A High-Resolution Radio Continuum Study Of The Dwarf Irregular Galaxy IC 10

1.4 MB·

by  J. Westcott

MNRAS000,1–15(2016) Preprint17January2017 CompiledusingMNRASLATEXstylefilev3.0
A High-Resolution Radio Continuum Study Of The Dwarf
Irregular Galaxy IC10
J. Westcott,1? E. Brinks,1 R.J. Beswick,2 V. Heesen,3,4 M.K. Argo,2,5 R.D. Baldi4,
7 D.M. Fenech,6 I.M. McHardy,4 D.J.B. Smith1 & D.R.A. Williams4
1 1CentreforAstrophysicsResearch,UniversityofHertfordshire,CollegeLane,Hatfield,AL109AB,UK
0 2JodrellBankCentreforAstrophysics,SchoolofPhysicsandAstronomy,TheUniversityofManchester,Manchester,M139PL,UK
2 3HamburgerSternwarte,UniversitätHamburg,Gojenbergsweg112,21029Hamburg,Germany
4SchoolofPhysicsandAstronomy,UniversityofSouthampton,SouthamptonSO171BJ,UK
n 5JeremiahHorrocksInstitute,UniversityofCentralLancashire,PrestonPR12HE,UK
a 6DepartmentofPhysics&Astronomy,UniversityCollegeLondon,GowerStreet,London,WC1E6BT,UK
J

3
2
AcceptedXXX.ReceivedYYY;inoriginalformZZZ
]
A
G ABSTRACT
Wepresenthigh-resolutione–MERLINradiocontinuummapsoftheDwarfIrregu-
.
h lar galaxy IC10 at 1.5GHz and 5GHz. We detect 11 compact sources at 1.5GHz, 5
p of which have complementary detections at 5GHz. We classify 3 extended sources
o- ascompactHII regionswithinIC10,5sourcesascontaminatingbackgroundgalax-
ies and identify 3 sources which require additional observations to classify. We do
r
t not expect that any of these 3 sources are Supernova Remnants as they will likely
s
a be resolved out at the assumed distance of IC10 (0.7Mpc). We correct integrated
[ fluxdensitiesofIC10fromtheliteratureforcontaminationbyunrelatedbackground
sources and obtain updated flux density measurements of 354 11mJy at 1.5GHz
1 and199 9mJyat4.85GHz.Thebackgroundcontaminationdoe±snotcontributesig-
v nificantl±ytotheoverallradioemissionfromIC10,sopreviousanalysisconcerning
1
itsintegratedradiopropertiesremainvalid.
7
5 Keywords: galaxies:dwarf–galaxies:ISM–ISM:supernovaremnants–ISM:HII
6 regions–techniques:highangularresolution–techniques:interferometric
0
.
1
0
7 1 INTRODUCTION part,toaddressthisbystudyingcompactSFproductsand
1
their relation to large scale radio emission across a wide
: Radio emission within normal galaxies originates from the
v range of galaxy type. The project is made up of two com-
evolution and death of massive stars ( 8M ) and hence
i ≥ (cid:12) plementarysurveysofnearbygalaxies;astatisticalsurveyof
X effectively traces recent star formation (SF; Condon 1992). 280galaxiestakeninsnapshotmode,comprisingtheentire
Since it is not subject to extinction by dust, radio emission
r Palomarspectroscopicbrightgalaxysamplenorthofdecli-
a shouldbeanidealtracerofSFwhichwouldbevalidoutto nation+20degrees,andadeepsurveyof6galaxies,thelatter
thehighestredshifts.Atthetimeofwriting,radioluminos- reachinganrmssensitivityof8and3µJybeam−1at1.5GHz
ityiscalibratedasaSFRindicatorviatheradio–FIRrelation
and5GHzrespectively.Thestatisticalsurveywilldetectand
(Bell2003),atightcorrelationwhichlinksbothFIRandra-
resolve radio emission from supernovae (SNe) and super-
dio emission to massive SF. Yet the physical origin of this
nova remnants (SNR) to provide a complete census of en-
correlationisnotfullyunderstood,withmanyseeminglyin-
ergeticSFproductsinthelocalUniverse.Athighersensitiv-
dependentfactorsbalancingouttoforma‘cosmicconspir-
ities,thedeepsurveywillalsobeabletodetectfaintradio
acy’ (Condon 1992; Appleton et al. 2004; Lacki et al. 2010).
emissionfromPlanetaryNebulae,HIIregionsandsuperstar
Thisthenmakesitdesirabletocalibratetheradio–SFrelation
clusters.Together,thesesurveyswillbeusedtocalibratethe
withoutrecoursetotheFIRdata.
SFrate(SFR)innearbygalaxiesonthebasisofcompactra-
TheLegacye–MERLINMulti–bandImagingofNearby
diosourcepopulations,independentofobscurationbydust.
Galaxies(LeMMINGs;Beswicketal.2014)surveyaims,in
ThenearbydwarfirregulargalaxyIC10(seeFigure1)was
observedduringthecommissioningphaseoftheupgraded
e–MERLINarrayandisthefirstgalaxytobeobservedaspart
? E-mail:[email protected] oftheLeMMINGsdeepsurvey.
©2016TheAuthors
2 J.Westcottetal.
Table1.KeyIC10properties Table2.JournalofIC10e–MERLINobservations
Property Value Reference ObsDate ObsFrequency TotalBandwidth Time
(yyyymmmdd) (MHz) (MHz) (h)
αJ2000 002017.34 –
δJ2000 +591813.6 – 2013Feb09 1510.65 512 7.64
GalaxyType IBm 1 2013Nov22 1526.65 512 16.24
Distance 0.7Mpc 2 2015Apr18 5072.50 512 21.06
SFRHα 0.039±0.001M(cid:12)yr−1 3 2016Feb21 5072.50 512 9.24
AngularSize(MajorAxis) 11.068 4 Notes:ObsFrequencyisthecentralobservedfrequencyandTimeis
AngularSize(MinorAxis) 7.012 4 thetotaltimeonsource.
PositionAngle 132◦ 4
ReferenceList:1:Nilson(1973),2:Sakaietal.(1999),3:Hunteretal. 2 OBSERVATIONSANDDATAREDUCTION
(2012),4:Jarrettetal.(2003).
Observations of IC10 were made using e–MERLIN (the
upgraded Multi–Element Radio Linked Interferometer
Network, an array consisting of 7 radio telescopes spread
acrosstheUK1)aspartofLeMMINGs(Beswicketal.2014),
ane–MERLINlegacyproject.Thedataweretakenat2wave-
bands; 1.5GHz (L–Band) and 5GHz (C–Band) in 4 observ-
IC10isthoughttobecurrentlyundergoingastarburst
ing runs summarized in Table 2. The Lovell Telescope was
phase(Leroyetal.2006),withopticalstudiesrevealingnu-
only available for the February 2016 observation, resulting
merous HII regions (Hodge & Lee 1990) and the highest
inasmallerfieldofviewbuthighersensitivitywhencom-
density of WR–stars observed in the Local Group (Massey
paredtotheotherobservingruns.Thecalibrationanddata
et al. 1992; Crowther et al. 2003). Moreover, stellar popula-
tionanalysisrevealsaburstySFhistorywiththemostrecent reductionwascarriedoutusingAIPS2,followingthecalibra-
tionstepsdetailedinthee–MERLINcookbook3.
burstfinishing 10Myrago(Hunter2001;Vaccaetal.2007;
∼
Sannaetal.2009).AbriefsummaryofthepropertiesofIC10
ispresentedinTable1.
2.1 InitialFlagging&Inspection
Recentradioinvestigationsintothelarge–scalegasmo-
tionsofIC10revealacomplexnature,consistingofalarge Beforeanycalibrationtookplace,weusedSERPent(Peck&
HIenvelopewithaninnerrotatingdiskandcounter–rotating Fenech2013)withdefaultsettingstoremoveanyobviousra-
component(Shostak&Skillman1989;Wilcots&Miller1998; dio frequency interference (RFI). SERPent is auto–flagging
Ashleyetal.2014).Theleadingexplanationforthesediffer- software developed in ParselTongue (Kettenis et al. 2006)
entkinematicalsubsystemsisthatIC10isstillaccretingpri- principally for use in the e–MERLIN pipeline (Argo 2015).
mordialgas(Wilcots&Miller1998),althoughnewevidence Wealsoconductedfurthermanualflaggingtoremoveother
hints towards a recent merger or interaction being respon- obviousRFIthatSERPenthadmissed.Thevisibilitiesfrom
sible for the unusual gas kinematics and current starburst eachdatasetwerethenvisuallyinspectedusingtheAIPStask
(Nidever et al. 2013; Ashley et al. 2014). Furthermore, nu- IBLED to further identify and flag time periods when the
merous bubbles and shells are observed in the large scale datawascorrupted.Weestimatethatoverall,weareflagging
HI distribution(Shostak&Skillman1989;Wilcots&Miller 52%ofthedataat1.5GHzand 10%at5GHz.
∼ ∼
1998),hintingthatIC10iscomingtowardstheendofitscur-
rentstarburstphase(seethebottom–rightpanelinFigure1).
Anumberofradiocontinuumstudiesfocusedonthenon– 2.2 1.5GHzData
thermalsuperbubbletowardstheSouth–EastofIC10(Yang
Both1.5GHzobservationsfollowedthesameobservational
&Skillman1993;Heesenetal.2015),whichisthoughttobe
set–up, which we describe here. The total bandwidth was
theresultofeitheracollectionofsupernovaremnants(Yang
splitinto8IFs(intermediatefrequencies),eachwith128in-
&Skillman1993)orasinglehypernovaevent(Lozinskaya&
dividualchannelsofbandwidth0.5MHz.Theprimaryflux
Moiseev2007).AtitscentreresidestheX–raybinarysystem,
calibrator, 3C286, was used to set the flux scale, the point–
IC10X–1,whichislikelyrelatedtothebirthandevolution
like source, OQ208, was used to determine the passbands
ofthesuperbubble(Bauer&Brandt2004;Brandtetal.1997).
and relative antenna gains and 0027+5958 was used as the
IC10islocatedatalowgalacticlatitude( -3◦),making
∼ phasecalibrator.Theobservationsstartedwith30minscans
distance measurements challenging with estimates ranging
ofboththeprimaryfluxcalibratorandpointsource,andthen
from0.5Mpcto3Mpc(Demersetal.2004;Kimetal.2009).
wentontoalternatebetweenobservationsofIC10andthe
Inthispaper,wewillassumeadistancetoIC10of0.7Mpc
phasecalibrator,spending5minonIC10and1minonthe
(1arcsec 3.4pc),aslistedinHunteretal.(2012);wescale
’
allliteraturemeasurementsusedheretothisdistance.
Thepaperisstructuredasfollows;inSection2wede-
1 A basic overview of the array can be found at http://www.
scribe our observations, data reduction and imaging meth-
e-merlin.ac.uk/summary.html
ods;inSection3wedescribeandanalyseoursourcedetec- 2 AIPS,theAstronomicalImageProcessingSoftware,isfreesoftware
tionmethodandpresentourresults;inSection4wedescribe availablefromtheNRAO.
thedetectedsourcesanddiscussouranalysisandwesum- 3 Available at: http://www.e-merlin.ac.uk/data_red/
marizeourresultsinSection5. tools/e-merlin-cookbook_V3.0_Feb2015.pdf
MNRAS000,1–15(2016)
High–ResolutionRadioContinuumStudyofIC10 3
Figure1.IC10atmultiplewavelengths.TopLeft:HαcontinuumsubtractedmaptakenfromHunter&Elmegreen(2004).TopRight:MapofIC10
at70µmtakenwithHerschelaspartoftheDwarfGalaxySurvey(Maddenetal.2013).BottomLeft:5GHzVLAC&DarraymapofIC10taken
fromHeesenetal.(2015).BottomRight:NaturallyweightedHImapofIC10takenaspartofLITTLETHINGS(Hunteretal.2012).Notehowthe
large-scaleholesandshellsintheHIdistributioncanbeeasilyidentifiedacrossmultiplewavelengths.
phase calibrator each iteration. We followed standard cali- because no reliable flux density measurement for OQ208
brationproceduretodeterminethecomplexgainsolutions couldbeobtainedduetoexcessiveRFI.
to apply to the data. The first and last 15channels in each Both 1.5GHz observations were calibrated indepen-
IFwereflaggedbecausetheyareintheleastsensitiveareas dently and then combined. We verified that both datasets
ofthepassbandandcontributemostlynoise.Severalrounds hadthesamefluxscalebyimaging8brightsourceswithin
ofphaseonlyself–calibrationwerecarriedoutonthephase the primary beam but outside IC10 (identified in prelimi-
referencesourceandonIC10itselftoimprovethequalityof naryimaging)andcheckingthatthefluxdensitymeasure-
thefinalmaps.Eachcalibrateddatasetwasweightedtogive ments agreed to within 10 percent error. The flux densities
moreweighttotelescopeswithalargercollectingarea,im- agreedfor7sources,showingthatourfluxscaleissatisfac-
provingsensitivity. torilyconsistentbetweentheobservations.Weconcludethat
The Cambridge antenna was not available for the theoutliermustbedowntovariabilitybetweentheobserva-
November 2013 observation, resulting in a slightly larger tiondates.AstheNovemberdatasetwastakenataslightly
synthesizedbeamasthisantennamakesupthelongestbase- higherfrequencythantheFebruarydataset,weusedtheAIPS
linesinthee–MERLINarray.Additionally,IF1wasflagged task BLOAT to ensure that both observations covered the
MNRAS000,1–15(2016)
4 J.Westcottetal.
(a) (b)
Figure2.Distributionsofvisibillitiesintheuv–planesforthee–MERLINobservations.(a)showsthecombined1.5GHzuv–planeand(b)shows
thecombined5GHzuv–plane.
samefrequencyrangebeforecombiningbothdatasetsusing imagedandanaturalweightingschemewasusedtomax-
thetaskDBCON.Thecombineduv–planeisdisplayedinFig- imisesensitivity.Asmallfieldcentredonthebrightsource
ure2(a). NVSS002108+591132wasaddedtothislisttoremoveitscon-
taminatingsidelobes.Theindividualsubfieldswerecleaned
downto2.5timesthermsnoiselevelofeachmapandwere
2.3 5GHzData thenmosaickedtogetherusingtheAIPStaskFLATNtopro-
duce a single widefield map of the entire disk. This wide-
The5GHzobservationsfollowedasimilarobservingsetup fieldmapwasusedtosearchforsources(seesection3.1)and
tothe1.5GHzobservations.Thetotalbandwidthwassplit blanksubfieldswithnoapparentsourceswereusedtotest
into 4 IFs, each with 128 individual channels 1MHz wide. the widefield maps completeness (see section 3.6). We im-
Theprimaryfluxcalibrator,3C286,wasusedtosettheflux age the detected sources separately at 5GHz to save time,
scale, the point source, OQ208, was used to determine the also using a natural weighting scheme to maximise sensi-
passbands and relative antenna gains and 0027+5958 was tivity.Wecorrecteachdetectedsourcefortheprimarybeam
used as the phase calibrator. IC10 and the phase reference individuallyusingthemodelsdescribedinsection3.2.This
were observed alternatingly, with 5min on IC10 to 3min wasdonetosavetimeaseachdetectedsourcewassufficently
onthephasecalibrator,finishingwith30minscansofboth smalltobecharacterisedbyasingleprimarybeamcorrection
theprimaryfluxandpointsourcecalibrators.Thefirstand factor.
last5channelswereflaggedbecausethisiswherethepass- Ourfinalmapsreachanoiselevelof26µJybeam−1 at
bandrapidlylosessensitivity.Oneroundofphaseonlyself– 1.5GHzand12µJybeam−1at5.0GHz.Thesenoiselevelsare
calibration on IC10 was performed to further improve the significantlyhigherthantheLeMMINGstargetsensitiviesof
dynamic range of the final maps. Again, both calibrated 8µJybeam−1and3µJybeam−1at1.5GHzand5GHzrespec-
datasetswereweighted. tively.ThiscanmainlybeattributedtothelackoftheLovell
TherewereseveralissueswiththeFebruary2016obser- telescopeinthemajorityofthepresentedobservationsasits
vation.TheMk2antennawasnotavailablefortheduration inclusionresultsinafactorof2improvementinsensitivity
ofthisobservationandreliablegainsolutionscouldnotbe (see e–MERLIN technical capabilities). Additionally, as the
found for the Darnhall antenna. The remaining data were arraywasstillinthecomissioningphaseformostoftheob-
combinedwithdatatakeninApr2015tofurtherreducethe servations,someantennaswerenotpresentfortheentiredu-
noiselevelinthefinalmaps.Thecombined5GHzuv-plane ration of an observation. It is also likely that the increased
isdisplayedinFigure2(b). noiselevelisduetotheconsiderableflaggingcarriedout(see
Section2.1)andthatlowlevelsofunflaggedRFIfurthercon-
tribute to the noise level in the final maps. Together, these
factorslargelyexplainthehighernoiselevel.Thesensitivity
2.4 Imaging
targetsarelikelytobehitinfutureLeMMINGsdeepobser-
IC10’sstarformingdiskwasimagedat1.5GHzwithamoa- vations provided the Lovell telescope will be included for
sicof19subfields,witheachsubfieldcovering0.03aside.The theentiredurationoftheseobservationsandthee–MERLIN
AIPStaskSETFCwasusedtogeneratethelistoffieldstobe arraywillbeoperatingatfullcapacity.
MNRAS000,1–15(2016)
High–ResolutionRadioContinuumStudyofIC10 5
Figure3.1.5GHzRadiocontoursofIC10.TopRight:CombinedVLAC+DarraymapofIC10at1.5GHz,takenfromHeesenetal.(2015).The
contoursaresetto-3,5,10,20,40,80,150,300,600xthe13µJybeam−1noiselevel.Therednumbersrepresentthelocationsofthedetected
e–MERLINsources,thebluesquaresrepresentspectroscopicallyconfirmedWRstarsfromCrowtheretal.(2003),theblackstarsrepresent
Chandra(resolution300)pointsourcedetectionsandtheblackupwardstrianglesrepresentXMM(resolution1300)pointsourcedetections.All
X–raydataaretakenfromWangetal.(2005).ThegridlinesarethesameasinFigure1.Surroundingplots:Naturallyweighted1.5GHze–MERLIN
detectionsfromthisstudy.Thebeamsizeforeachimageis00.036x00.024andisdisplayedinthebottom-leftofeachmap.Thecontoursoneach
sourcearesetto-3,3,6,10,20,40xthe26µJybeam−1noiselevel.Thecontoursalsocorrespondtobrightnesstemperaturessetat690,1400,
2300,4600and9200K.Eachfieldmeasures600aside.Anidentifyingsourcenumberislocatedinthetop–leftcornerofeachcontourmap.
3 RESULTS sionwithinobservedgalaxies,essentiallyhighlightingcom-
pactstar–formingproductsandcontaminatingbackground
Radio emission from star–forming galaxies originates from
sources.Inthissectionwepresentoure–MERLINdetections
two main sources, thermal emission from HII regions and withinIC10andtesttherobustnessofourdetections.
non–thermalemissionfromCosmicRayelectrons(CRe)that
havebeenacceleratedinSNRshockfronts.Itisunlikelythat
anydetectionsarerelatedtoX–raybinariesorPlanetaryNeb-
3.1 SourceDetection
ulae(Fender&Hendry2000;Leverenzetal.2016)asatthe
distanceofIC10,thesesourceswouldhavefluxdenstiesof We used the widefield maps described in Section 2.4 to
order 1µJy. Very long baseline interferometers, such as e– searchforcompactradiosources.Weusethemapat1.5GHz
MERLIN, effectively resolve out nearly all extended emis- to search for sources because not only does this map have
MNRAS000,1–15(2016)
6 J.Westcottetal.
Figure3.(Continued.)AdditionalNotes:ThemapofSource7hasbeenconvolvedtoaresolutionoftwicethesynthesizedbeamtoshowthelow
surfacebrightnessemission.Thecontoursonsource7aresettothesamelevelsasintheothercontourmaps,butthenoiselevelinthiscaseis
53µJybeam−1.Thecontourscorrespondtobrightnesstemperaturessetat170Kand340Kinthiscaseonly.
Table3.PropertiesofSourcesDetectedinIC10
SourceNo IAUDesignation αJ2000 δJ2000 S1.5GHz S5.0GHz PeakTB SpectralIndex
(hhmmss) (ddmmss) (mJy) (mJy) (K)
1∗ WBBHJ002017+591839.7 002017.390 +591839.68 8.06±0.82 1.02±0.11 5200 −0.3±0.1
2∗ WBBHJ002027+591728.7 002026.952 +591728.66 3.39±0.35 2.28±0.24 7700 −0.1±0.1
3∗ WBBHJ002015+591853.9 002014.988 +591853.88 2.23±0.25 - 2600 −1.0±0.1
4∗ WBBHJ002027+591706.1 002027.160 +591706.12 1.83±0.20 0.36±0.06 4900 −0.4±0.1
5 WBBHJ002008+591540.2 002008.348 +591540.21 1.29 0.14 0.28 0.04 >11000 1.3 0.2
± ± − ±
6 WBBHJ002027+591843.7 002026.807 +591843.69 0.99 0.11 - >8500 1.9
± ≤−
7∗ WBBHJ002018+591740.9 002018.249 +591740.92 0.93±0.14 - 700 −0.4±0.1
8 WBBHJ002019+591802.3 002019.334 +591802.33 0.87 0.16 - >7700 2.0
± ≤−
9 WBBHJ002033+591942.9 002033.390 +591942.89 0.49 0.11 - >4100 1.0
± ≤−
10 WBBHJ002008+591955.2 002008.137 +591955.23 0.37 0.07 0.14 0.03 >3200 0.8 0.3
± ± − ±
11 WBBHJ001951+591644.2 001951.498 +591644.20 0.26 0.06 - >2000 1.1
± ≤−
Notes:Sourceswitha*areextendedsourcesandtheirspectralindices(italicized)arederivedfromthelow–resolutionspectralindexmaps
presentedinHeesenetal.(2011).Allotherspectralindicesarederivedfromthee–MERLINobservations.Thepeakbrightnesstemperatures
havebeenderivedfromthe1.5GHzobservations.
awiderfieldofview,itismoresensitivetoextendedemis- cross-matchedtheseremainingcandidateswithlowerreso-
sionthanthecorresponding5GHzmaps(seeFigure2).Fur- lutionfeaturesfromKarlG.JanskyVeryLargeArray(VLA)
thermore,anysourcesdominatedbynon–thermalemission maps at 1.5GHz (Heesen et al. 2015; see Figure 3) to iden-
mechanisms will be brighter at 1.5GHz due to the power– tifyspuriousdetections.Thelowerresolutionmapshavea
lawradiospectrathesesourcesexhibit.WeusedAegeanas 13µJybeam−1 rmsnoiselevel,andso,wewouldexpectto
oursourceextractor(Hancocketal.2012)withapeakflux observea5σe–MERLINdetectioninthesemaps.Wedeter-
densitythresholdof5timesthermsnoiselevel,σrms,tofind mine2ofthee–MERLINdetectionstobespuriousandend
candidates.Thesourceextractorretrieved26sourcesinto- upwithasampleof13sourceswhichwedisplayinFigure
tal, of which 15 are coincident with IC10’s main disk. We
MNRAS000,1–15(2016)
High–ResolutionRadioContinuumStudyofIC10 7
Figure4.Naturallyweighted5GHze–MERLINdetectionsfromthisstudy.Thebeamsizeforthisimageis00.014x00.006andisdisplayedinthe
bottom–leftofeachcontourplot.Thecontoursoneachsourcearesetto-3,3,6,10,20,40xthe12µJybeam−1noiselevel.Eachfieldmeasures
200onaside.Anidentifyingsourcenumberislocatedinthetop-leftcornerofeachcontourmap.
3.Wediscussthecompletenessandreliabilityofoursource with a FWHM of 30’ for the 1.5GHz observations (exclud-
detectionmethodinSection3.6. ingtheLovellantenna)andaFWHMof5.6’forthe5GHz
observations(includingtheLovellantenna).
3.2 SourceFluxMeasurement
3.3 BrightnessTemperature
We determine integrated flux densitites for the unresolved
and resolved sources using 2 different methods. For unre- Assuming an optically thick source, the expected observed
solvedsourceswefitGaussiansthroughtheuseoftheAIPS brightness temperature for free–free emission from HII re-
taskJMFIT.Forresolvedsources,wegenerateamaskbyonly gions is of order 10000K. Electrons radiating via non–
considering emission associated with each source above a thermalmechanisms,however,leadtoobservedbrightness
3σrmslevel,whereσrmsisthelocalmaprmsnoiselevel(mea- temperaturesthatcanbemuchhigher(e.g.,1012K);bright-
suredoffsource).Wethenintegratethemaskedemissionto ness temperatures could therefore be used to aid in source
findtheintegratedfluxdensity. classification.
Theerrorsfortheintegratedfluxdensitymeasurements Assuming a Rayleigh–Jeans black–body curve, we cal-
aredeterminedusing: culate the brightness temperature, T , for each detected
B
sourceusing:
σ = (σ )2+(σ )2 (1)
Total Map Scale c2 S
q T ν (2)
JMFIwThfoerreuσnMreaspoilsveedithsoeurrecqeusaolrtiotitsheequfiatletroro√rNreσturmrsnfeodrrbey- B≈W2hνe2rkeBΩk(cid:18)is10t2h6eJyBWolt−z1mma2nnHzco(cid:19)nstant, c is the speed of
solvedsources(N isthenumberofindependentbeamsthat B
light, ν is the central frequency of the combined 1.5GHz
fitwithinthemaskedimage).σ istheuncertaintyinthe
Scale observations (1518MHz), Ω is solid angle that the synthe-
fluxscalewhichistakentobe10percent.
Wecorrecttheintegratedfluxdensityfromeachsource sized beam subtends and Sν is the measured flux density
inJy.Thederivedbrightnesstemperatureisonlycorrectfor
forprimarybeamattenuationindividuallybyassumingthe
sourcesthatareresolved,i.e.,whenthesynthesizedbeamis
e-MERLINprimarybeamcanbeparameterizedbyaGaus-
sian,asdisplayedinthee–MERLINtechincalcapabillities4, completelyfilledwiththeemittingsource.Asitstands,our
brightnesstemperatureforextendedsourcesislikelytobean
underestimate,duetousmissingfluxasaresultofthelack
4 Availableathttp://www.e-merlin.ac.uk/tech/ ofshortbaselinesintheseinterferometricobservations.The
MNRAS000,1–15(2016)
8 J.Westcottetal.
To find spectral indices for resolved sources, we make
Table4.AngularSizesofExtendedandResolvedSourcesat1.5GHz
use of the spectral index maps provided in Heesen et al.
(2011).Thesemapsaretakenatamuchlowerresolutionthan
SourceNo MajorAxis MinorAxis Positionangle
thepresentede–MERLINobservations,andhencecanonly
(00) (00) (◦) giveusageneralideaaboutthespectralindexoftheresolved
1* 3.34+0.30 1.85+0.26 138.96+13.37 sourcescombinedwithemissionfromtheirimmediatesur-
0.63 0.34 10.99
2* 1.86−+0.12 1.21−+0.12 14.93+−17.18 roundingenvironment.
0.34 0.27 19.65
3* 2.49 −0.38 0.96−+0.14 71.35−+9.16
4* 1.46±+0.09 0.90−+00..0249 9.51+−116..4796
0.33 0.17 10.32
7* 1.80+−0.07 1.25+−0.06 48.93−+12.46
0.21 0.20 11.37
8 0.694 −0.094 0.590 −0.080 123.65−33.87
± ± ±
Notes:Thesuperscript*intheSourceNocolumnindicatesthatthe
3.6 Completeness&Reliability
sourceisextended.Thepositionangleofeachsourceismeasured
anti-clockwisefromtheNorth. WecarriedoutMonte–Carlosimulationsat1.5GHztoassess
thecompleteness,C,andreliability,R,ofoursourcedetec-
brightnesstemperatureobservedforunresolvedsourceswill tionmethod.Ateachband,wetookablankfieldfromthe
alsobeanunderestimateasthesynthesizedbeamislarger widefield imaging (i.e., a noise map) and randomly added
thanthetrueangularsizeofthesource.Furthermore,ourra- 400 Gaussian sources, each the same size as the naturally
dioobservationsareprobingopticallythinconditions,result- weighted synthesized beam. We assigned a flux density to
inginafurtherreductionoftheobservedbrightnesstemper- eachofthesesources,drawnfromacontinous,uniformprob-
atures.Therefore,thebrightnesstemperaturesderivedboth ability distribution between 2 and 12 times the rms noise
forallsourcesshouldbetreatedaslowerlimits.Wepresent levelofthemaps.WethenusedAegeantorecovertheplaced
thepeakbrightnesstemperatureforeachdetectedsourcein sources, setting a detection threshold at 3 times the map
Table3anddisplaybrightnesstemperaturecontoursforthe rmsnoiselevel.Asinouranalysis,onlysourcesexceeding
resolvedsourcesinFigure3. athresholdof5σareusedtodeterminethestatisticalcom-
pletenessofoursourcedetectionmethod.Thisprocesswas
repeated1000timestoensureastaticallyrobustanswer.
3.4 AngularSizes Wesplitourdataintofluxbins,eachofwidth0.5times
themaprmsnoise.Wedefinethecompleteness&reliability
We measure the angular size, position angle and sky co-
foreachfluxbinas:
ordinates of each extended source by fitting an ellipse to
the3σrmscontour.Weoptimizethefitbyminimizingtheχ2
statistic,assumingtheerrorateachpointonthecontouris
thesame.Weobtainerrorsinthefitviathe∆χ2method,i.e., C = NM (3)
bymodifyingeachindividualparametertogainχ2asafunc- NP
tionofsaidparameterwhilstfixingallotherparametersto
thebestfit.Thequoted1σerrorsaredefinedbytheintersect
where∆χ2fromtheminimizedχ2isequalto1. N
For marginally resolved and unresolved sources, we R= M (4)
N
R
fit 2–D Gaussian functions using the AIPS task JMFIT to
determine the angular size, position angle and sky co-
WhereN isthenumberofmatchedsources(amatch
M
ordinates.Theangularsizepropertiesforextendedandre-
is counted when the recovered source is spatially located
solvedsourcesat1.5GHzaresummarizedinTable4andthe
within 1 synthesized beam of the placed source), NP is
skyco-ordinatesarepresentedinTable3.
the number of placed sources and N is the total number
R
of matched sources recovered by the source extractor that
3.5 SpectralIndices alsowithinmatchedinflux(within1σrms uncertainty).The
completenessisessentiallyameasureofthefractionofreal
Ideally,forunambiguoussourceclassification,werequirea sources our source extractor detects and the reliability is a
combination of reliable spectral indices (Sν ∝ να), to gain measureofhowoftenspuriousdetectionsmaybemistaken
ameasureofthespectralenergydistributionforeachcom- forrealsources.
pactsource,andmapsmadeatmultipleepochswithalong TheresultsfromthisanalysisaresummarisedinFigure
enoughtimebaselinetomeasureSNRexpansion( 10yrs: 5.Wearecompletedowntoapproximately7σandreach50%
∼
McDonaldetal.2001;Beswicketal.2006;Fenechetal.2008). completenessatapproximately5σ.Thereliabilityplottellus
These current observations are the highest resolution thatour5σdetectionsareveryrobustanditisunlikelythat
mapsofIC10yet,andhence,wehavenopreviousobserva- anyofthesesourcesarespuriousdetections.Modifyingthe
tionstoobserveanyexpansion.Furthermore,torecoverre- detectionthresholdresultsinatranslationofthecomplete-
liablespectralindicesforresolvedsources,werequireboth ness distribuion presented in the upper panel of Figure 5,
observationstohavesignificantlyoverlappinguv-planes.For wherethechosenthresholdbroadlymatcheswherethecom-
ourcurrente–MERLINobservationshowever,thisisnotthe pletenessreaches50%.Changingthethresholdto,forexam-
case(seeFigure2)andwecanonlygainreliablespectralin- ple3σ,increasesthecompletenessatlowfluxlevels,how-
dices for sources that are unresolved at both 1.5GHz and ever,thisdoesnotresultinanincreaseinthereliability,as
5GHz. additionalsourcesmaybespurious.
MNRAS000,1–15(2016)
High–ResolutionRadioContinuumStudyofIC10 9
Figure5.Completenesssandreliabilityplotsforthecombined1.5GHzobservationsofIC10.Theuncertaintyinallbinsis<5%.Inthelower
plot,averticalreddashedlinerepresentsthelowerfluxlimittowhichwecaninvestigatethereliability.
4 DISCUSSION gion.Thebrightnesstemperatureforthissourceisoforder
8000K,whichisroughlywhatisexpectedforanHIIregion.
4.1 IndividualSourceNotes
Therefore,itislikelythatthissourceisacompactHIIregion
Inthissectionwebrieflydiscusseachsourceinturn,high- withinIC10.
lightingrelevantinformationtoaidinitsclassification. Source3(WBBHJ002015+591853.9):Thissourceispar-
Source1(WBBHJ002017+591839.7):Thisisthebright- ticularly interesting. It has an unusual extended morphol-
estsourcethatwedetectinoure–MERLINobservationsand ogy, no counterpart in either the Hα or 70µm maps and
italsohasthelargestangularsize.Thereisevidenceinthe doesnotappeartobeassociatedwithanydiffuseHI emis-
presentedcontourmapsfora‘cleanbowl’,hintingthatwe sion.Thissourcedoesnotappeartoberelatedtoanystar-
aremissingfluxfromthissourceduetothelackofshortspac- forming regions within IC10, but it is spatially coincident
ingsinthee–MERLINarray.Wefindthatthissourceisspa- withaChandraX–raysource.Asthissourcealsohasasteep
tiallycoincidentwithemissionpeaksintheHα,70µmmaps low–resolutionspectralindex(α 1.0),determinedfrom
∼ −
andlowerresolution5GHzand1.5GHzmaps.Thissource lowerresolutionmapsintheliterature,itislikelytobeacon-
alsofallsbetween2holesinthelarge-scaleHIemission(see taminatingbackgroundgalaxy.Thissourcehasalowerthan
Figure 1). We derive a brightness temperature of approxi- expected brightness temperature of roughly 3000K, which
matly5000Kforthissource,whichislowerthanexpected. couldbeduetohighopticalthicknessalongthelineofsight
Itisunlikelythatthisisduetoahighopticalthicknessalong to this source or due to missing flux from short baselines.
thelineofsighttothesource,andisprobablyduetomissing Future VLA A–Array 10GHz observations would provide
flux.Thelowresolutionspectralindexfromtheliteratureis areliablespectralindexforthissourceandhelpdetermine
flat(α 0.3).Inviewoftheabove,weclassifythissource whetheritresideswithinIC10.
∼−
asacompactHIIregionwithinIC10. Source4(WBBHJ002027+591706.1):LikeSource2,this
Source2(WBBHJ002027+591728.7):Thissourceisspa- source is coincident with emission in the Hα and 70µm
tiallycoincidentwithpeaksofemissionintheHαand70µm maps,thelowerresolution5GHz,and1.5GHzmapsaswell
mapsaswellaslowerresolution5GHz,and1.5GHzmaps. aswithapeakintheHIdistribution.Source4islocatedon
Itisalsospatiallycoincidentwithacurrentmajorstarform- theoutskirtsoftheNon–Thermalsuperbubble(seeSection
ingregionwithinIC10(Heesenetal.2015),withnumerous 4.5),andtherearemanyWRstarsinthisgeneralregionofthe
WRstarsnearby.Moreover,thissourceislocatedwithinthe galaxy. We determine a brightness temperature of roughly
highestpeakoftheHIdistribution,indicatingthatSFispos- 5000K,whichislowerthanexpectedandcouldbeduetothe
siblybeingfuelledatthislocation.Thelow-resolutionspec- samereasonsasdiscussedforsource1.Thissourcehasarel-
tralindexfromtheliteratureforthissourceis-0.1,whichis ativelyflatlowresolutionspectralindex(α 0.4)andis
∼ −
consistantwithadominatingthermalcontributioninthisre- likelytobeacompactHIIregionwithinIC10.
MNRAS000,1–15(2016)
10 J.Westcottetal.
Source5(WBBHJ002008+591540.2):Thissourceisun- North–East of IC10’s main disk. The source appears to be
resolvedandlocatedtowardsthesouthernoutskirtofIC10’s extendedtotheEastinFigure3,butvisualinspectionsug-
main disk. There are no corresponding sources at Hα or gests this to be a noise peak. This source appears isolated
70µm.Asthissourceisunresolved,thederivedbrightness fromIC10’smaindisk,withnocorrespondingemissionin
temperature must be treated as a lower limit and the true eithertheHαor70µmmaps.Thissourceisfaintwithalower
brightnesstemperaturemustbegreaterthan10000K.Ifthis limittothebrightnesstemperatureof4000K,whichisprob-
sourceisofnon-thermalorigin,weexpectthatitmusthave ablyduetothesourcefillingasmallfractionofthesynthe-
an angular size much smaller than the synthesized beam. sizedbeam.Duetoitsisolation,weclassifythissourceasa
Furthermore, this source has a steep spectral index (α backgroundgalaxy.
∼
1.3)asdeterminedfromoure–MERLINobservationsand Source10(WBBHJ002008+591955.2):Thissourceisan-
−
itisalsocoincidentwithaChandraX–raysource.Itislikely otherunresolvedsourcelocatedtowardstheNorth–Westof
thatthissourceisabackgroundAGN. IC10’smaindisk.Thissourcelookslikeitcouldbeassoci-
Source6(WBBHJ002027+591843.7):Thissourceisun- atedwiththenorthernringintheHIdistribution(seeFigure
resolvedandlocatedtowardstheNorth-EastofIC10’smain 1)butitappearstobeoffset.Thereisnocorrespondingemis-
disk.SimilarlytoSource5,thereisnocorrespondingemis- sionineitherHαor70µmmaps.Wederivealowerlimitto
sion in either Hα or 70µm maps. We derive a lower limit thebrightnesstemperatureof3000Kwhichispossiblydue
forthebrightnesstemperatureof8000K,butthisislikelyto tothesourcefillingasmallfractionofthesynthesizedbeam.
be due to the source filling a small fraction of the synthe- This source appears to be coincident with a low resolution
sizedbeam.Thissourceisspatiallylocatedclosetobutsepa- XMMX–raysource,butduetothelowresolution(1300,Wang
ratefromaChandraX–raysource.Itissurprisingthatwedo etal.2005)wecannotsaywithcertaintythatthissourceisthe
notdetectthisbrightsourceinthepresented5GHzobserva- originoftheX–rayemission.Thiscurrentevidencesuggests
tions,resultinginanextremelysteepspectralindexestimate thatthissourceisafaintbackgroundsource.
(α 1.9).Fromthisevidence,itislikelythatthissourceis Source11(WBBHJ001951+591644.2):Thissourceisan-
≤−
abackgroundsource. other faint, unresolved source that is located towards the
Source7(WBBHJ002018+591740.9):Thissourcehasa WestofIC10’smaindisk.Thissourcefallswithinextended
resolved,diffusestructurewithasharppeakabovethe5σrms emissioninthe70µmmapbuttheHαmapdoesnotextend
threshold.InFigure3wedisplayacontourmapthatwecon- thisfar.Thissourceislikelytobeabackgroundgalaxyasit
volvedtoaresolutionoftwicethenaturallyweightedsyn- hasasteepspectralindex(α< 1.1)andtherearenocoinci-
−
thesisedbeamtohighlightthisdiffuseemission.Wecarried dentcompactsourcesatanyotherwavelength.Itcouldalso
out the same analysis as for the other sources but with a be a peak in a more diffuse HII region but this is unlikely
beamthatwastwicethesize.Thepeakbrightnesstemper- given the spectral index. The lower limit to the brightness
ature for this source was found to be just 700K, and this temperature is still quite low ( 2000K) which is possibly
∼
should be treated as a lower limit. This source is located duetothesourcefillingasmallfractionofthesynthesized
roughlyinthecentreofIC10’smaindisk,withcorrespond- beam.Furtherinvestigationwillberequiredtoproperlyclas-
ingpeaksofemissionintheHα,70µmandthelowresolu- sifythissource,butcurrentevidenceindicatesitislikelyto
tionradiocontinuummaps.Moreover,thissourcehasaflat beabackgroundgalaxy.
low–resolutionspectralindex,suggestingthatthissourceis
possibly an HII region within IC10 although more investi-
gationwillberequiredtofullyclassifythissource.Thereis 4.2 ExpectedBackgroundSourceCount
eemasitssoifonthaistaso3uσrcrmes. Ilefvtehlisloecmatiesdsio∼n2i.s500altsoowaasrsdosctihateedsowutihth– Wederivetheexpectednumberofbackgroundsourcescoin-
source7,itispossiblethatthissourceisayoungSNRandwe cidentwithIC10byanalysingsourcecountsfromthelitera-
areobservinghighbrightnessfeaturesintheexpandingshell ture.WefitthesourcecountsfromMassardietal.(2010)with
(seeSection4.4).Future,moresensitive,e–MERLIN1.5GHz a 4th order polynomial5. This was chosen as it is the low-
andVLA1.5GHzA–Arrayobservationswillberequiredto estorderpolynomialthatrepresentsthedatawell.Wethen
determine if this source has an extended component. VLA applycorrectionsforcompleteness(seeSection3.6)andend
10GHzA–Arrayobservationscouldalsobeusedtoderivea upwithanestimateforthebackgroundcountof14.8 3.8
±
resolvedspectralindextoaidinclassifyingthissource. sources in a circular field of radius 5’. It is likely that this
estimate is a slight overestimate as any nearby extended
Source8(WBBHJ002019+591802.3):Thissourceispar-
sourceswillberesolvedout,butthisisexpectedtoonlyaffect
tiallyresolvedandlocatedtowardsthecentreofIC10.This
sourceiscoincidentwithdiffuseemissionintheHαmapand brightersourceswhichcontributelittletotheoverallcounts.
InaregionofthesamesizecentredonIC10(seeSection3.1)
withpeaksinthe70µmandlowresolutionradiomaps.We
wedetect15sourceswhichisconsistentwiththehypothe-
do not detect this source in the presented 5GHz maps, re-
sultinginaverysteepinferredspectralindex(α 2.0).We sisthatalldetectedsourcesarebackgroundgalaxies.Asdis-
≤− cussed in Section 4.1, it is clear from comparison with Hα
present a lower limit for the brightness temperature of or-
and70µmmapsthatatleast3ofourdetectionsresidewithin
der8000K.Thissourcecouldeitherbeabackgroundsource
IC10,butitisobviousfromthisexercisethatthemajorityof
orapeakinamoreextendedHIIregion.Deepere–MERLIN
thedetectedsourcesareprobablybackgroundgalaxies.
5GHz observations would be required to obtain a reliable
spectralindexforthissourceandtodetermineitsmember-
shiptoIC10.
Source 9 (WBBH J002033+591942.9): This source is 5 The raw data is available from http://web.oapd.inaf.it/
another compact, unresolved source located towards the rstools/srccnt/srccnt_tables.html.
MNRAS000,1–15(2016)