An optical search for supernova remnants in the nearby spiral galaxy NGC 2903

0.84 MB·English

by  E. Sonbas

Astronomy&Astrophysicsmanuscriptno.ngc2903paper7 (cid:13)c ESO2009
February6,2009
An optical search for supernova remnants in the nearby spiral
galaxy NGC 2903
(ResearchNote)
E.Sonbas1,2,A.Akyuz1,andS.Balman3
9
0
0 1 UniversityofCukurova,DepartmentofPhysics,01330Adana,Turkey
2 2 SpecialAstrophysicalObservatoryofR.A.S.,Karachai-Cherkessia,NihnijArkhyz,369167Russia
n 3 Dept.ofPhysics,MiddleEastTechnicalUniversity,06531Ankara,Turkey
a
Received-;accepted-
J

6
ABSTRACT

]
A Aims.Wepresenttheresultsofanopticalsearchforsupernovaremnants(SNRs)inthenearbyspiralgalaxyNGC2903.
Methods. Interference filterimages and spectral data were taken in March 2005 with the f/7.7 1.5 m Russian Turkish Telescope
G (RTT150)atTUBITAKNationalObservatory(TUG).Spectraldatawereobtainedwiththe6mBTA(BolshoiAzimuthalTelescope,
. Russia).WeusedtheSNRidentificationcriterionthatconsistsofconstructingthecontinuum-subtractedHαandcontinuum-subtracted
h [SII]λλ6716,6731imagesandtheirratios.
p Results.FiveSNRcandidateswereidentifiedinNGC2903with[SII]/Hαratiosrangingfrom0.41-0.74andHαintensitiesranging
- from9.4×10−15 to1.7×10−14ergscm−2s−1.ThisworkrepresentsthefirstidentificationofSNRsbyanopticalsurveyinNGC2903.
o
Wepresentthespectrumofoneofthebrightcandidatesandderivean[SII]/Hαemissionlineratioof0.42forthissource.Inaddition,
r
t the weak [OIII]λ5007/Hβ emission line ratio in the spectrum of this SNR indicates an old oxygen-deficient remnant with a low
s
propagationvelocity.
a
[
Keywords.SupernovaRemnants(SNRs)–SpiralGalaxies,NGC2903

1
v
2 1. Introduction Extragalacticsearches for SNRs were first obtainedfor the
4 MagellanicCloudsbyMathewson&Clarke(1973).Theywere
6 Supernovaremnants(SNRs)areimportantformanyofthetheo- thefirsttousethe[SII]/Hαemissionlineratiosforopticaliden-
0 riesofinterstellarmedium(ISM)becausesupernovaexplosions
tification of SNRs. Blair et al. (1981),Smith et al. (1993),and
. andtheireventualdispersionofejectedmaterialhavetheeffect
1 Blair&Long(1997,2004)alsousedsamemethodsuccessfully.
0 of enriching the ISM with the material processed in stellar in-
9 teriors.In a typicalHII region,the sulfuris in the formof S++ A number of nearby spiral galaxies have already been
0 becauseofthestrongphotoionizationfluxofthecentralhotstar observed to identify SNRs using optical observations (e.g
: orstars.Therefore,the[SII]/Hαratioistypically≈0.1-0.3for D′Odorico,Dopita&Benvenuti1980;Braun&Walterbos1993;
v
HII regions. Outside the HII region, there are not enough en- Magnieretal.1995;MF97;Matonicketal.1997;Gordonetal.
i
X ergeticfreeelectronstoexciteS+andtoproduceforbidden-line 1998,1999;Blair & Long1997,2004)and X-rayobservations
r ([SII]λλ6716,6731)emission.Forthatreason,nearlyalldiscrete (Pence etal. 2001;Ghavamianet al. 2005).Radio searchesfor
a emissionnebulaewith[SII]/Hα≥0.4areshock-heatedandthey extragalacticSNRs have been conductedby Lacey et al. 1997;
areprobablySNRs. Lacey & Duric (2001) and Hyman et al. (2001). SNR surveys
have also been carried out at optical, radio, and X-ray wave-
In several prior works, the motivation for observing SNRs
lengthsbyPannutietal.(2000,2002,2007).
in nearbygalaxies(Matonick& Fesen 1997(hereafter MF97);
Pannutietal. 2000,2002)andin ourGalaxy(Mavromatakiset In this work, we searched for SNRs in the nearby spiral
al. 2002; Fesen et al. 1997, 2008) has already been discussed. galaxy NGC 2903 using the criterion [SII]/Hα ratio is ≥ 0.4.
ThesampleofGalacticSNRsisquitelarge,andinterstellarex- NGC 2903is an SB(s)d type galaxywith a 61◦ inclinationan-
tinction and uncertain distances cause selection effects. These gle, and 17◦ position angle at a distance of 9.4 Mpc to NGC
problems are much less significant in extragalactic samples. 2903 has been adopted for this paper (Bresolin et al. 2005). It
Assumingthedistancetoagalaxyisknown,alltheSNRsareat was observed at radio (Williams & Becklin 1985; Tsai et al.
thesamedistancefromus,sosomepropertiescanbecompared 2006),infrared(Simonset al.1988;Williams& Becklin1985;
directly. Also, the foreground extinction is generally low, thus Alonso - Herrero, Ryder & Knapen 2001), X - ray (Fabbiano,
relativepositionsofSNR samplesaredeterminedaccuratelyin Trinchieri & MacDonald 1984; Mizuno et al. 1998; Junkes &
anextragalacticsurvey.ByknowingthepositionsoftheSNRs, Hensler 1996; Tschoke, Hensler & Junkes 2003), and optical
theirdistributionsarecalculatedrelativetoHIIregionsandspi- wavelengths (Bresolin et al. 2005). Multiwavelength observa-
ralarms.PossibleSNRprogenitorshavebeeninvestigatedfrom tionsofNGC2903impliesthatithasaverycomplexstructure
thesedistributions(MF97;Blair&Long1997,2004). withknotsinthenucleus.Theknots,called“hot-spots”,contain
manyearlytypestars(Okaetal.1974;Simonsetal.1988).Tsai
Sendoffprintrequeststo:E.Sonbas et al. (2006) report subarcsecond-resolution VLA (Very Large
2 E.Sonbasetal.:AnopticalsearchforsupernovaremnantsinthenearbyspiralgalaxyNGC2903(RN)
Array)imagingofNGC2903.Theyfoundsevendiscreteradio doublet lines and to extract and eliminate contributions of two
sources in the central 15′′ × 15′′ of galaxy at the wavelengths [NII]λλ6548,6583linesneartheHαline.
of6and2cmtoalimitingintegratedfluxdensityof0.2and0.3
mJy,respectively.Theyidentifiedoneoftheirsources(sourceD)
3. Resultsanddiscussion
as a candidate radio SNR. Their detected sourcesmeet at least
oneofthe followingcriteria:5σdetectionatthepeakintensity ToidentifytheSNRcandidatesinNGC2903,weusedthetech-
atonewavelengthand4σemissiondetectionatboth6and2cm. nique where continuum-subtractedHα and [SII] λλ6716,6731
Theorganizationofthepapermaybedescribedasfollows:. images were constructed and then [SII]/ Hα image ratios were
InSect.2wediscussboththeimagingandspectroscopicobser- made. Finally, regionsthat have image ratio values ≥ 0.4 were
vationsthatwereconductedaspartofthisstudy,aswellasthe identifiedascandidateSNRs(Blair&Long1997).
accompanyingdatareduction.TheSNRidentificationtechnique, Preliminary SNR candidates were found by blinking be-
searchresults,anddiscussionaredescribedinSect.3. tween continuum-subtracted[SII] and Hα subfield images. We
only displayed a region about 2′ on a side at one time for vi-
sualinspectionofthefieldstosearchforcandidates.Anybright
2. Observationsanddatareduction feature in the continuum-subtracted [SII] image was checked
againstthecontinuum-subtractedHαtomakesurethestarswere
2.1.Imaging not poorly subtracted. If the feature in the [SII] image looked
brighterthan it was in the Hα image, we marked it as an SNR
NGC2903wasobservedin2006Marchwiththe1.5mRussian
candidate.EachpreliminarySNR candidatewas a possibletar-
TurkishTelescope(RTT150)atTUBITAKNationalObservatory
getforfollow-upspectralobservation.Inthelatterstep[SII]and
(TUG) in Turkey.Imageswere taken with TFOSC (TUBITAK
Hα,continuum-subtractedimageswereusedtomeasurethetotal
FaintObjectSpectrographandCamera)2048×2048pixelCCD
countsforeachSNRcandidate.We selecteda circularaperture
with a plate scale of 0′′.39 pixel−1, giving 13′.3 × 13′.3 FOV.
in continuum-subtractedimagesto sumtheADU (Analogueto
We used narrowband interference filters centered on the lines
Digital Units) counts. Afterwardsa concentricannuluswas se-
of [SII] & Hα + 2 continuum filters to remove starlight from
lected to determinethe backgroundcountsto subtractfromthe
Hα and [SII] images. Characteristics of the interference filters
aperture sum. The aperture sizes used to measure fluxes were
usedintheseobservationsarelistedinTable1.Anobservation
constrained by the seeing that was attained during our inter-
logoftheimagingdataisshowninTable2forthisgalaxy.The
ference filter imaging observations(namely 1.9′′, which corre-
datawerereducedusingESO–MIDAS(TheEuropeanSouthern
spondsto∼87pcfortheassumeddistancetoNGC2903of9.4
ObservatoryMunichImageDataAnalysisSystem)softwareen-
Mpc).Becauseofthedifferencebetweenseeingandpixelscales,
vironment.SeveralHα,[SII],andassociatedcontinuumimages
wedidnotincluderadiicalculationsfortheSNRcandidatesthat
of each galaxy field were combined to obtain deeper field im-
wedetected.Tocorrectfluxvaluesforinterstellarextinction,we
agesinordertoincreasethesignal-to-noiseratioforthefaintest
useddatafromCardelli,Clayton,andMathis(1989).
objects. As shown in Table 2, 20 exposures (5400 seconds in
Using the SNR identification technique described above, 5
totalfortwofilters) werecombinedfortwoobservationnights.
SNR candidates were detected in NGC 2903 with [SII]/Hα ≥
StandardstarsfromthelistofOke(1974)andStone(1977)were
0.4.Resultsofourpresentobservationswithcandidatesandcor-
observed each night to determine the flux conversion factors.
rected flux ratios from the imaging analysis of these SNRs are
Bias framesanddomeflats were also observed.Eachexposure
listedinTable3.ThemeasuredHαfluxofSNR4issmallerthan
wasbias-subtracted,trimmed,andflat-fielded.Thecosmicrays
the other four optically-identified SNRs by factors of approxi-
were removed from each [SII] and Hα image. The SNR can-
mately 2-3. We caution that calculations involvingthe Hα line
didates overlaid on DSS (Digitized Sky Survey) images of the
mayhaveslightcontaminationfromthe[NII]lines.We carried
spiralgalaxycanbefoundinFig.1.
outthesecalculationsinanenvironmentwithalimitingfluxsen-
sitivity level of 3.1×10−15 erg cm−2 s−1 for our imaging obser-
2.2.Spectroscopy vationsofNGC2903.Thissensitivitylimitwas determinedby
choosingastructurethathasminimummagnitudeinthegalaxy
ThespectraldataofonebrightSNRwereobtainedwiththeop- field. Then the total counts from the same circular aperture of
ticaltelescopeBTA-6m(BolshoiAzimuthalTelescope,Russia) SNRsusedareconvertedtogalaxyfluxvalueusingthespectro-
2008 April. The SCORPIO (Spectral Camera with Optical scopicstandardstarfluxmentionedabove.
Reducer for Photometricaland InterferometricalObservations) OnlyonefieldwasobservedinNGC2903.Itcoversatotal
spectrograph was used in BTA with a CCD 2048 × 2048 pix- fieldof12′×6′.InFigs.2-3weshow4′×4′subfieldsofNGC
elsinsize.Weused1′′ slitwidth,anda3500-7200Åspectral 2903.NoSNRswerefoundinthesouthernhalfofthegalaxy.We
rangewasassumedforSCORPIOwith10Åspectralresolution. notethatthe southfieldofthe galaxyconsistsofmainlybright
The IDL codes and IRAF packages were used to perform the HIIregionscomparedwiththenorthfield(Bresolinetal.2005).
basicreductions,flux,andwavelengthcalibrationsandinterstel- ThismightbeoneofthereasonsfortheabsenceofSNRsinthis
larextinctioncorrection.Spectrophotometricstandardstarsfrom region.The detectedSNR distributionin NGC 2903resembles
Oke(1974)andStone(1977)catalogswereobservedeachnight. theskeweddistributionofSNRsseeninNGC2403byMatonick
Derivedfluxesfromthespectrophotometricstandardswereused etal.(1997).TheyalsoexplaintheabsenceofSNRsinthenorth
tofindthefluxesforspectrallinesinourSNRspectrum.Biases, fieldoftheimageofNGC2403inasimilarfashion.
halogenlampflats,andFeArorNeoncalibrationlambexposures AmongthefivecandidatesdetectedinNGC2903,wewere
were obtained for each observation set. Finally [SII] and Hα able to observe SNR4 spectroscopically and derive a specific
emissionlinefluxesweremeasuredusingsplotroutineinIRAF. line ratio of [SII]/Hα of 0.42. The optical spectrum of SNR4
To obtainspectral data of SNRs, the slit position was arranged isshowninFig.4.LineintensitiesrelativetoHβ,theE ,and
(B−V)
so thatbotha brightstar andthe SNRwere insidethe slit. The Hαintensity valueforthisspectrumgivenin Table 4.We used
aim of spectral observationswas to resolve [SII] λλ6716,6731 thisspectrumtocalculatelineratiosfor[SII]λ6761/[SII]λ6731
E.Sonbasetal.:AnopticalsearchforsupernovaremnantsinthenearbyspiralgalaxyNGC2903(RN) 3
and [OIII]λ5007 / Hβ. We calculated electron density, N , us- As noted by MF97, Braun & Walterbos (1993) have esti-
e
ing [SII]λ 6761/[SII]λ6731line ratio and the Space Telescope matedthatabouthalfofallSNeareofTypeIb/corTypeII(that
ScienceDataAnalysisSystem(STSDAS)tasknebular.temden. is,producedbythedeathsofmassivestars);inturn,onlyhalfof
Whenanelectrontemperaturevalueisgiven,thistaskcalculates alloftheseSNearelocatedinregionswithenoughambientden-
theelectrondensity,basedonthefive-levelatomapproximation sitytoproduceadetectableSNR.Thismeansthat,onlyabouta
explainedinthetask.Thelineratioof[SII]λ6716/[SII]λ6731> quarterofallSNeventsmayleaveeasilydetectableopticalrem-
1.46gives the low density limit correspondingto N ≤ 10 cm3 nants.InourcasewehavedetectedfiveSNRs,indicatingthatthe
e
(Osterbrock 1989). Assuming an electron temperature of T = totalnumberofSNeinNGC2903is∼20,abouthalfofwhich
104 K, the calculated N value is 360, which is not so atypical (∼ 10)wouldleave remnants.Thisnumbercouldalso betaken
e
forsuchgalaxies(forexample,SNR19inNGC2403,Matonick asanupperlimitforobservationswithmuchmoreimprovedvis-
et al. 1997). For SNR4, we detected only a weak [OIII]λ5007 ibilityandseeingconditions.However,onlyabouthalfofthem
/ Hβ emission line ratio of ∼0.1indicatingan oxygen-deficient (∼5)wouldoccurineasilydetectableregions.Alltheseprovide
remnant, which corresponds to a low propagation velocity be- acceptableexplanationsforourobservations.Iftheopticallyvis-
lowthelimitof≤100km/s(Smithetal.1993),whichshutsoff iblelifetimeofatypicalSNRisabout20,000years(Braunetal.
thenebularshocks.Therearemanyexamplesofsuchweakline 1989),it would also give us an SN occurrencerate of about 1
ratioswithpooroxygencontentinanumberofnearbygalaxies per 1000 yrs. When we compared this rate to MF97 SN rates,
(Blair&Long2004;MF97). wefindquitegoodoverlapforthecaseofNGC5585,whichhas
thesamenumberofSNRs.Inthesamework,forgalaxieswith
We searched for X-ray counterparts to the SNRs that
lower and higher SNR numbers, this rate goes proportionally
were found by our optical observations. Twenty one X-
higherandlower.
ray point sources were determined in the position and
Intheiranalysis,MF97alsopresentthemodevaluesofthe
extension of NGC 2903 in the Master X-Ray Catalog
measuredHαintensitiesforthedetectedSNRsamplesfromnu-
(http://heasarc.nasa.gov/W3Browse/all/xray.html). We found
merousgalaxiestoseeevidenceofanyselectioneffectsandbi-
only one positional coincidence with the Master X-ray cata-
ases(seetheirTable19).Usingthegalaxydistancestheyshow
log sources taking a 30′′ positional error circle around the ob-
thelogofthemodeoftheHαluminosity,L(Hα) ,islargerfor
jects.SNR3fallsintheerrorcircleof1RXSJ093210.2+212947. mode
moredistantgalaxies.Thismeansthat,ifthedistanceofgalaxies
However,thetabulatedpositionerrorofthecatalogsourcewas
increases,itismuchmoredifficulttodetectthefainterSNRs.In
18′′,whichwaslowerthanourassumederror,oncetheerrorcir-
ourcase,wecalculatedthattheL(Hα) valuecouldbetaken
clediminishedinsize,therewefoundnocorrelationofourcan- mode
as ∼1.2×1038 erg s−1 (since we have detected a few SNRs, we
didate SNR3 with the source. This catalog contains data from
were onlyable to calculate an averagevalue for Hα intensities
the Position Sensitivity Proportional Counter (PSPC) onboard
andconsideredthis asourmodevalue.Thiswas also the prac-
ROSAT (Rontgen Satellite) and Imaging Proportional Counter
ticebyMF97forgalaxieswithalownumberofSNRs).Witha
(IPC) onboardEinstein observatories. IPC provides an angular
distanceof9.4Mpc,NGC2903followsthesametrendtoward
resolutionof ∼30′′ (FWHM) at ∼1 keV andthe limiting sensi-
higherL(Hα) valuestogowithgreaterdistances.
tivity rangefrom ∼5×10−14 to ∼3×10−12ergcm−2s−1 in the 0.3 mode
- 3.5 keV energy band (Gioia et al. 1990). And ROSAT PSPC
minimum sensitivity lies around a few times 10−13 - 2×10−14 Acknowledgements. WethanktheTUBITAKNationalObservatory(TUG)and
SpecialAstrophysicalObservatory(SAO)fortheirsupportwithobservingtime
ergcm−2s−1 intheenergyband0.1-2keV(Morleyetal.2001). andequipment.AlsowewouldliketothanktoIGPP(InstituteofGeophysical
PSPChasprovided∼30′′(FWHM)on-axisangularresolutionat PlanetaryPhysics)atUCR(UniversityofCaliforniaRiverside)forprovidingus
1keV(Trumper,1984).Theselimitingfluxessetanupperlimit someoftheinterferencefilters.WealsothankananonymousrefereeandM.E.
Ozelfortheirvaluablecommentsanddiscussions.
ontheluminosityofsourcesthatwouldbedetectedinthiscat-
alogasafewtimes1038 ergs−1 atthedistanceof9.4Mpc(for
NGC2903).Giventhata maximumradiativeX-rayluminosity
References
ofanSNR(withashocktemperatureof0.1keVorabove)will
beafewtimes1038−39ergs−1(seePanutti,Schlegel,Lacey2003; Alonso-Herrero,A.,Ryder,S.D.,Knapen,J.H.2001,MNRAS,322,757
Schlegel&Panutti2003;Holtetal.2003),itisonlynormalthat Blair,W.P.,Kirshner,R.P.&Chevalier,R.A.1981ApJ,247,879
Blair,W.P.&Long,K.S.1997,ApJS,108,261
thereare no SNRs amongthe Master X-rayCatalog sourcesin
Blair,W.P.&Long,K.S.2004,ApJS,155,101,121
thevicinityofNGC2903.Tschokeetal.(2003)havealsofound
Braun,R.,Goss,W.M.,Lyne,A.G.1989,ApJ,340,355
18 sources in the vicinity of NGC 2903. We checked for any Braun,R.&Walterbos,R.A.M.1993,A&AS,98,327
positional coincidence with these sources using an error circle Bresolin,F.,Schaerer,D.,GonzlezDelgado,R.M.,Stasinska,G.2005,A&A,
of 30′′, but found no correlation. Given our results for SNR4, 441,981
Cardelli,J.A.,Clayton,G.C.&Mathis,J.S.1989,ApJ,345,245
sincewederivenostrong[OIII]emissionandour[OIII]λ5007/
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expectedthatX-rayimagingwithChandra,withimprovedan-
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gularresolutionandsensitivity,couldprovidevaluableimprove- Gioia,I.M.,Maccacaro,T.,Schild,R.E.,Wolter,A.,Stocke,J.T.,Morris,S.L.,
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positionalaccuracy)amongthesesources. R.2001,ApJ,551,702
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Junkes,N.&Hensler,G.1996,InternationalConferenceonX-rayAstronomy Table 1. Characteristics of the interference filters used in our
andAstrophysics:Ro¨ntgenstrahlungfromtheUniverse,459 observations
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Name λ FWHM
Lacey,C.K.&Duric,N.2001,ApJ,560,719
Wavelength(Å) Å
Magnier,E.A.etal.1995,A&A,114,215
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Oka,S.,Wakamatsu,K.,Sakka,K.,Nishida,M.,&Jugaku,J.1974,PASJ,26,
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GalaxyName Date Filter Exposure
Oke,J.B.1974,ApJS,27,21
(s)
Osterbrock,D.E.1989,S&T,78,491
Pannuti,T.G.,Duric,N.,Lacey,C.,Goss,W.M.,Hoopes,C.G.,Walterbos,R. NGC2903 2006March4/5 [SII] 3×600
A.M.,Magnor,M.A.2000,ApJ,544,780 2006March4/5 Hα 3×600
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Pence,W.D.,Snowden,S.L.,Mukai,K.&Kuntz,K.D.2001,ApJ,561,189 2006March5/6 Hα 6×600
Schlegel,E.M.,Holt,S.S.,Petre,R.2003,ApJ,598,982
2006March5/6 continuum-6446 4×300
Schlegel,E.M.,Panutti,T.G.2003,AJ,125,3025
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Sersic,J.L.1973,PASP,85,103
Simons,D.A.,Depoy,D.L.,Becklin,E.E.,Capps,R.W.,Hodapp,K.-W.&
Hall,D.N.B.1988,ApJ,335,126 Table3.NewopticalSNRcandidatesdetectedinNGC2903.
Smith,R.C.,Kirshner,R.P.,Blair,W.P.,Long,K.S.,&Winkler,P.F.1993,
ApJ,407,564
Stone,R.P.S.1977,ApJ,218,767 SNRName RA DEC [SII]/Hα I(Hα)
Trumper,J.1984,PhyS,7,209 (J2000.0) (J2000.0) (ergcm−2s−1)
Tsai,Chao-Wei,Turner,J.L.,Beck,S.C.,Crosthwaite,L.P.,Ho,P.T.P.,Meier, SNR1 9:32:12.5 +21:32:30 0.41 9.4E-15
D.S.2006,AJ,132,2383 SNR2 9:32:13.7 +21:30:48 0.74 1.7E-14
Tschoke,D.,Hensler,G.&Junkes,N.2003,A&A,411,41 SNR3 9:32:10.7 +21:29:19 0.57 1.6E-14
Wynn-Williams,C.G.&Becklin,E.E.1985,ApJ,290,108
SNR4 9:32:12.5 +21:29:06 0.42 5.4E-15
SNR5 9:32:11.1 +21:28:23 0.53 1.1E-14
Table 4. Relative line intensities and observational parameters
forSNR4
Line SNR4
Hβ(λ4861) 100
OIII(λ4959) -
OIII(λ5007) 11
NII(λ5200) -
He(λ5876) -
OI(λ6300) -
OI(λ6364) -
NII(λ6548) 30
Hα(λ6563) 280
NII(λ6583) 98
SII(λ6716) 62
SII(λ6731) 55
E 0.032
(B−V)
I(Hα) 5.4E-15ergcm−2s−1
[SII]/Hα 0.42
E.Sonbasetal.:AnopticalsearchforsupernovaremnantsinthenearbyspiralgalaxyNGC2903(RN) 5
Fig.1.AlltheSNRsthataredetectedinourstudyareindicated Fig.4. Optical spectrum of SNR4 in NGC 2903 obtained with
ontheimagesextractedfromDigitalSkySurvey(DSS).Figure BTA.Identifiedlinesareindicatedinthefigure.
showsNGC2903withthe5newSNRcandidatesfoundinthis
worklabeledwithcircles.
Fig.2. The figure indicates the SNR candidates (SNR1 and
SNR2)detectedinthisworkwithinNGC2903overlayedona∼
4′ssubfieldofcontinuum-subtracted[SII]image.
Fig.3. Thefigure indicatesthe SNR candidates(SNR3, SNR4,
andSNR5)detectedinthisworkwithinNGC2903overlayedon
a∼4′subfieldofcontinuum-subtracted[SII]image.