276)◦.
Field Imager (AAT/WFI)at Siding Spring Observatory (SSO) in
Thesymbolsdenotethenumberofpointingsperfield.Theresultantfields
NewSouthWales,Australia.Mountedatthetelescopeprimefocus,
havebeenselectedbasedondataqualityandwhileideallyfourpointings
thecameraconsistingofeight4k×2kCCDswith0.2295arcsecper
wouldcorrespondtoabout1squaredegreeofsky,observationsundertaken
pixel,coversafieldofviewapproximately33’×33’perpointing. duringFebruary2006hadonly7outof8CCDsavailableinthearray.
Theobservationsweretakenoverfourobservingruns,thefirst
onthe22nd-25th January2004,thesecondon30th January-1st
February2004,thethirdonthe14th-16thofAugust2004andthe by Schlegel1 was used to determine the extinction for each star.
fourthonthe1st-5thFebruary2006.Allofthefieldswereobserved This program interpolates the extinction from the dust maps of
withtheg(WFISDSS#90)andr(WFISDSS#91)filters.These Schlegel,Finkbeiner,&Davis (1998). Using the several standard
were chosen to minimise the fringing effects that can be present fieldsobservedpernightprovidesacomparisonforthecalibration
when observing with other filters. Each exposure consisted of a ofthephotometrytobedetermined.Thestandardsareusedtode-
single600secondexposurewiththegfilterandtwo450secondex- rivetheCCDzeropointsfromwhichallthemagnitudesaredeter-
posureswiththerfilter.Twoexposureswereperformedinrsoas mined (Irwin&Lewis2001). A catalogue of each colour-band is
toavoidpreservingcosmicraysandoverexposingthebrighterstars producedbythepipelineforeachpairedexposureofgandr.Non-
whenusingasingle900secondexposure.Eachnight,twilightflats stellarimagesarerejected;however, nearthelimitingmagnitude,
weretakenalongwithbiasanddarkframesforcalibrationandthe galaxies begin toappear stellar-likeand thusbecome a source of
closestLandoltStandardStarfieldtoourtargetwasobservedevery contaminationinthedataset.
twohours. Inthismanner,theremoval ofinstrumentalsignatures
andprecisephotometriccalibrationcouldbeachieved.Theseeing
at the SSO can vary from 0.9′′ to3.0′′ thus heavily affecting the
4 DATAPREPARATION
limitingmagnitudeofthedata.Althoughsomefieldswerelostdue
topoorweatherconditions,onlythefieldswiththebestphotometry 4.1 DetectingNon-GalacticStructure
havebeenpresentedhere.
SearchingforadditionalstructurewithintheColour-Magnitudedi-
The present survey was designed as a continuation of the
agrams (CMDs) of the AAT/WFIsurvey requires an understand-
MonocerosRingSurveyobservedwiththeIsaacNewtonTelescope
ingoftheinherent structurespresentwhenobservingthroughthe
WideFieldCamera(Connetal.2005a).Thefieldswerechosento
Galactic disc. The Milky Way can be roughly divided into four
beroughlyseparatedby20degreesinGalacticLongitudewithad-
components:Thindisc,Thickdisc,HaloandBulge.TheThindisc
justments made on the final location to ensure that the field was
isessentiallytheplaneoftheGalaxycontainingthemajorityofthe
placed to ensure minimal dust extinction. Altogether the present
stars and is where the spiral structure is also present. The Thick
surveyhasobservedfieldsfroml=(193-25)◦,acrosstheGalactic
discislessdensethantheThindiscandhasagreaterscaleheight
bulge; thispaper reports only onthe resultsof those fieldsinthe
outoftheplane.TheHaloisconsideredtobeasmoothspherical
regionl=(193-276)◦.
distributionofstarsaroundthecentreofgravityoftheGalaxyand
Thispartofthesurveyconsistsof16fields.Mostfieldsareap-
extendsoutbeyond20-30kpc.TheBulgeisthecentralregionof
proximatelyonesquaredegreeinsize(fourWFIpointings).How-
theGalaxy,includingtheBar,andcontainsaverydenseoldstellar
ever some fields are a combination of one, two or three point- population. The Besanc¸on synthetic Galaxy model2 (Robinetal.
ingsdependingonthetimeavailableandqualityofdataobtained.
2003)allowsforthesecomponentstobeconsideredseparatelyas
Thesinglepointingfieldsmakeupastripofobservations linking
shown in Figure 2. Each component is revealed in a distinct re-
(240,−9)◦ with(240,+10)◦.Asummaryofthefieldlocationsand
gionoftheCMD,allowingforatleastapreliminaryestimateofits
areaoftheskyobservedwiththepreliminaryresultsofthissurvey
origins when interpreting the CMDs fromthe observational data.
isshowninFigure1andTable1.
It should be noted that the Besanc¸on model employs a Thin disc
A specialised version of the CASU data reduction pipeline
cut-offat∼14 kpcfromtheGalacticcentre.Thevalidityofsucha
(Irwin&Lewis2001)wasusedtoperformthede-biasingandtrim-
cut-offisdisputedinLo´pez-Corredoira(2006).
ming, vignetting correction, astrometry and photometry. The flat
fielding of the science frames used a master twilight flat gen-
erated over each entire observing run. To account for the dust 1 http://www.astro.princeton.edu/∼schlegel/dust/data/data.html
extinction in the fields the dust getval.c program supplied 2 http://www.obs-besancon.fr/www/modele
4 BlairConn et al.
Table1.SummaryoftheobservationsofCanisMajorTidalStreamwiththeAAT/WFI,orderedinascendingGalacticlongitude(l).Thevariationbetweenthe
totalareacalculationsof2004and2006areduetofailedCCDsinthearrayshrinkingthefieldofview.
Fields Regions AverageSeeing TotalArea MonocerosRing Average Date
(l,b)◦ perfield (arcsec) (deg2) E(B-V)
(193,−21)◦ 4 1.3 1.21 Maybe 0.08 30/01/04
(218,+6)◦ 3 1.3 0.8 Yes 0.01 03/02/06
(220,−15)◦ 4 1.0 1.21 Yes 0.22 31/01/04
(220,+15)◦ 3 1.3 0.91 Yes 0.04 25/01/04
(240,−9)◦ 3 1.0 0.91 Yes 0.18 24/01/04
(240,−6)◦ 1 1.0 0.3 No 0.40 31/01/04
(240,−4)◦ 1 1.0 0.3 No 0.99 31/01/04
(240,−2)◦ 1 1.0 0.3 No 1.10 31/01/04
(240,+2)◦ 1 1.0 0.3 No 0.79 31/01/04
(240,+4)◦ 1 1.0 0.3 Yes 0.28 31/01/04
(240,+6)◦ 4 1.2 1.21 Yes 0.13 31/01/04
(240,+10)◦ 2 1.3 0.61 Yes 0.10 01/02/04
(245,−9)◦ 4 1.6 1.21 Yes 0.14 30/01/04
(260,−10)◦ 3 1.3 0.91 No 0.21 01/02/04
(273,−9)◦ 2 1.4 0.53 No 0.29 03/02/06
(276,+12)◦ 3 1.1 0.8 Yes 0.09 01/02/06
Figure2.ThetoppanelsshowtheHessplots(apixelatedColourMagni-
tudediagraminwhichtheresultant imageisthesquarerootofthenum-
berdensity)of(240,−2)◦fieldfromthesyntheticmodeloftheGalaxyby Figure3.OntheleftistheAAT/WFICMDofthefield(l,b)=(220,+15)◦,
Robinetal.(2003)beingsplitintoitsvariousGalacticcomponents-Thin
andontherightisthesamefieldasproducedbytheBesanc¸onsynthetic
Disc,ThickDiscandHalo.Thisillustrateswherethevariouscomponentsof
Galaxy model. The model outputs are CFHTLS-Megacam (AB) Photo-
thegalaxylieontheCMD(Colour-MagnitudeDiagram).Thelowerpanels metric system converted to Sloan g′ and r′ using Equation 1. The fidu-
arefromthemodelfield(193,−21)◦ andshowhowthecomponentsvary
cial sequence is placed with zero offset and uses the raw SDSS data
furtherawayfromtheplaneoftheGalaxy.
(Newbergetal.2002)fromwhichthefiducialwascreated.
Asanexample,Figure2showsthebreakdownoftheGalactic
components asseenintwoonesquaredegreefieldsat(240,−2)◦ the CMDs that are observed. The closest Main Sequences, being
and(193,−21)◦.AnalysingFigure2revealsthat,asexpected,fields brighterinapparentmagnitude,formatthetopoftheCMDswith
closertotheplanehavemorestarsthanthoseawayfromtheplane. thesubsequentMainSequences,beingfainter,forminglowerdown
Thismanifestsinthesmoother CMDsinthetoppanels, withthe on the CMDs. The different stellar components of the Thin and
bottom panels containing fewer Thin and Thick disc stars. The Thickdiscshowsthesignificantchangeindensitybetweenthetwo,
structure of each component is due to the homogeneity of the giving them different characteristics intheCMDs. The Thindisc
Galaxy and as a result a Main Sequence forms at every distance beingnarrowanddensehasasmallerscaleheight (typicallyesti-
incrementalongthatlineofsight.SoastheMilkyWayisprobed matedatseveralhundredparsecsthick)whichformsamuchtighter
deeper, thecollectiveweightof theMainSequencessumtoform sequenceintheCMD.ThemorediffuseThickdiscbeingmoreex-
TheAAT/WFIsurveyof theMonocerosRingandCanis MajorDwarfgalaxy 5
sidered out to 100 kpc (heliocentric distance, R ) which en-
HC
sures that the Halo population extends below the magnitude cut-
off of the dataand hence does not introduce additional structure.
The synthetic fields contain a broad magnitude range (−99,99)
for each passband with zero extinction and the inclusion of all
ages/populations and luminosity classes. This then provides as
completeapictureoftheregionofinterestasthemodelcansupply.
Italsoallowsustoapplycolouranddistancecutsatourdiscretion.
Themodelcanoutput directlytothebandsg &r in
CFHT CFHT
theCFHTLS-Megacam(AB)3 photometricsystemwhichneedto
beconvertedtotheSDSS(g′,r′)systemviaEquation1.
′
r =r
CFHT
g′= gCFHT −0.1480×rCFHT (1)
1−0.1480
Frei&Gunn (1994) report that no conversion is needed be-
tween SDSSpassbands (g′,r′) inthe AB system to SDSS(g′,r′)
in the Vega system. However, Figure 3 shows that when using
onlyABmagnitudes,theAAT/WFIdataintheVegasystemdoes
not match the model in the AB system. A fiducial of the ridge-
linefromtheoriginal SDSSdetection of theMonoceros Ring by
Newbergetal. (2002) can be seen to lie blueward of the bluest
Figure4.OntheleftistheAAT/WFICMDofthefield(l,b)=(220,+15)◦,
edgeof thedata(See§4.1.2).So,usingtheresultsof Ibataetal.
andontherightisthesamefieldasproducedbytheBesanc¸onsynthetic
Galaxy model. Themodel isnow the Sloang′ andr′ intheABsystem (2003),whocomparedoverlappingINTandSDSSfieldstodeter-
mine a colour conversion between the two systems, the model is
converted to the INT/WFC g and r in the Vega Photometric system via
Equation2.Thefiducialsequencehasalsoundergonethesamecolourcon- nowshiftedtotheVegasysteminthisway.Thisisapplicabletoour
version. dataasboththeINTdatasetandourAATdatasethavebeenreduced
usingthesamepipeline(withsmallmodificationstoallowfordif-
ferences betweenthe telescopes) and nosuch study has been un-
tendedoutoftheplane(severalkiloparsecs,thick)formsabroader dertakenwiththeAAT/WFIinstrument.Theresultantcolourtrans-
sequence. Comparing the top and bottom panels in Figure2 also formationtotheINT(g,r)is:
showstheimportanceoftheangleoftheobservationswithregard
theGalacticPlane. (g−r)=0.21+0.86×(g′−r′)
Detecting non-Galactic components then involves visually ′ (2)
g=g +0.15−0.16×(g−r)
comparingthestrongsequencesdetectedintheobservationswith
whatisexpectedfromthemodels.TheMonocerosRingislocated TheeffectofthistransformationcanbeseeninFigure4.The
beyond the edge of the Thick disc, and thus should be seen as a converted SDSSfiducial,correctedviaEquation2, isnow anex-
coherent sequence below the last sequence of the Thick disc, as cellent fit to the strong sequence in these data. The photometric
canbeseeninFigure12ofNewbergetal.(2002).Differentiating systemofthemodelCMDisnowthesameastheAAT/WFICMD
it from the Halo component is simpler due to the Halo not hav- and no further changes have been applied to the model or data.
ing any strong Main Sequences present below the Thick disc in AllfiguresinthispaperwillbeshowninthesameformatasFig-
therangeofourCMDs.So,anystrongsequencebelowtheThick ure 4, utilising Equations 1 and 2, with the resultant (g,r) from
disc will most likely be of non-Galactic origins. Although given thesyntheticGalaxymodelconsideredthesameastheextinction
the recent hypothesis on the MRi being attributed to the flare of corrected observational dataset (g◦,r◦). The Besanc¸on synthetic
the disc, this will also need to be considered. Another possibil- Galaxymodel employsdifferentdensityprofilesforeachcompo-
ityisthatanon-sphericalhalocouldbemisinterpretedasa“non- nentoftheGalaxy.ThesehavebeenoutlinedinConnetal.(2005a)
Galactic” feature in the outer Thick disc. Comparisons with the and in more detail in Robin&Creze (1986); Robinetal. (1996);
Besanc¸onsynthetic galaxy model reveals that theirspheroid den- Robin,Reyle´,&Cre´ze´(2000)andReyle´&Robin(2001).
sitydistributionhasaflatteningof0.76andapowerindexof2.44
(Robin,Reyle´,&Cre´ze´ 2000)whichismoderatelynon-spherical.
4.1.1 MagnitudeCompleteness
Theynoteasignificantdegeneracybetweentheseparameterswhich
could allow for spheroids with a (c/a) of 0.85 (close to spheri- Most of the fields presented here, consist of several overlapping
cal) or 0.6 (quite oblate). As discussed in the following sections, subfields, see column 2, Table 1. The completeness of this sam-
the synthetic galaxy model does not introduce structures into the ple isdetermined in asimilar manner to that of the2MASS All-
outer Thickdiscandsotheinfluence of anon-spherical halocan SkyPointSourceCatalogue(Skrutskieetal.2006).Thisapproach
beassumedtobeminimal.TheexistenceoftheCanisMajordwarf determines thefraction of starsthat aredetected inboth overlap-
galaxyiscurrentlybeingscrutinisedandsotheCMDsoftheCMa pingimagesasafunctionofmagnitude.Sobymatchingthestars
regionwillbeusedtoinvestigatetheinfluenceoftheGalacticwarp withinthoseoverlappingregionsandcalculatingtheproportionof
on thestellarpopulations present thereand whether it isaviable
solutiontoresolvingthedwarfgalaxydebate.
ThesyntheticCMDsfromtheBesanc¸onmodeldataarecon- 3 http://www.cfht.hawaii.edu/Instruments/Imaging/MegaPrime/
6 BlairConn et al.
Determiningavaluefortheerrorassociatedwithsuchamea-
Table2.Parametersusedtomodelthecompletenessofeachfield,ordered
inascendingGalacticlongitude(l).mcistheestimated50%completeness surement is dependent on several factors. Most predominant of
level for each filter with λ describing the width ofthe rollover function theseiswhether thefieldshavebeencorrectlycalibratedwithre-
(Equation3). gard totheir photometry and taking into account the dust extinc-
tionpresentwithinthefields.Thedustcorrectionforthisdatawill
Fields(l,b) mc(g◦) mc(r◦) λ always over-correct for stars within the Galaxy, because the dust
value is based on the entire cumulative impact of the dust along
(193,−21)◦ 22.40 21.80 0.40
(218,+6)◦ 23.40 22.40 0.50 that line-of-sight (Schlegel,Finkbeiner,&Davis 1998). The stars
(220,−15)◦ 22.90 21.90 0.30 inthissurveydonotresideattheendofthatline-of-sightandso
(220,+15)◦ 23.60 22.60 0.30 willbeover-correctedinthedustextinctionprocess.Inmostofthe
(240,−9)◦ 23.60 22.50 0.40 fields,thelevelsofdustaresufficientlylowthatthedifferencebe-
(240,+6)◦ 24.00 22.70 0.40 tweenthedustvalueusedandthe“correct”valueshouldbesmall,
(240,+10)◦ 22.65 21.70 0.25 seeTable1.
(245,−9)◦ 23.30 22.20 0.60 To determine whether the colour transformations applied to
(260,−10)◦ 22.80 21.80 0.40 thedata set correspond to reliabledistance estimates, threefields
(273,−9)◦ 23.40 22.20 0.50 to which there are distance estimates to these structures from
(276,+12)◦ 23.70 22.50 0.40 other surveys have been analysed. Those are (l,b) = (220,+15)◦,
(240,−9)◦ and(245,−9)◦.Thefirstfieldisveryclosetotheorig-
inal Monoceros Ring detection of Newbergetal. (2002) and the
matchedstarsineachmagnitudebinwithrespecttothetotalnum-
second and third are part of the Canis Major detection fields of
berofstarsobserved,anestimateofthecompletenessismade.This
Martinetal.(2005a).TheMRiisknowntohaveadistanceof11.0
producesaphotometriccompletenesscurvewhichcanfitapproxi-
kpcintheNewbergetal.(2002)field,at(220,+15)◦ itisalsolo-
matelybytheequation:
catedat11.0 kpc.FortheCMafields,thedistancedeterminedby
1
CF = (3) Martinetal.(2005a)isabout7.2 kpc.Thepresentsurveylocates
1+e(m−mc)/λ thecentreofthestrongsequenceat7.3 kpcor−0.9magnitudesof
wheremisthemagnitudeofthestar,mc isthemagnitudeat50% offset.Importantly then, eachdistance estimatehereisconsistent
completenessandλisthewidthoftherolloverfrom100%to0% withindependentmeasurementsofthosestructures.
completeness.Thevaluesusedtomodeleachfieldcanbefoundin Havingunderstoodtheerrorsinvolvedinboththedetermina-
Table2. tion of the photometry, extinction correction and the fiducial se-
Althoughthecompletenessofoursurveyisnotakeyproblem, quence, manually placing this fiducial at the centre and two ex-
attemptingtocharacteriseitdoesprovideamannerinwhichwecan tremesofanacceptablefit“by-eye”providesarangeofdistances
comparethedataqualityofthevariousfields.Ingeneral,thisallows overwhichthisstructureresides.Giventhelargeerrorsinvolved,
anestimateofthemagnitudeatwhichthedatabecomesuntrustwor- these distances can only be considered indicative of the true dis-
thy. Additionally, sincethe model ismostly used tohelp identify tance. However, the range of magnitude offsets defining the ex-
the major structures in the CMDs, it is unnecessary to apply the tremesdogiveasenseofthewidthofthestructure.Severalfields
completeness function to the model. This is because those struc- haveonlyonedistanceestimate,aslocatingtheextremesisnotpos-
turesaretypicallywellawayfromthe50%completenesslimit.It sibleduetothedataqualityorthenarrownessofthesequence.The
isalsoimportanttonotethatwhilethismethoddoesnotaccountfor dominantstrongsequenceineachfield,whichcouldbeattributed
starsincrowdedfields,noneofoursurveyfieldscanbeconsidered toeithertheThin/ThickdiscorCMaoverdensityhasonlyasingle
crowdedandsothisapproachisvalidfortheentiredataset. distance estimate, corresponding to the faintest edge of that fea-
ture.Thisisdue,ingeneral,tothelackofadefiniteupperedge,see
Table3.
4.1.2 EstimatingtheDistanceandAdditionalCalibration
DeterminingthedistancetotheCanisMajorandMonocerosRing
4.1.3 SignaltoNoiseEstimationTechnique
sequences isachieved bytaking theridge-lineof the detectionin
the SDSS S223+20 field [Newbergetal. (2002), Figure 12] and Estimatingthesignal tonoise ratio(S/N)oftheMonoceros Ring
creatingafiducialsequence.TheAAT/WFIusesSDSSfiltersand Main Sequence found in the data has been attempted for several
sothefiducialsequence canbedirectlyusedonthedatawiththe fields.ThecriteriaforS/NdeterminationisthattheMRiMainSe-
colourtransformationsdescribedinEquation2. quenceissufficientlydistinctfromtheCMa/Discsequencetoavoid
TheheliocentricdistanceestimateoftheSDSSS223+20de- potential contamination. The model field is assumed to represent
tectionisassumedtobe11.0 kpc(Newbergetal.2002),thisthen thepropertiesofthebackgroundGalacticstarswhichshouldbere-
is the zero offset distance. Since the fields have been extinction movedtohighlighttheadditionalMRiMainSequence.Themodel
correcteditisassumedthatonlydistancevariationsarethecause needstobeadjustedfirsttomoreaccuratelyreflectthepropertiesof
for any deviation in magnitude from this location. This method thedataandthensubtractedtorevealtheexcessMRistars.Before
doesnotaccountforpossibledifferencesinageormetallicitybe- subtractionthemodelisdegradedtomatchboththecompleteness
tweenMainSequences.Theheliocentricdistanceiscalculatedus- profileofthedataasshowninTable2andtherelevantmagnitude
ingEquation4andassumingtheSunis8.0 kpcfromtheGalactic errorproperties.Toensurethatsimilarnumbersofstarsarepresent
centre,thegalactocentricdistanceisfoundfromsimpletrigonom- inboththedataandmodelpriortosubtractionanadditionalscaling
etry. factorisapplied.Theseprocessesresultinthedataandthemodel
beingessentiallyidenticalwiththeexclusionoftheadditionalMRi
offset Main Sequence. The S/N isestimated by dividing the number of
RHC =11.0ׄ10 5.0 « (4) starsinthefeaturebythePoissonnoiseduetothemodellednum-
TheAAT/WFIsurveyof theMonocerosRingandCanis MajorDwarfgalaxy 7
inthemodelandtheCMaMainSequenceisinferredtobeassoci-
atedwiththe strongest Main Sequence inthe CMD. Thefiducial
isthenplacedontheCMDandshiftedinmagnitudetobealigned
“by-eye”withtherespectivefeatures.Amoreaccuratetechniqueis
unneccessarygiventheinherentuncertaintiesalreadypresentina
distanceestimateofthiskind.Itisimportanttonotethatthefidu-
cialsplaced onthe dominant featuresof the CMD arenot neces-
sarily considered detections, but rather placed to provide insight
into all the features present in the CMDs. For the dominant and
strongestsequenceineachCMD,onlythefainteredgehasbeenfit
withthefiducialsequenceprovidingasingledistanceestimate.For
thefaintercoherentsequences,boththeupperandlowerextremes
have been fitted giving a range of distances to that structure, see
Figure5.Thecompletelistofthemagnitude offsetsanddistance
estimatesiscontainedinTable3.Thebrighter,nearerormoredom-
inantsequencehasbeenlistedundertheheadingMW/CMa(Milky
Way/CanisMajor)toillustratethatdifferentiatingbetweenthetwo
isnotstraightforwardwhenusingCMDs.Althoughsomefieldsare
mostlikelytocontainonlyMilkyWaystars,thereareseveralthat
arepossiblyamix,orcompletelydominatedbyCMastars.Forthis
reason,thisstructureisleftambiguouslyidentifiedinTable3.The
fainter,moredistantorlessdominantsequenceislistedunderthe
headingMRi.Tounderstandthewidthsofthestructuresandhence
Figure 5. Hess plot (a pixelated Colour-Magnitude diagram where the
grayscaleisthesquarerootofthenumberdensityforthatpixel)of(l,b)◦ anestimationoftheerrorsforeachfield,thistableshouldberefer-
=(220,+15)◦(Left)andthecorrespondingBesanc¸onmodel(Right).This enced.ThefinalstructureofinterestaretheBluePlumestarswhich
figureillustratesthealignmentofthefiducialwithboththedominantMain arelocatedaround18thmagnitudeandinthecolourrange(g−r)
SequenceintheCMDassociatedwithbrighternearbystarsandthefiducial = 0.0 - 0.3. These stars can be seen clearly in Figure 10 and are
alignedwiththeMRifeature.Asillustrated,thedominantMainSequence discussedfurtherin§6.2.2.
isonlydefinedbyitsfainteredgewhiletheMRifeatureislocatedalong
thecentreoftheMainSequence.Forthisfeaturetheoffsetsemployedare
−2.5and0.0mag.Toavoidunnecessarily cluttering thedataCMDs,the
following figures only show the placement of the fiducial onthe model.
5.1 MonocerosRegion
Thisallowsadirectcomparisonwithmodelandhighlightswherethenew
featuresliewithrespecttotheexpectedGalacticcomponents. Thefourfieldspresentedhereareincloseproximitytotheoriginal
detections of the Monoceros Ring as presented in Newbergetal.
(2002).Importantly,thepresentsurveyhassampledbothsidesof
berofstarsintheregion4.Onlyfivefieldshavebeensuitablefor
theplane,findingtheMRitobepresentacrosstheGalacticplane.
thisestimatenamely(l,b)◦=(218,+6)◦,(220,+15)◦,(220,−15)◦,
ThishasimplicationsregardingwhethertheMRicouldbeaphe-
(240,+10)◦ and (276,+12)◦, see Table 3. These fields have MRi
nomenarelatedtotheGalacticwarpandflare.
detectionswhichareeasilydistinguishedfromthedominantMain
SequencesintheCMDandthusaresuitableforthistechnique.The
remainingdetectionsaretooclosetotheCMa/Discpopulationto
easilymeasuretheirS/Nratiowiththismethod. 5.1.1 Fieldsat(193,−21)◦
ThisisthefurthestfieldWestfromCanisMajor(towardstheanti-
centredirection)thatisincludedinthissurvey(Figure6).Despite
5 SURVEYFIELDS
thegoodseeingandareacoveredinthisfield,manyofthefainter
Thelocationof eachfieldisshowngraphicallyinFigure1.Each starshavebeenlostduetotherelativeproximityoftheMoon.This
fieldpresentedinthissectionshowstheCMDswiththeappropri- has removed a lot of the detail present in other fields of similar
atefiducialsequencetakenfromtheoriginalNewbergetal.(2002) size.Usingthefiducialsequence onthetwomainfeaturesinthis
detectionasdescribedintheprevioussection.TheCMDsthatwe CMD,weobtain twodistance estimates.Thefirstusinganoffset
haveusedaredensitymapsoftheunderlyingdistributionwiththe of−1.2magcorrespondingtoaheliocentricdistanceof∼6.3 kpc.
squarerootofthenumberofstarsperpixelbeingpresentedinall Thesecondmoredistantfeature,somewhatmoretentative,isfound
oftheCMDsinthispaper,calledaHessdiagram.Thismethodpro- at an offset of 0.5 mag, or RHC ∼ 13.8 kpc. Given the lack of
videsbettercontrastofthestructuresespeciallyinhighstardensity clarityregardingthepotentialMonocerosRingfeatureinthedata,
regions.Inthefollowingsections,thedistanceestimatestothema- noattempthasbeenmadetomeasurethespreadofdistancesover
jor features present in each CMD are provided with an analysis whichitisvisible.Indeed,itisuncertainwhetherthisissimplythe
of these results in the Discussion section. The principal features strongoverdensityofstarsseenatthefaintblueendofthemodel
of the CMD have been identified by visual comparison with the CMD.ThestarslocatedatthefaintblueendoftheCMDaremost
Besanc¸onmodel.TheMRiMainSequenceisevidentbyitsabsence likelymisclassifiedgalaxiesandareunlikelytorepresentanyreal
Galacticstructure,theyhaveaconsiderableerrorincolourascan
beseenbytheerrorbarsontherighthandsideofthepanel.The
4 Parametersavailableonrequest:[email protected] 50%completenessing◦forthisCMDis22.4mag.
8 BlairConn et al.
Table3.SummaryoftheobservationsofCanisMajorTidalStreamwiththeAAT/WFI,orderedinascendingGalacticlongitude(l).
Fields(l,b)◦ MRioffset MRidist MRiS/N MW/CMaoffset MW/CMadist
&width(mag) &width(kpc) estimate Loweredge(mag) (kpc)
(193,−21)◦ 0.5 13.8 - −1.2 6.3
(218,+6)◦ 0.0+−00..335 11.0+−11..94 ∼34 −1.5 5.5
(220,−15)◦ 0.2+−00..33 12.1+−11..76 ∼32 −1.4 5.8
(220,+15)◦ 0.0+−00..335 11.0+−11..66 ∼14 −2.5 3.5
(240,−9)◦ 0.4 13.2 - −0.6 8.3
(240,−6)◦ - - - - -
(240,−4)◦ - - - - -
(240,−2)◦ - - - - -
(240,+2)◦ - - - - -
(240,+4)◦ 0.1 11.5 - −1.4 5.8
(240,+6)◦ 0.3+−00..33 12.6+−11..96 - −1.2 6.3
(240,+10)◦ 0.5+−00..33 13.8+−21..17 ∼22 −2.2 4.0
(245,−9)◦ 0.5 13.8 - −0.6 8.3
(260,−10)◦ - - - −0.3 9.6
(273,−9)◦ - - - −0.3 9.6
(276,+12)◦ 0.3 12.6 ∼26 −2.0 4.4
Figure 6. Hess plot (a pixelated Colour-Magnitude diagram where the Figure 7. Hess plot of (l,b)◦ = (218,+6)◦ (Left) and the corresponding
grayscale isthesquarerootofthenumberdensity forthat pixel)of(l,b) Besanc¸onmodel(Right).ThefigureisotherwisethesameasFigure6.The
=(193,−21)◦ (Left)andthecorrespondingBesanc¸onmodel(Right).The sequencesfittedhereareoffsetby−1.5magand0.0+−00..335mag.Thehelio-
synthetic GalaxymodelistakenfromtheBesanc¸ononlinegalaxymodel centricdistanceoftheseoffsetsare∼5.5 kpcand11.0+−11..94 kpc.TheCMD
website.Themodelhasadistanceintervalof100 kpcfromthesuntoen- is50%completeatg◦=23.4mag.
surenoartificialcutsenterintotheCMDs.ThemodelisselectedinUband
andthenconvertedtog,rusingcolourcorrectionsfromtheINT/WFCweb-
site.Therearetwostructuresinthisfield,thestrongsequencebeginningat
g◦∼19.0andafainter coherent sequence atabout 20th-21st magnitude.
TheoffsetsrequiredtofitthefiducialsequencefromNewbergetal.(2002) sequence and 0.0+0.35 mag for the fainter coherent sequence.
are−1.2magand0.5mag.Heliocentrically, this corresponds to6.3and Theseresultindist−a0n.c3eestimatesof∼5.5kpcand∼11.0+1.9 kpc
13.8 kpc.The50%completenessing◦forthisCMDis22.4mag. respectively.TheCMDis50%completeatg◦ =23.4ma−g1..G4iven
thebroadnatureoftheMilkyWaysequence, onlythedistanceto
5.1.2 Fieldsat(218,+6)◦ thelowerboundaryisstated.TheS/NestimatefortheMRiinthis
fieldis∼34.Thisvalueishigherthanexpected,probablyduetothe
AtthesameGalacticlongitudeastheoriginalMonocerosRingde- closeproximityofthedominantMainSequence.Themodeldoes
tection made by Newbergetal. (2002), the CMD is presented in notcleanlysubtractthisfeatureandsosomecountsremaintoboost
Figure7.Thetwofiducialsareoffsetby−1.5magforthebrighter thesignaltonoiseestimate.
TheAAT/WFIsurveyof theMonocerosRingandCanis MajorDwarfgalaxy 9
Figure8.Hessplotof(l,b)◦ =(220,−15)◦ (Left)andthecorresponding Figure9.Hessplotof(l,b)◦ =(220,+15)◦ (Left)andthecorresponding
Besanc¸onmodel(Right).ThefigureisotherwisethesameasFigure6.The Besanc¸onmodel(Right).ThefigureisotherwisethesameasFigure6.The
sequencesfittedhereareoffsetby−1.4and0.2+−00..33mag.Theheliocentric sequencesfittedhereareoffsetby−2.5and0.0+−00..335mag.Theheliocentric
distance oftheseoffsets are5.8and12.1+−11..76 kpc.Atg◦ =22.9magni- distanceoftheseoffsetsare3.5and11.0+−11..66 kpc.The50%completeness
tudes,theCMDis50%complete. ing◦forthisCMDis23.6mag.
5.1.3 Fieldsat(220,−15)◦ latitudeshasdistortedtheCMDs;however,thereisstillinformation
inthesefieldsandforthisreasontheyhavebeenleftin.Thedustis
AstheSouthern counterpart for theoriginal Monoceros Ringde-
tectionmadebyNewbergetal.(2002),theCMDfor(220,−15)◦is moreprominentintheSouthernfieldsascharacterisedbythelesser
distortionoftheCMDsintheNorthernfields.Allofthefieldsfrom
presentedinFigure8.Thetwofiducialsareoffsetby−1.4magfor
thebrighterstrongersequenceand0.2+0.3 magforthefainterco- b=−6◦tob=+6◦wereobservedunderthesameconditionswith
−0.3 the(240,+10)◦fieldobservedthefollowingnight.
herentsequence.Theseresultindistanceestimatesof∼5.8kpcand
∼12.1+−11..76 kpc respectively. At g◦ = 22.9 magnitudes, the CMD
is50% complete. Thefainter sequence isfound tohavea S/N∼ 5.2.1 Fieldsat(240,−9)◦
32,althoughthisestimateisprobablycontaminatedbythenearby
brighterMainSequence. ThisfieldiscentredonthelocationoftheputativecoreoftheCanis
Majordwarfgalaxy(Figure10).Theverystrongsequencerunning
thelengthoftheCMDcanbefitalongthefaintedgewithafidu-
5.1.4 Fieldsat(220,+15)◦
cialoffsetby−0.6magwhichcorrespondsto∼8.3 kpc.Placingit
Thisfield,Figure9,istheclosesttotheoriginalMonocerosRing roughlyalongthecentreofthefeaturerequiresanoffsetof−0.9
detectionmadebyNewbergetal.(2002)takenduringthepresent magor∼7.6 kpc.InConnetal.(2005b),thepresenceoftheMRi
survey. Thetwofiducialsareoffset by−2.5magfor thebrighter inthebackgroundoftheCMaoverdensitywasrevealed.Thiswas
strongersequenceand0.0+0.3 magforthefaintersequence.These determinedtobeatadistanceof13.5±0.3 kpc.Indeed,justbelow
−0.35
result in distance estimates of ∼3.5kpc and ∼11.0+1.6 kpc re- thesequencedominatingtheCMDtheredoesseemtobeanexcess
−1.6
spectively. Being only five degrees from the fields presented in ofstarswhichmaybeanothercoherentsequence,itscontrastislow
Newbergetal. (2002), the distance to the MRi here is the same duetothedominatingeffectoftheCMasequence.Anoffsetof0.4
astheirdistanceestimateof11 kpc.The50%completenessing◦ magisneededtoalignthefiducialwiththisfeature,corresponding
for this CMD is 23.6 mag. A S/N ∼ 14 is found for the fainter toaheliocentricdistanceof13.2 kpc.Giventhematchthismakes
sequence. withtheAAT/2dFdetectionthisisconsideredatentativedetection.
TheCMDis50%completeatg◦=23.6mag.
5.2 CanisMajorRegion
5.2.2 Fieldsat(240,−6)◦
The following fields are part of a strip of observations linking
(240,−9)◦to(240,+10)◦.Thefieldsfromb=−6◦tob=+4◦are In Figure 11, the strong sequence is still present in this field al-
singlepointings,while(240,+6)◦ hasfourpointingsfillingouta though distorted by the increased level of dust and possible non-
∼1◦×1◦fieldand(240,+10)◦ hastwopointings.Theseobserva- photometricconditions.TheBlue-Plumestars(see§6.2.2)arestill
tionsprovideaglimpseastohowtheGalaxyprofilechangesacross easilyvisibleandwhilethereiscontentionovertheirorigins,they
theGalacticplane.ThehighdustextinctionofthelowerGalactic arestillindicativethat thegeneral structurespresent here areun-
10 BlairConn etal.
Figure 10. Hess plot of (l,b) = (240,−9)◦ (Left) and the corresponding Figure11.AsforFigure6,Hessplotof(l,b)=(240,−6)◦ (Left)andthe
Besanc¸onmodel(Right).AsforFigure6.Thestrongsequenceinthisfield correspondingBesanc¸onmodel(Right).NooffsetisplacedonthisCMD
isdeterminedtobeat∼8.3kpcfromanoffsetof−0.6magwiththefiducial duetotheobviousdistortionpresentwhichismostlikelyduetotheincrease
sequence.Belowthisstrongsequenceisanexcessofstarsatag◦=20-21 industandnon-photometricconditions.TheCMDdoesshowallthesame
magand(g◦−r◦)∼0.5.Thisexcessfollowsbelowthestrongsequence featuresaspresentinthe(240,−9)◦field.Consistingofonlyoneexposure,
asitincreasesincolour.Fittingafiducialtothisexcessatanoffsetof0.4 the50%completenesslevelhasnotbeencalculated.
magadistanceof13.2 kpcisobtained.Thisinexcellentagreementwith
estimateddistancetotheMRiof13.5kpcasderivedintheAAT/2dFsurvey
ofConnetal.(2005b).ThisisdeemedatentativedetectionoftheMRiin
thisfield.TheCMDis50%completeatg◦=23.6mag.Notethepresence
ofBPstarsatg◦.18.
changedfromthepreviousfield.Consistingofonlyoneexposure,
the50%completenesslevelhasnotbeencalculated.
5.2.3 Fieldsat(240,−4)◦
Thisfield(Figure12),aswiththoseclosesttoplane,isheavilyaf-
fected by dust extinction. In particular, it has the second highest
dust levelsin our survey, where E(B-V)istypically around 0.99.
Again, despite the loss of structure in this field, the CMD still
shows evidence for a Blue Plume population although it appears
alittleweaker thanthepreceding (240,−6)◦ field.TheCMD has
beenleftinthelocationasdeterminedbythecalibrationprocessof
theCASUpipeline.Thehighdustlevelsarethemostlikelycause
forthedistortionontheMainSequencespresentintheCMD.No
completenessestimatehasbeenmadeforthisfield.
5.2.4 Fieldsat(240,−2)◦ Figure12.Hessplots of(l,b)=(240,−4)◦ (Left)andthecorresponding
Besanc¸on model (Right). The high dust levels in this field E(B-V)∼1.4,
Thedustextinctioninthisfield,Figure13,isextremelyhigh,typ- hasdistortedtheresultantCMD.AlthoughtheexactfeaturesoftheCMD
ically around an E(B-V)∼1.1. There is a faint suggestion of the areindistinguishable, thepresenceoftheBluePlumestarsremainsobvi-
presence of Blue Plume stars, although less so than in the previ- ous,althoughseeminglyweakerthanthe(240,−6)◦field.Nocompleteness
ousfield.Thestrongsequencevisibleinthisregionofskyisstill estimatehasbeenmadeforthisfield.
apparent.However,itisimpossibletoprovideadistanceestimate
tothisstructure.Thelimitingmagnitudeofthisfieldismostlikely
heavilyaffectedbythedustaccountingforitspositionwithregard
theCMDsintheremaining survey locations. Asfor thetwopre-