Mon.Not.R.Astron.Soc.000,1–14(2012) Printed29January2013 (MNLATEXstylefilev2.2) New Galactic supernova remnants discovered with IPHAS L. Sabin1,2⋆, Q.A. Parker3,4,5, M.E. Contreras1, L. Olgu´ın6, D.J. Frew3,4, M. Stupar3,4,5, 3 R. Va´zquez1, N.J. Wright7, R.L.M Corradi8,9, R.A.H Morris10 1 1InstitutodeAstronom´ıa,UniversidadNacionalAuto´nomadeMe´xico,Apdo.Postal877,22800Ensenada,B.C,Mexico. 0 2InstitutodeAstonom´ıayMeteorolog´ıa,DepartamentodeF´ısica,CUCEI,UniversidaddeGuadalajara,Av.Vallarta2602,C.P.44130,Guadalajara,Jal.,Mexico 2 3MacquarieUniversityResearchCentreinAstronomy,Astrophysics&Astrophotonics,Sydney,NSW2109,Australia n 4DepartmentofPhysicsandAstronomy,MacquarieUniversity,Sydney,NSW2109,Australia a 5AustralianAstronomicalObservatory,POBox296,Epping,NSW1710,Australia J 6Depto.deInvestigacio´nenF´ısica,UniversidaddeSonora,Blvd.RosalesEsq.L.D.Colosio,Edif.3H,83190Hermosillo,Sonora,Mexico 7 7Harvard-SmithsonianCenterforAstrophysics,60GardenStreet,Cambridge,MA02138,USA 2 8InstitutodeAstrof´ısicadeCanarias,E-38200Tenerife,Spain 9DepartamentodeAstrof´ısica,UniversidaddeLaLaguna,E-38200Tenerife,Spain ] 10AstrophysicsGroup,DepartmentofPhysics,BristolUniversity,TyndallAvenue,Bristol,BS81TL,UK R S . h Received2012June11.Accepted2013January25. p - o r ABSTRACT t s Aspartofa systematic searchprogrammeofa 10-degreewide strip ofthe NorthernGalac- a tic plane we present preliminary evidence for the discovery of four (and possibly five) [ new supernova remnants (SNRs). The pilot search area covered the 19–20 hour right as- 1 cension zone sampling from +20 to +55 degrees in declination using binned mosaic im- v ages from the INT Photometric Hα Survey (IPHAS). The optical identification of the can- 6 didate SNRs was based mainly on their filamentary and arc-like emission morphologies, 1 their apparently coherent, even if fractured structure and clear disconnection from any dif- 4 fuse neighbouring HII region type nebulosity. Follow-up optical spectroscopy was under- 6 taken, sampling carefully across prominent features of these faint sources. The resulting . 1 spectra revealed typical emission line ratios for shock excited nebulae which are charac- 0 teristic of SNRs, which, along with the latest diagnostic diagrams, strongly support the 3 likelySNR natureof thesesources:G038.7-1.3(IPHASX J190640.5+042819);G067.6+0.9 1 (IPHASX J195744.9+305306); G066.0-0.0 (IPHASX J195749.2+290259) and G065.8-0.5 : v (IPHASX J195920.4+283740). A fifth possible younger, higher density nebula SNR can- i didate, G067.8+0.5 (IPHASX J200002.4+305035)was discovered ∼5 arcmins to the west X ofIPHASXJ195744.9+305306,andwarrantsfurtherstudy.Amulti-wavelengthcross-check r fromavailablearchiveddataintheregionsofthesecandidateswasalsoperformedwithafo- a cusonpossibleradiocounterparts.Aclosepositionalmatchbetweenpreviouslyunrecognised radiostructuresatseveralfrequenciesandacrossvariouscomponentsoftheHαopticalimage datawasfoundforallSNR candidates.ThislendsfurtherdirectsupportfortheSNR nature of these objects. Evolved SNRs may have very weak and/orhighly fragmentedradio emis- sion which could explain why they had not been previously recognised but the association becomesclearincombinationwiththeopticalemission. Keywords: HαSurvey–ISM:Supernovaremnants. 1 INTRODUCTION enrich the interstellar medium (ISM) in heavy elements (such as oxygen,ironandnickel)andothernucleosynthesisproducts.These Supernova remnants(SNRs) areamong themajor contributors to ejecta play an important role in the observed chemical composi- thechemicalregenerationoftheGalaxy.Indeed,followingtheex- tionofthelocalISMandthestellarsystemsthatmaysubsequently plosion of their high mass progenitor stars in the case of core- be formed from it. SNRs are also responsible for the release of collapse supernovae (∼80%of allsupernovae) ortheType-Iabi- largeamountsofenergy,generatingshockwavesthathighlightor nary route (∼20-25% of all supernovae, Bazinetal. 2009), they “shape”thesurroundinginhomogeneousISMwiththeconcurrent production of hot, dense and bright ionised zones. The detection andstudyofthoserapidlyevolvingremnants(afewthousandyears ⋆ E-mail:[email protected](LS) 2 L. Sabinet al. old)arecrucialfortheunderstandingnotonlyofstellarevolution ofextendedopticalemissionwithanHαsurfacebrightnessdown ofhighmassstarsinthecaseofcore-collapsesupernovabutalso, to≃2×10−17ergcm−2s−1arcsec−2(≃3Rayleighs).Thismakes moreglobally,fortheunderstandingofGalacticabundance gradi- the survey sensitive to evolved, low surface brightness nebulae ents,kinematicsandchemistry. suchasplanetarynebulae(PNe),includingthoseinteractingwith The most recent and complete compilation of SNRs in the the ISM (Sabinetal. 2010; Wareingetal. 2006), symbiotic stars GalaxyhasbeenassembledbyGreen(2009)whichcurrentlycom- (Corradietal. 2010), Wolf-Rayet (WR) shells (Stock&Barlow prises274recognisedandverifiedobjects.However,accordingto 2010), Herbig-Haro objects, proplyd-like objects (Wrightetal. Tammannetal.(1994)weshouldexpectaround1000SNRstobe 2012),andotherkindsofresolvednebulaeaswellasSNRs. visible at any one time inour Galaxy. Thisdiscrepancy could be Thisisnot thefirst timeadeep, narrow-band optical survey accountedforbythelack(anddifficulty)ofdetectionatbothends has been used to search for new SNRs as previously mentioned. ofSNRevolutionwheneithertheSNRistoocompact/distanttobe Stuparetal.(2007,2008,2011)presentedthediscoveryofasignif- seenorwhenitisonthevergeof completelydissipatinginto the icantnumberofnewopticallyidentifiedandconfirmedSNRsbased ISM.ThemajorityoftheSNRshavebeenhistoricallydetectedvia on data from Southern Galactic plane Hα Survey (Parkeretal. radio observations and the subsequent determination of the non- 2005). The authors also published a catalogue of newly uncov- thermal natureof theemission wherethetypicallyobserved neg- eredopticalcounterparts for24known GalacticSNRspreviously ative spectral index implies synchrotron emission. More recently identifiedonlyfromradioorX-rayobservations(Stupar&Parker optical detectionsof new GalacticSNRshavebeenpossible (e.g. 2011). Boumisetal. (2005, 2008, 2009) also presented the first Stuparetal.2008)evenintheabsenceofpreviousradioevidence optical detections and spectroscopic confirmation for a number due tothe advent of high resolution, high sensitivity Hαsurveys of SNRs based on CCD data. While also in the northern sky such as those described by Parker et al. (2005) and Drew et al. Fesen&Milisavljevic(2010)discoveredopticalfilamentsforSNR (2005). G159.6+7.3whichhadnopreviousradiodetection. Green (2004) highlighted the most important radio surveys Inthispaperwereportthepreliminaryresultsofasystematic traditionallyusedforuncoveringSNRs.TheseincludetheEffels- searchfornewSNRsfromtheIPHASsurvey.Radioobservations berg2.7GHzsurvey(Furstetal.1990)witharesolutionof ≃4.3 (acrossalargefrequencyrange)arewidelyusedtoidentifyandim- arcmin, the Sydney University Molonglo Sky Survey (SUMSS) ageSNRssoinadditiontothecharacteroftheionisedopticalemis- at 843 MHz (Bocketal. 1999) with a resolution of 43×43 arc- sionpresented,thedetectionofradioemissionfromtheexpanding seconds cosec δ,theParkes-MIT-NRAO (PMN)radio continuum SNR shell is another important tracer, especially if it is possible surveyat4.85GHz(Griffith&Wright1993)witharesolutionof to measure a negative spectral index to indicate synchrotron ra- ∼ 5arcminfortheSouthernsurveyand∼3arcminfortheNorth- diation. Weexamined archival data from the Westerbrock North- ern counterpart undertaken at the same frequency on the Green- ernSkySurvey(WENSS)at325MHz(Rengelinketal.1997),the banktelescope(GB87;Condonetal.1994)andfinallytheNRAO NRAOVLASkySurvey(NVSS)at1.4GHz(Condonetal.1998) VLA Sky Survey (NVSS) at 1.4 GHz (Condonetal. 1998) with andtheGreenBank(87GB)4.85GHzsurvey(Gregoryetal.1996) a resolution of 45 arcseconds. These surveys are able to distin- tolookforradiocounterpartstoourIPHAStargets.Furthermore, guishSNRsoveravarietyofangularscalesdepending onthere- thehighenergy releasedbytheinitialsupernova explosion (typi- spectivebeamsizes.ForareviewofSNRsinthePMNsurveysee callyabout1051 ergsfollowingBurke&Graham-Smith2002)re- Stupar,Filipovic´,Parker&etal.(2007). sults in the generation of X-ray emission which can be detected Small,andthereforeyoung(ordistant),SNRsmaynotbeop- by X-ray space telescopes such as the previous ROSAT and cur- ticallyrecognisedduetothepresenceofhighlyvariableextinction rentChandraandXMMNewtonspaceobservatories.Archivaldata atlowGalacticlatitudeswheremostSNRsareexpectedtoreside. fromthesesurveyshavealsobeeninterrogatedtolookforpossible OtherkindsofSNRsthatarelikelytobemissedarethosehighly X-raycounterparts,bothdiffuseandcompact.Asearchforinfrared evolved SNRswithlow surface brightness and largeangular size counterparts was also undertaken using most of the available IR which are usually also highly fragmented. Indeed, radio surveys surveys (e.g. WISE, GLIMPSE) but this did not return any con- are generally not very sensitive to extremely evolved SNRs (e.g. vincingcorrespondence. Stupar&Parker 2011, Stuparetal. 2008) where the detected ra- Thepaperisorganisedasfollows.In§2wepresentthenew diofragmentsareoftenunrecognised. Furthermore,intheoptical IPHASopticaldetectionsoftheSNRcandidates.In§3wepresent regime,thetypicallylowemissionlevelsfromGalacticSNRsmake thespectroscopic follow-upofthecandidates. In§4somemulti- themdifficulttoidentifywhileconfusion withtheubiquitous HII wavelength,principallyradiocounterpartstotheopticalimagesare regionsisalsoaproblem.Theseselectioneffectshaveresultedina discussed,whilein§5wepresentadiscussionandourconclusions. seriousbiasintheglobalstudyofSNRsandtheirproperties. Aspartofasystematicefforttohelpaddressthiswearetak- ing advantage of the recent, deep narrowband INT Photometric 2 IPHASDETECTIONSOFNEWSNRCANDIDATES Hα Survey of the Northern Galactic plane (IPHAS; Drewetal. 2005, Gonzalez-Solaresetal. 2008) to look for “previously hid- Thisinitialsearchfornew,opticallydetectableSNRsconcentrated den” but optically detectable SNRsin a similarway to those un- ontherightascensionrangefrom19h–20h.Thisisthefirstmajor coveredbyStuparetal.(2008)intheSouth.IPHAS,whichstarted IPHASzonewhichhasbeensystematicallyandcarefullyscanned in 2003, is now essentially complete. It used a wide-field CCD fornewemissionnebulaeofallkinds,includingPNeandHIIre- camerawithr,iandHαfilterstoprovide anaccuratephotomet- gions.ThissearchformedamajorcomponentofthethesisofSabin ricsurvey. IPHAScovers1800 deg2 of thenorthern plane across 2008.Aseparatepaperdedicatedtothefirst100confirmedPNere- a±5degreelatitudestripandwasinitiatedasthedirectcounter- sultingfromthissearchisalsointrain(Sabinetal.2013,inprep.). parttotheAAO/UKSTHαsurveyoftheSouthernGalacticplane Note that the full IPHAS sky area (1800 deg2) has been divided (Parkeretal. 2005). The survey has a nominal resolution of 0.33 into2deg2 Hα-r(continuum removed) image mosaicsthat facil- arcsec.pix−1andthenarrow-bandcomponentallowsthedetection itatedetection andrecognition of extended emissionnebulae. We IPHAS Galacticsupernovaremnants 3 usedmosaicscalefactorsof15×15pixels(5arcsec) and5×5 Table1.Estimatedcentralpositionsandangularsemiminorandmajoraxes pixels(1.7arcsec).Thefirstbinninglevelwaschosentoenablethe forthenewSNRcandidatesfoundviavisualinspectionoftheIPHASmo- detection of low surface brightness objects (down to the IPHAS saicsinthe19–20hourRAzone.TheRA/DECpositionsareJ2000. limit) (Sabinetal. 2010) over large angular scales while the sec- ondwasusedtodetectintermediatesizenebulae,i.esmallerthan ≃15–20arcsecindiameter. IPHASXID GalacticID RA(J2000) DEC(J2000) Angularsize TheSNRcandidateswereselectedbasedontheiropticalmor- hms 0’” (arcmin) phologicalcharacteristics.Theywereconsideredasplausiblecan- J190640.5+042819 G038.7-1.3 19:06:40.5 +04:28:18.8 18.6×31.8 didatesiftheycouldbeidentifiedasdetachedbutcoherentfilamen- J195744.9+305306 G067.6+0.9 19:57:44.9 +30:53:05.6 45.2×49.2 tarystructuresorasdiscreteroundshellsorarcsintheIPHASHα-r J195749.2+290259 G066.0-0.0 19:57:49.2 +29:02:58.8 24.6×31.0 mosaicimages.TheHαBalmeremissionlineisanexcellenttracer J195920.4+283740 G065.8-0.5 19:59:20.4 +28:37:40.0 6.2×10.4 ofionisedgaswhilethecontinuumsubtractedimagesenhancethe J200002.4+305035 G067.8+0.5 20:00:02.4 +30:50:35.0 7.2×5.4 contrastevenfurther.Alreadycataloguedsources,includingknown SNRs are also scrutinised as additional, previously unrecognised emission components to these sources may become evident. For Table 2. Summary details of the spectroscopic observations of the SNR examplemostknownSNRswereuncovered onlyfromradiodata candidates.Theformalwavelengthrangesofthespectraareincluded butwecannow alsoreveal optical counterparts tomanyof these remnantsforthefirsttime(Stupar&Parker2011). IPHASXID Telescope Date λcoverage Exposure Spectroscopic validation of newly identified optical compo- A˚ (seconds) nents of perhaps widely dispersed SNR candidates is important fortheirproperidentificationbutthiscanbedifficulttorealizein J190640.5+042819 SPM2.1m 07/06/2010 ≃4330–7530 1800 crowded zones such as the Galactic centre where several SNRs J195744.9+305306 SPM2.1m 21/08/2011 ≃4330–7530 1800 J195749.2+290259 SPM2.1m 22/08/2011 ≃4330–7530 1800 (and HII regions) can overlap (Green 2009, their figure 3). Our J195920.4+283740 INT2.5m 18/04/2008 ≃3500–8960 3×1200 careful search over the preliminary RA zone resulted in a set of J200002.4+305035 SPM2.1m 22/08/2011 ≃4330–7530 1800 fourorpossiblyevenfivecandidateSNRsbeinguncovered.These are described individually below with summary details presented inTable1andnamedaccordingtothenomenclatureadoptedforall ture of the”duplicated” structure which may represent some evi- IPHASextendedobjects(IPHASXJHHMMSS.s±DDMMSS). dence of symmetrical mass ejection. Further study is needed but OurfirstcandidateSNRIPHASXJ190640.5+042819wasse- this is beyond the scope of this paper. The structure of IPHASX lected due to its well defined semi-circular emission of radius J195749.2+290259isagaintypicalofoptical(andradio)SNRsand ∼9.3′×15.9′togetherwithsomeelongatedtailsoffilamentaryfea- wenotethesimilaritybetweenthisobjectandthewellknownSNR turesparticularlytotheSouth.Thesemorphologicalstructuresclas- IC443whichalsoexhibitsfaintfilament(G189.1+3.0; seeFesen sifieditasagoodSNRcandidate(Fig.1–Top). 1984). ThesecondcandidateisIPHASXJ195744.9+305306(Fig.2– The last selected SNR candidate is IPHASX Top)whichhastheappearanceofafracturedovalringwithsome J195920.4+283740 which reveals an elongated and closed shell internalemissionspursemanating fromtheEasternandNorthern structure at faint isophotes over a region of radius 186×312 arc- edges.Ithasaprojectedangularradiiof∼22.6′×24.6′.Thereare seconds (Fig.5–Top). Thenebulaisgloballyvery faint except for several HIIregionsinthegeneral vicinityof thiscandidateSNR, theenhancededgeseenintheEasternside. including Sh2-98and Sh2-97 whicharefound within ∼0.5deg butthesearewellseparatedfromtheSNRcandidate.However,the most northerly faint rim of the SNR appears somewhat detached from the main shell and comes within 4 arcminutes of the outer 3 SPECTROSCOPICOBSERVATIONSAND rimofthecircularHIIregiondirectlytotheNorthsosomeover- CONFIRMATORYRESULTS lappingofemissionherecannotbecompletelyruledout.Thereis As part of our general programme of IPHAS follow-up spec- alsoaknownSNRsG067.7+01.8,located∼1degreewest.Clearly, troscopy, we obtained low resolution confirmatory spectra of thisgeneralareahostssomelargenebulae.Nevertheless,themain these candidate SNRs. Our first SNR spectrum was of IPHASX structureremainsclearlydetachedfromanyofthesurroundingHII J195920.4+283740 takenonthe2.5mIsaacNewtonTelescopeon regionsandwecanconsiderIPHASXJ195744.9+305306asasin- LaPalma(CanaryIslands)inApril2008whiletheremainingcan- gleobject. didateswereobservedusingthe2.1-mtelescopeattheSanPedro Acoherent,completelyseparate,ellipticalemissionstructure MartirObservatory(OAN-SPM1)betweenJune2010andAugust was also found ∼5′ to the east of IPHASX J195744.9+305306 2011.Asummaryofthesespectroscopicobservationsaregivenin (Fig.3–Top)whichwelabelIPHASXJ200002.4+305035 andwas Table2whilethespectraarepresented inFig.6and theresulting includedinourinvestigationasitcouldbeayounger,morecom- emissionlinemeasurementsareshowninTable3. pactSNRorperhapsevenanevolvedPN. Onthe2.5mINTweusedthelowresolutionspectrographIDS WealsoidentifiedIPHASXJ195749.2+290259(Fig.4)which withtheEEV10detectorandR300Vgrating.Thiscoverstherange has a filamentary partial shell morphology with a bright emis- ≃3500A˚-8960A˚ witharesolutionof≃1.8A˚ thoughthedatabe- sion arc to the North. A fainter apparent morphological duplica- low3600A˚ isofnovalueduetotheatmosphericcut-off.Theslit tionofthisstructure ∼5′ totheNorth-Westisalsoevident which must be related given their strong similarity in appearance. Such similar morphological repetition is also seen in parts of the well 1 The Observatorio Astrono´mico Nacional at the Sierra de San Pedro known Vela SNR.We cannot probe deeper into a detailed expla- Ma´rtir(OAN-SPM)isanational facility operated bytheInstitutodeAs- nation at this stage as we lack the data to fully establish the na- tronom´ıaoftheUniversidadNacionalAuto´nomadeMe´xico. 4 L. Sabinet al. lengthwas3.3arcminutesandtheseeingduringtheobservations Animmediateconsequenceofsuchfaintnebulosityistheinability variedbetween∼1.8and∼2.2arcseconds.Theexposuretimefor toeasilydetectothercommondiagnosticemissionspeciessuchas the observation of IPHASX J195920.4+283740, which is a rela- theFelines(Fesen&Hurford1996). tivelyfaintnebulawassetto3×1200sonaportionofitsbrighter Anotherissuerelatedtothefaintnessofourtargetsisthedis- Easternedge.WenotethedatabluewardofHβinthespectrumdid crimination between true SNRs and other morphologically simi- notshowanyemissionlinesduetothelowS/N. lar nebulae such as Wolf-Rayet (WR) shells, symmetric HII re- Fortheremaining2.1-mSanPedroMartirtelescopeobservations gions or even low excitation zones in PNe, as these nebulae can we used a Boller & Chivens spectrograph and Thomson & Mar- exhibitsimilaremission-lineratiosinsomecases.Anoverallbody coni CCDs(2048×2048 pixels of size 14µm). An East-West slit of evidence is used to aid in the identification of likely SNRs orientationwasadopted withthe5arcminutelengthslitcarefully andtoremovecontaminants.Thiscomprisestheopticalmorphol- positionedonthemostprominentnebulararcsorfilamentstogive ogy, spectroscopic signature, local environment, presence of X- betterS/Nbutalsochosen toallow sufficientskyforbackground ray sources internal to the nebulae and any corroborating radio subtraction.Incaseswhere,despiteourbestefforts,itwasnotpos- emission. While other shock-excited nebulae such as supershells sibletoextractsufficientdecentskywealsotookbackgroundspec- (Hunter 1994; Skeltonetal. 1999) and giant outflows associated tra in relatively”blank” areas located in the immediate surround- withHerbig-Haroobjects(Reipurthetal.1997;Maderetal.1999) ings of the nebula. This was particularly the case for IPHASX canhavesimilaremissionline-ratiostoSNRs,thesmall(lessthan J195744.9+305306 (B&Cpositions).Theobservationswereob- thedegree)angularsizesofourcandidates,theirfilamentarymor- tainedundervarying seeingconditionstypicallyaround2arcsec- phologies and the lack of star-forming activity in their vicinities onds and were made with the 400 line/mm grating. This gave a stronglymilitatesagainstthesealternativeinterpretations. spectralcoverageof≃4330A˚–7530A˚ witharesolutionof≃5A˚ sufficienttoencompassmostopticalemissionlinesofinterest.The 3.1 Newemissionlinediagnosticdiagrams databluewardofHβ inthesespectradidnotexhibitanyemission linesduetothelowS/Nandextinction.Thedatareductionofall The ability to separate out different classes of astrophysical ob- spectrawasdonewiththeusualIRAFroutines.Theextinctioncor- jectaccordingtotheirobservedemission-linesratiosiswellestab- rectionappliedtoourspectrawasperformedusingtheFitzpatrick lished,e.g.thewell-knownSabbadin,Minello&Bianchini(1977) andMassaextinctioncurve(Fitzpatrick&Massa2007)forRV = ‘SMB’ Hα/[N II] versus Hα/[S II] diagnostic diagram or the 3.1. Baldwin,Phillips&Terlevich(1981)‘BPT’[OIII]λ5007/Hβ ver- Inordertoderivetheerrorsonthemeasurementsweconsider sus[SII]/Hαdiagnosticdiagram.Wehavebeenrefiningandupdat- mainlytwosourcesoferror.ThefirstisrelatedtotheCCDreadout ingthesediagnosticdiagramsusingcarefullyvettedspectroscopic noiseand photon noise relativetotheskyand objects. Thiserror datafromtheliterature,supplementedwithourownflux-calibrated isderivedviatheestimationofthevariance(σ2)ateachpointof spectroscopy forasignificant sampleof objectsof various kinds. the spectrum (i.e. at each wavelength). The ”variance spectrum” Thenew figurespresented here forthefirsttimearebetterpopu- obtained is processed the same way as for regular data spectra latedversionsofthediagramspresentedbyFrew&Parker(2010). in IRAF assuming a non-correlated error approximation (Taylor ThesenewplotsincludemanymoreGalacticSNRsandothertypes 1997).Thereforethestatisticalerrorscorrespondingtoeachspec- ofnebulaethanpreviousefforts.Byonlyutilisingthemostreliable trallineareextractedandregistered.Thesecondsourceoferrorwe data and identifications, a clearer picture of the loci of different consideredhereislinkedtothefluxcalibrationandthedata were object typesisestablishedcompared towhat hasbeen previously processedfollowingViironenetal.(2011). available. The usual Hα, [NII]λλ6548,6583 and [SII]λλ6716,6731 In this paper we therefore base our SNR spectroscopic con- emissionlineswereusedtoidentifythenatureofthenebulaebased firmations on their observed strong emission line-ratios used in onthecriteriadefinedbyFesenetal.(1985)[hereafterF85]andre- conjunctionwiththesepowerful,updateddiagnosticdiagramscre- finedmostrecentlybyFrew&Parker(2010).Themostconspicu- ated by one of us (DJF), based on an extensive database of ousopticalfeaturesforthediscriminationofSNRsaretherelative spectroscopic measurements. The line-ratio data for the Galactic strengthof[SII]linescomparedtoHα.Indeedalarge([SII]6716+ SNRswastaken from ourown unpublished spectroscopy as well [SII]6731)/Hαratio>0.4–0.5(seeF85)isakeyprobefortheoc- as numerous literature sources, with notable contributions from currenceoftheshockcharacteristicsoftheturbulentandenergetic Fesen&Kirshner (1980), Fesenetal. (1982, 1985), Blairetal. environmentofSNRswhichisnotgenerallyfoundinHIIregions (1991),andBoumisetal.(2005,andreferencestherein).TheLMC orPNe. SNR flux data is mostly from Payneetal. (2008, and references The [SII]ratioisused as adiagnostic to determine electron therein) and the line fluxes for Galactic PNe are taken from the densitiesinionizedplasmasandformostSNRsthisratioislikely sourcessummarisedinFrewetal.(2012).TheHIIregiondataare tobeinthelowdensitylimitwitharatioof∼1.4.Inaddition,the mostly taken fromthe references given inFrew&Parker(2010), presence ofrelativelystrong[NII]lines(with0.5<([NII]6548+ supplemented with unpublished data utilized by Boissayetal. [NII]6583)/Hα<1.5 following F85) as well as the presence of (2012). The labelled domains include approximately 95% of the moderate[OI]emission(λλ6300,6363)arealsogoodindicatorsof objectsofthatclasslistedinourdatabase.Afullerdescriptionof alikelySNRnature.However,thelattercancauseproblemsinlow- theconstructionandcontentofthesenewdiagnosticdiagramswill resolution spectraasthese linesarealsoquite strong inthenight be presented in their entirety in an upcoming paper (Frew et al., skymakingtheirdeconvolutionfromtheirbrightskylineequiva- 2013, in preparation, F13 hereafter). Our new sources are over- lentsvery difficultunless thereisastrong velocitycomponent in plotted on these figures which demonstrates clearly how they lie theSNR[OI]lines.Thisisespeciallygermaneiftheextendedneb- inthezonespopulatedbyconfirmedSNRs(seeFigure7andFig- ulosity extends along the entire length of the slit. Our main lim- ure8). itationthough istheinherent faintness of thetargetseven inHα, WeemphasisethatevolvedGalacticSNRshavecompositions whichalsoexplainstheirnon-detectioninpreviousopticalsurveys. dominated by swept-up ISM, and show a clear abundance gra- IPHAS Galacticsupernovaremnants 5 dient in the Galaxy (Binetteetal. 1982; F85); the most metal- uesof41and11(scaledtoHβ=100),supportingtheSNRclassifi- rich examples in the inner Galaxy tend to plot at the lower- cation(seeFigure6andTable3).However,theobservedlineratios left of the SNR domain in the Hα/[N II] versus Hα/[S II] di- fromthespectraforpointingAareclosertothoseexpectedfora agram (Figure 7). Since the IPHAS candidates also plot here, HIIregion(Table3).Notethatpointing‘A’fallsonanebularcon- this suggests that they are metal rich, which is consistent with densationinternaltotheproposedmainSNRshellandcouldsim- the Galactic sight-lines for these objects. At any plausible dis- plybeanunrelatedcompactHIIregion,as,unlikethetwointernal tance, they are located interior to the Solar Circle. This diag- emission spursto theNorth, thisblob does not appear connected nostic plot also illustrates how the Galactic objects tend to plot to the main arcuate shell. Furthermore, the spectral characteris- separately from the Magellanic Cloud remnants, a consequence ticsof thisregion (slitpositionA) aredifferent tothose obtained of the lower nitrogen abundance in the latter (Meaburnetal. fromslitpositionsBandCwithinsteadaratherlow[SII]/Hαratio 2010; Leonidakietal. 2012). We also note that many of the (0.24) which does not advocate for a shock regime and very dif- youngest core-collapse Galactic SNRs (e.g. Fesen&Kirshner ferent[NII]/Hαratios,addingfurtherweighttoitsidentificationas 1982;Leibowitz&Danziger1983;Winkler&Kirshner1985)are adiscreteHIIregion.Nevertheless,givenwehavenoindependent chemically stratified, and show clear nitrogen overabundances estimateofthedistancesofthesecomponentswecannotcategori- in many of their filaments (Chevalier&Kirshner (1978, 1979); callydiscardalinkbetweenthisputativecompactHIIregionand MacAlpineetal.(1994,1996);Isenseeetal.2012). the surrounding main SNR shell. Additional information such as TheindividualspectroscopicdetailsofeachIPHAScandidate distance estimationsand thekinematics of both structures arere- aredescribedinthefollowingsubsections. quiredtofullyestablishanypossibleconnection. The information brought by the extinction data would posi- tion the filaments/structures related to pointing A and B at rela- 3.2 IPHASXJ190640.5+042819 tivelyclosedistancefromeachotherwithrespectivec(Hβ)values of0.82and0.86.PointingC,withc(Hβ)=1.26,wouldthenappear Forthiscandidate(refertotheimagesinFigure1)ournewoptical asaindependentelementbutwecannotdiscardalinktoIPHASX spectrumgivesratiosof[NII]/Hα=1.48(eliminatinganypossible J195744.9+305306sincepointings‘B’and‘C’sharethesameSNR confusion with a HII region) and [SII]/Hα=1.15, clearly placing spectroscopicclassification. theionisedgasintheshockregime.Placingtheseemissionlinera- InsummarySNRcandidateIPHASXJ195744.9+305306falls tiosintheF13andF85diagnosticdiagrams, indicatesthesource inageneralareaofHIIregionsincludingaprobablycompact HII liessquarelyintheregionoccupiedbySNRs.Theforbiddenoxy- region(pointingA)superposedonthemainSNRshell.HIIregions gen emission lines[OI]λλ6300,6364 which arerelatively strong projectedonSNRsarenotunexpectedandourcombinedimaging in SNR (contrary to HII regions and PNe) are also present and andspectroscopicdatastillprovidestrongevidencethatIPHASX were measured with extinction-corrected fluxes of 34.1 and 34.5 J195744.9+305306(G067.6+0.9)isatrueSNR. respectively(forHβ=100andHα=286).The[OI]λ6300lineflux shouldbe3×the[OI]λ6364fluxaccordingtothetheoreticalratio of Einstein’sA coefficients. Theobserved values betray sky sub- 3.4 IPHASXJ200002.4+305035 tractionproblemsduetothepresenceofthesameoxygenlinesin thesky-backgroundthatmakesthemdifficulttoextractproperlyas The spectral characteristics exhibited from pointing D (object thenebularemissionfilledmostoftheslit.Althoughweproceeded shown in Figure 3) including from its location in the F13 dia- with great care when removing the sky, there is clearly signifi- gram,andaquitehighbutnotconclusive[SII]/Hαratio,thatthis cant uncertainty inthefluxmeasurement of theselines. IPHASX apparently coherent, oval nebulae may be an additional compact J190640.5+042819 is our first confirmed SNR which can then SNRwhichwetentativelydenoteasIPHASXJ200002.4+305035 be named according to the standard Galactic SNR nomenclature orG067.8+0.5. It islocated 5′ totheEastof themoreextensive (GD.d±D.d)asSNRG038.7-1.3. nebulosity linked to IPHASX J195744.9+305306 and is clearly detached from it. The spectrograph slit was placed E-W across the middle of the Western enhanced edge of the optical nebula. 3.3 IPHASXJ195744.9+305306 The absence of the Hβ line prevents us from deriving the ex- tinction. Note that it isunrelated tothe nearby compact (FWHM The large projected optical angular extent of IPHASX ∼1.62 arcseconds) PN candidate IPHAS J195956.42+304823.8 J195744.9+305306 (∼24.6′×22.6′- see Figure 2) led us to (2MASSJ19595642+3048238) reportedbyViironenetal.(2009). obtain several different pointings during the spectroscopic obser- IPHASXJ200002.4+305035 alsoshowsthehighestelectronden- vations so as to obtain a more representative sample of spectra sityofallthetargetswithne−=620±74cm−3whichmaybeanin- across different possible components of this large source. Those dicationofyouth.Furtherstudyofthisemissionnebulaisrequired arenotedasA,BandCinthetoppanelofFigure2.Thereisan toclarifyitsnaturebutanSNRidentificationisnotunwarrantedat additional pointing D which was made on the faint, oval shaped, thisstagefromtheopticalimageryandspectroscopyalone. apparently detached nebula ∼5′ to the east of the main structure andapproximately8′×6′ indiameter(Figure3).Thisturnsoutto beaninterestingsourceinitsownrightandanotherpossibleSNR 3.5 IPHASXJ195749.2+290259 candidate(seebelow). Figure7indicatesthatthespectrafrompointingsBandpar- ThespectroscopicanalysisofthiscandidateshowninFigure4,in- ticularlyCprovidelineratioscompatiblewithanSNRoriginwith dicatesalikelySNRidentification(inthiscasethenebulaisnamed a clear signature of shock-heated emission with [SII]/Hα ratios G066.0-0.0) either using the diagnostic diagrams from F13 (Fig- of 0.57 and 0.99 respectively (Figure 8, Table 3)- see also the ure 7) or the conditions fixed by F85. The observed diagnostic diagram by Phillips&Cuesta (1999). Pointing B also shows the emissionlineratiosof[SII]/Hα=0.83(clearlyintheshockregime [OI]λλ6300,6364 emission lines with respective dereddened val- following F85), [NII]/Hα=1.9 (clearly eliminating a HII region 6 L. Sabinet al. identification)and[SII]6717/6731=1.4indicatinganelectronden- sityclosetothelowdensitylimit,allfitwellwithinthedefinition ofanSNRfollowingF85.Wenoticethat,similarlytoG067.6+0.9 (IPHASXJ195744.9+305306: pointingsAandC),wedidnotde- tectanyclear[OI]emission(seeFigure6). 3.6 IPHASXJ195920.4+283740 ThenebulapresentedinFigure5,iswellconstrainedspectroscop- ically to fall within the SNR regime using the criteria of F85 ( [SII]/Hα=0.90and[NII]/Hα=1.25)andF13(seeFigure7).How- ever, anew heliumrichemission point source possiblyrelatedto a WR star has been identified close to the enhanced bow-shaped easternemissioncomponentbyCorradietal.(2010),anddenoted asIPHASJ195935.55+283830.3.Possibleassociationofthenebu- laewiththisunusualstarwouldthrowdoubtonitsSNRnatureand mightimplythattheshellresults,atleastpartly,toapastejection from a massive star. We note that the elongated and asymmetric filamentarymorphology ofIPHASXJ195920.4+283740 doesnot show the ring-like structure(s) generally displayed by WR nebu- lae(Stock&Barlow2010,Marston1995),whichlendssomesup- port to an SNR classification. However, the IPHAS nebula does not display the typical SNR filamentary morphology either. The SMBlog(Hα/[NII])versuslog(Hα/[SII])diagnosticdiagramaf- terFrew&Parker(2010)isusefulindiscriminatingSNRandWR shellemission-linesignaturesastheytendtooccupydifferentloci inthediagram.InthecaseofIPHASXJ195920.4+283740,theob- ject plots well inside the SNR area and away from the WR shell / HII region domain. Nevertheless, the presence of a helium rich starsoclose tothe enhanced easternedge of thenebula issuspi- cioussuchthatanon-SNRorigincannotbecompletelyruledout, despitetheindicativeemission-lineratiosandcorroboratingradio signatures(seelater). Figure1.Thetoppanelpresentsabinned(5arcsecond/pixels)Hαimageof 4 MULTI-WAVELENGTHDATA IPHASXJ190640.5+042819fromtheIPHASmosaicsoverlaidwith6cm contoursfromthe87GBsurvey.Contoursgofrom0.01to0.14Jy/beam 4.1 G038.7-1.3–IPHASXJ190640.5+042819 inintervalsof0.02Jy/beam.ThereisanexcellentmatchbetweentheHα andtheradiodata.Thepeakfluxhas90mJy.Thebottomimagepresents TheregionofskycontainingthisSNRwascoveredbytworadio theNVSS1.4GHz(21cm)radio image. Thepositional matchwithHα telescopes, the 64 m Parkes telescope as part of the PMN 6 cm andthe6cmemissionisobviousontheeasternside.Duetotheimproved survey(Griffith&Wright1993)andthe91mNRAOGreenBank resolutionof∼45arcseconds,thewholeSNRshell(delimitedbythewhite telescope and included within the 87 GB survey (Condonetal. arrows)canbenoticedwhichenables anupdatedSNRIDasG038.7-1.3 1989). Radio emission is clearly seen and follows quite well the basedonthegeometric centre oftheshell.TheNVSSradio contours go opticalnebularmorphologyasshowninthetoppanelofFigure1. from0.001to0.05Jy/beaminintervalsof0.005Jy/beam.Thepositionof TheyellowcontoursoverlaytheHαgreyscaleimageandshowthe thespectrographslitisindicated. 4.85GHz(6cm)radiofluxfromthe87GBsurveyastheresolu- tionofthistelescopeatthisfrequency(∼3.5′)issomewhatbetter thaninPMNsurvey(∼5′).Theradiodataseemstoindicateamuch from0.001to0.05Jy/beaminintervalsof0.005Jy/beam.Theinte- largerextenttotheSNRthanisevidentfromtheopticaldataalone. gratedfluxforthisregionprovidesvaluesof90mJyfor87GBand Notethat thisradioemission had not been previously recognised 109.2mJy and 119.9 mJy (21 cm) for the two NVSS frequencies as being part of any coherent structure so we make the associa- available.Unfortunatelyitisproblematictoconcludeifthesemore tionhereforthefirsttime.Theradiocontoursgofrom0.01to0.14 compactNVSSemissionpeaksarerelatedtotheSNRorarefrom Jy/beaminintervalsof0.02Jy/beam. aseparateGalacticmJysourceorevenabackgroundextragalactic TheSNRisalsoseenat1.4GHz(21cm)withintheNVSSsur- double-lobedradiosource.Evenifthisdouble-peakedsourceisun- vey(Condonetal.1998)andshowninthebottompanelofFigure1. relatedtotheSNRtheNVSSimagestillshowsacomplete,though Comparison with the 87 GB and PMN data shows that both the fragmentedSNRshellof∼25′indiameter.Arrowsareaddedtothe 6cmand21cmsurveysexhibitstrongemissionpeak(s)atthesame figuretofacilitaterecognition.ThisenablesanupdatedSNRIDof location(RA=19h07m20s,DEC=4o32’).However,theNVSSim- G038.7-1.3basedonthenewgeometriccentreoftheshell(notewe age has much better resolution (45 arcseconds) and resolves two retaintheoriginalIPHASopticalIDbuttheRA/DECoftheshell compactemissionpeaksatthesamelocation.Theradiocontoursgo centre is approximately 19h 06m 35s +04o 35’ 00”). Due to the IPHAS Galacticsupernovaremnants 7 C A B Figure 3. Top panel: The Hα-r image mosaic of IPHASX J200002.4+305035 with 15× pixel binning overlaid with NVSS con- tours in green ranging from 0.0015 to 0.004 Jy/beam. The E-W slit locationofthis‘D’pointingwasplacedmid-wayuptheWesternenhanced edge of the shell. This detached nebula is at ∼5′ East of IPHASX J195744.9+305306. Wenotethatthe NVSS1.4GHzemission ismainly coincident with the bright optical interacting front on the West. In the bottompanelthe87GB6cmemission,overlaidasyellowcontoursranging from0.015to0.05Jy/beam,isdistributedoverthewholenebulaincluding out to the faint opposing optical emission rim directly to the West. The Figure 2. Top panel: The Hα-r image mosaic of IPHASX E-Wslitpositionisindicated. J195744.9+305306 with 15×pixel binning overlaid with NVSS radio contours ingreen ranging from0.0008to0.0027Jy/beam. The redE-W barsindicatethelocationoftheslitsforthedifferentspectralpointingsA,B andCwithlengthsof5′.Thenebulaisinagenerallycomplexenvironment whichmakesithardertodefinethereallimits ofthenewSNRinHαat theNorthernfainterrim(sampledbyslitC).Thediagonaldarkstreakisan imageartefact(possiblyanasteroidtrail).Bottompanel:Asabovebutnow ferent frequencies) would be another clear proof to support their withthelowerresolution6cm87GBradiosurveydataoverlaidinyellow SNRstatus.Howeverthiswasnot possiblewiththeexistingdata contours whichrange from0.004to0.02Jy/beam. Asinthe caseofthe due tothenon-compatibility between the radiosurveys (different 21cmNVSSdatathe6cm87GBradioemissionfollowstheedgesofthe resolutions/beam sizes) and because of the generally fragmented SNRveryclosely andespecially alongtheSouthEastpartofthenebula natureoftheremnants.Newanddeeperradioobservationsarere- wheretheopticalemissionisstrongest.Theoptical-radiooverlapisnotas quiredtoobtaindecentintegratedradiofluxesfromwhichreliable completeasinthecaseoftheSNRIPHASXJ190640.5+042819. spectralindicescouldbeobtainedandhencetodemonstratetheex- istenceofnon-thermalradioemission.Thisisbeyondthescopeof lowresolutionat6cmfromthe87GBsurveywecouldnotmatch thispaper. emission withthe NVSS shell at 21 cm apart from at the South- AsingleXraysourceisfoundwithintheconfinesoftheSNR, Eastedge.WealsonotethatReichetal.(1984)detectedonlyone ROSAT1RXSJ190709.4+043100. Similarlytopulsars,aconnec- source(similartothe6cm87GBsurveydata)attheSNRlocation tioncouldbemadeviathelocalisationofX-raysourcesinthecen- usingtheEffelsberg100mdishwitharesolution∼4.3′. tralpartoftheSNRbutthissourceisnotlocatedclosetothegeo- Weemphasize, andthisisalsovalidfortheremainingradio metriccentremakinganydirectlinkunlikely.Furthermore,thereis analysisoftheotherobjects,thatmeasuringavalidspectralindex noindicationofanyextendedX-rayemissioninternaltotheshell forourSNRcandidates(i.e.obtainingreliableradiofluxes atdif- whichcouldindicateaPlerion. 8 L. Sabinet al. Figure 4. Image of new IPHAS SNR IPHASX J195749.2+290259. We showtheopticalIPHASHαimageoverlaidwithyellow6cm87GBcon- tourswhichrangefrom0.0014to0.14Jy/beam.Thereisaclearcorrespon- dence between the optical and radio emission particularly on the North- Westrim.Inthiscasewedidnotfindanyclearlyassociated1.4GHzemis- sion. AgaintheE-Wslitpositionismarked. 4.2 G067.6+0.9–IPHASXJ195744.9+305306 ThetoppanelinFigure2showstheHα-rimagemosaicofIPHASX Figure 5. Top panel: Full resolution Hα mosaic of potential new SNR J195744.9+305306with15×pixelbinningoverlaidwithNVSSra- IPHASX J195920.4+283740 with the 21 cm NVSS (1.4 GHz) contours diocontoursrangingfrom0.0008to0.0027Jy/beam.Thishigher emissionoverlaid.Thebottompanel,asabovewithnowthe6cm87GB resolutionNVSS21cmemissiondirectlyandclearlyoverlapswith radioimage(4.85GHz)contoursemissionoverlaid.Thosetwomapsindi- the strongest optical emission to the SSE. The complex environ- catesthatwhiletheeasternenhancededgeismappedclearlybythe1.4GHz emission,the4.85GHzradioemissionisperfectlycoincidentwiththefull mentsurroundingthenebulamakesithardertodefinethereallim- opticalimagewhichmaypartiallyreflectthelowerresolutionofthe87GB its of the new SNR in Hα. The red E-W bars indicate the loca- data.Aheliumenrichedemissionlinestarislocatedclosebutinternalto tionoftheslitsforthedifferentspectralpointingsA,BandCwith theeasternarcofthenebulae. Spectrograph slitposition markedE-Was lengthsof5′.ThebottompanelofFigure2isthesameexceptwith before. the6cm87GBradioemissionoverlaidascontoursrangingfrom 0.004to0.02Jy/beam.Heretheradioemissionappearscoherently distributedalongtheSouthernrimofIPHASXJ195744.9+305306. onagrey-scale15×pixelbinnedIPHAS25×20′Hαimageinthe Therearealsosomesmaller,sparseemissionblobsoutsidetheneb- toppanelofFigure3duetoitshigherresolution.Thebottompanel ula which are probably unrelated. Detection of radio emission at isasabove with87 GB data overlaidas yellow contours ranging two frequencies across components of the optical emission lends from0.015to0.05Jy/beam. Whilethe6cm87GBemissionap- strongsupporttoalikelySNRidentificationwhencombinedwith pearstoentirelycovertheopticalnebulaincludingaweakexten- theindicativespectroscopic signatures.Agroup ofX-raysources siontotheEasternside,thehigherresolutionNVSSdataismore has been identified within the boundaries of the proposed new localizedandcoincidesmostcloselywiththeWesternenhancement SNR. Except for one located in the North-East (detected with oftheovalshell(apossiblebow-shock?). the ROSAT all-sky survey catalogue of optically bright OB-type stars,Berghoeferetal.1996)andrelatedtothedouble/multiplestar HR7640, themajorityoftheseX-raysourcesareconcentrated at 4.4 G066.0-0.0–IPHASXJ195749.2+290259 ∼11.5′WestfromthegeometriccentreoftheSNRandaremostly A multi-wavelength investigation of archival data reveals radio ROSATbrightsources(Vogesetal.1999)unrelatedtotheSNR. emissionat6cmfromthe87BGsurvey.Figure4showstheIPHAS Hαgrey-scaleimageoverlaidwithyellow6cmcontoursranging from0.0014to0.14Jy/beamthatappearstofollowtheedgesofthe 4.3 G067.8+0.5–IPHASXJ200002.4+305035 SNRespeciallytotheNorth–EastandNorth–Westthoughitisalso ThepossiblenewSNRPHASXJ200002.4+305035hasclearradio somewhatfragmented.Aninterestingpointisthelocationonly≃ detectionsinboththeNVSSand87GB.WepresenttheNVSSdata 31′totheSouth–EastofanotherofournewIPHASidentifiedSNRs overlaidasgreencontoursrangingfrom0.0015to0.0040Jy/beam G065.8-0.5(IPHASXJ195920.4+283740)furtherdiscussedbelow. IPHAS Galacticsupernovaremnants 9 TheE-Wslitpositionofthespectroscopicobservationisindicated [NII]/Hα∼0.62.However,wedeterminethatthiscompactnebular bythewhitehorizontalbar. region,whichislocatedinternaltothemainnebulashellandtar- getedaspointing‘A’islikelytobeanunrelatedHIIregion.Thesec- ondisIPHASXJ2000002.4+305035.Inthiscasethespectrumstill 4.5 G065.8-0.5–IPHASXJ195920.4+283740 displaysan[NII]/Hα> 0.5whichisatypicalofHIIregions.The [OI]6300and6364A˚ emissionoftenseeninopticalremnantsare For this SNR candidate an excellent match between the known difficulttounambiguouslyidentifyinmostcasesduetolowspec- NVSSradiosourceNVSS195938+283832(27.0mJyat1.4GHz) tralresolutionsemployedmeaningthattheselinescannotbeeasily and thebright Easternenhanced arcuate rimof thispossible bow resolvedorsubtractedproperlyfromthestrongnightskylinesof shockwasfound(seetoppanel ofFigure5).Thereisalsoavery thesamespeciesduetothelackofsignificantlinevelocitydiffer- goodcoincidencebetweenthe4.85GHz6cm87GBradioemis- ence. Consequently, among the group of likely SNRs, only three sionand theentireoptical nebulaextent including thefainterop- show[OI]6300A˚ and/or[OI]6364A˚ withanycertaintythoughthe posingrimtotheWest(seebottompanelofFigure5).Whentaken lineintensitiesthemselvesareonlyindicativeandarenotrecorded togetherwiththeopticalemissionlineratios,whichplacetheneb- inthecanonical3:1ratio. ulaintheSNRregimeofthevariousdiagnosticdiagrams,anSNR identification seems likely. However, the situation is complicated It is also important to emphasise that shock emission is not by possibleassociation withahelium enriched emission-line star inherent solely to SNRs and can result from stellar winds and (Corradietal.2010)closetobutinternaltotheEasternarc traced interactions such as those produced by young-stellar objects and bytheopticalandNVSSdata.Moreinvestigationisneededtoas- some PNe and WR stars. However, in general WR shells do not certainthetruenatureofthisnebula. show strong [SII] emission, and largely plot in separate regions of Sabbadin,Minello&Bianchini (1977) (SMB) type diagnostic plots. In order to further corroborate our results we also per- 5 DISCUSSIONANDCONCLUSIONS formed a multi-wavelength investigation based on searches of Wereportthediscoveryoffourorpossiblyfivenew,likelySNRs online archives including all available radio data from the vari- followingasystematicandcarefulvisualsearchofthecontinuum ouscurrent on-line surveys. Radio emission, mainly at4.85 GHz removed Hα mosaics of the IPHAS survey over the 19-20 hour (6 cm) from the 87 GB radio survey, was found in all cases RA zone. These small-to-medium angular size Galactic nebulae with a decent positional and structural match to components of and SNR candidates (i.e. with major axes ranging from ∼6–50′) the optical morphology. In most cases there were also detections were selected primarily on the basis of theiroptical morphology. from the higher resolution 1.44 GHz (21 cm) NVSS survey data PlausiblecandidateSNRsneedtoexhibit coherent filamentaryor which provides clearer and more direct associations. These radio shell-like characteristics which are common features of currently matchesareparticularlygoodinthecasesofG038.7-1.3(IPHASX knownopticallyidentifiedSNRs.Therelativelysmallangularsize J190640.5+042819), G065.8-0.5 (IPHASX J195920.4+283740) of these objects compared with the degree-size SNRs found for andG067.8+0.5(IPHASXJ200002.4+305035)whichsupportsthe many Galactic remnants (e.g. referto on-line catalogue of Green SNR nature of those objects. The remaining two SNRs display – http://www.mrao.cam.ac.uk/surveys/snrs/snrs.data.html) implies morediscontinuous radioemissionbutstillwithcomponents that thatthesenewlydiscoverednebulaecouldbemoredistantorper- follow the most intense regions of optical emission. Full overlap haps younger, assuming they have not had the time to expand to betweentheradioandopticaldataisnotanecessaryconditionto the dissipation phase. In the latter case a connection between an demonstrate association, particularly in the case of faint nebulae SNRand ayoung pulsar might be realisedif they originatefrom andpossiblelocaldustanddensityvariationscanaffectthevisibil- core-collapseevents. ityofopticalemissionacrossthetrueextentoftheSNR. Ourspectroscopicfollow-upofthesecandidatesallowedusto Another source of confusion with possible SNR identifica- identifythemallaslikelySNRsusingthestandarddiagnosticemis- tions of optical nebulae resides in the diffuse ionised gas (DIG) sionlineratiosdefinedbyFesenetal.(1985)andthenewandmore emission (or warm interstellar medium, WIM) as its morphol- comprehensive diagrams presented firstby Frew&Parker(2010) ogy can be confused with our equally faint SNR candidates. andwithupdatedandimprovedversionsofthesefigurespresented Since the DIG is generally photo-ionised, as are HII regions hereforthefirsttime.Aprobleminthistypeofinvestigationlies (Domgorgen&Mathis 1994), the separation of the DIG with inthegenerally faintoptical emissionandlow surfacebrightness shock-ionised objects such as SNRscan be easilydone from our ofsuchevolvedSNRswhichmakestheirspectroscopicinvestiga- opticalspectroscopy usingthelimitingconditions forbothstates: tiondifficulton2mclasstelescopes.Nevertheless,inthisprelim- [SII]λλ6716,6731/Hα ≃ 0.4 and [NII]λλ6548,6583/Hα ≃ 0.5 inarystudy,theprimarycombinationofopticalimageryandspec- (seeforexampleGalarzaetal.1999,theirfigure8).Asmentioned troscopy and their locations in the emission line diagnostic plots previously,ourline-ratiosclearlyindicatethatexceptforregion‘A’ lendstrongsupporttotheSNRoriginoftheexaminedcandidates. in IPHASX J195744.9+305306 (which is indeed likely an unre- In several cases this has been further corroborated by the recog- latedHIIregion)andtheambiguousIPHASXJ2000002.4+305035, nitionofpreviouslyun-cataloguedradioemissionusuallyatmore all the targets show shock-ionisation characteristics of SNRs ( thanonefrequencyfromarchivaldatawhichisclearlyassociated [SII]/Hα>∼0.4and[NII]/Hα>∼0.5.ADIGclassificationcan withtheopticalnebulosities. thereforebeconfidentlydiscardedforthesesources. ThespectroscopicdatashowtypicalSNRdiagnosticlinesex- Figure 6 and Table 3 indicate a relatively strong hibitedbyallcandidatesandindicatesthepresenceofshockswith [OIII]5007A˚ which is known to be a good indicator of higher [SII]/Hαlineratiosgreaterthan0.4–0.5.Theobserved[NII]/Hα shockspeed.Raymond(1979)showedthatthepresenceofthisline lineratiosarealsogenerallyhigh(> 1.2)whichruleoutanypos- iscoincidentwithshockspeedgreaterthan80km/s.Thistrendis sible confusion with HII regions. There are two exceptions. The confirmedwhencomparingourresultswiththetheoreticalmodels firstisforpointing‘A’ofIPHASXJ195744.9+305306 whichhas by Allenetal. (2008) which indicate shock speeds between 200 10 L. Sabinet al. Figure6.BlueandredspectralregionsofinterestforthefiveIPHAStargets.Themainemissionlinesarelabelledandthefluxesarenormalized.Thespectra havebeencroppedforabetterdisplayofthenebularlines buttheareaofthegapisemptyoflinesorareinsidethenoiselevel.WerecallthatIPHASX J200002.4+305035alsodesignatesthepointingD. and1000 km/sintheSNRwhere[OIII]5007A˚is“well”detected. tificationsinthefuture.Consideringthereturnfromasingle1-hour This does not apply to the C slit-position of J195744+305306 IPHASRAzonewemightexpectdozensofnewGalacticSNRcan- wherethe[OIII]5007A˚ lineishardlydetectableandmostprobably didatestobeuncoveredbasedontheiropticalemissionsignatures. mixed withnoise. Those high values would help to constrain the Thiswouldaddsignificantlytotheknown Galacticpopulationof evolutionary phase of the SNR observed, as although we are not evolvedSNRs. dealingwithSNRsintheirultimatephaseofdissipationintheISM wearenotdealingwithyoungoneseither. ACKNOWLEDGEMENTS OurresultsonthefirstSNRsidentifiedfromIPHASarepre- liminary, and each nebula warrantsadditional follow-up. Ouron- WethankboththeanonymousrefereeandP.Boumisforvaluable goingsearchfornewSNRcandidatesisbasedoncarefulandsys- comments and suggestions that have contributed to significantly tematic examination of the IPHAS survey image mosaics of the improvingthequalityofthepaper.Thisprojecthasbeensupported GalacticplaneandishighlylikelytoprovidemanymoreSNRiden- byPAPIIT-UNAMgrantIN109509.Thispapermakesuseofdata