Astronomy&Astrophysicsmanuscriptno.jiang_hc3n_v11 cESO2017 (cid:13) January3,2017 ∗ ⋆ HC N Observations of Nearby Galaxies 3 (ResearchNote) Xue-JianJiang1,5,Jun-ZhiWang2,YuGao1,5,andQiu-ShengGu3,4,5 1 Purple Mountain Observatory & Key Laboratory for Radio Astronomy, Chinese Academy of Sciences, 2 West Beijing Road, Nanjing210008,China;Email:[email protected] 2 ShanghaiAstronomicalObservatory,ChineseAcademyofSciences,80NandanRoad,Shanghai200030,China 7 1 3 SchoolofAstronomyandSpaceSciences,NanjingUniversity,Nanjing210093,China 0 2 4 KeyLaboratoryofModernAstronomyandAstrophysics(NanjingUniversity),MinistryofEducation,Nanjing210093,China n 5 CollaborativeInnovationCenterofModernAstronomyandSpaceExploration,Nanjing210093,China a J 2 Received 2016monthday;accepted 2016 monthday ] A ABSTRACT G Aims.WeaimtosystematicallystudythepropertiesofthedifferenttransitionsofthedensemoleculargastracerHC Ningalaxies. 3 . Methods. We have conducted single-dish observations of HC N emission lines towards a sample of nearby gas-rich galaxies. h 3 HC N(J=2-1) wasobserved in 20 galaxies withEffelsberg 100-m telescope. HC N(J=24-23) was observed in nine galaxies with p 3 3 the10-mSubmillimeterTelescope(SMT). - o Results.HC3N2-1isdetectedinthreegalaxies:IC342,M66andNGC660(> 3σ).HC3N24-23isdetectedinthreegalaxies:IC r 342,NGC1068andIC694.ThisisthefirstmeasurementsofHC3N2-1inarelativelylargesampleofexternalgalaxies,althoughthe t detectionrateislow.FortheHC N2-1non-detections,upperlimits(2σ)arederivedforeachgalaxy,andstackingthenon-detections s 3 a isattemptedtorecovertheweaksignalofHC3N.ButthestackedspectrumdoesnotshowanysignificantsignsofHC3N2-1emission. [ TheresultsarealsocomparedwithothertransitionsofHC3Nobservedingalaxies. Conclusions.ThelowdetectionrateofbothtransitionssuggestslowabundanceofHC3Ningalaxies,whichisconsistentwithother 1 observational studies.Thecomparison betweenHC NandHCNorHCO+showsalargediversityintheratiosbetweenHC Nand 3 3 v HCNorHCO+.MoreobservationsareneededtointerpretthebehaviorofHC Nindifferenttypesofgalaxies. 3 2 1 Keywords. galaxies:active–galaxies:ISM–galaxies:evolution–ISM:molecules 3 0 0 1. Introduction asphotodissociationregions(PDRs)dominatedbyyoungmas- . 1 sivestars, X-raysdominatedregions(XDRs) inducedbyactive Molecular lines play an essential role in our understanding 0 galacticnucleus(AGNs),andshockwavesbycloud-cloudcolli- of star formation activity and galaxy evolution. With molecu- 7 sions(Aladroetal. 2011;Greveetal. 2009;Aladroetal. 2011; 1lar lines of different species and their different transitions, not Costagliolaetal.2011;Vitietal.2014). :only the chemical composition of the interstellar medium can v be investigated, but other important physical parameters, such i Oneoftheinterstellarspeciesthatbenefitsfromtheupgraded Xas temperature, pressure, density, and non-collisional pumping facilities is cyanoacetylene(HC N). HC N was firstly detected 3 3 mechanism can be derived as well (e.g., Henkeletal. 1991; r in 1971 at 9.0977 GHz (J=2-1) in the Galactic star forming aEvans 1999; Fukui&Kawamura 2010; Meier&Turner 2012; region Sgr B2 (Turner 1971). The critical density of HC N is 3 Meieretal. 2014). New facilities providing wide band and comparable to the widely-used dense gas tracer HCN and can highly sensitive instruments are making weak line surveysand alsotracedensemoleculargasaroundstarformingsites.HC N 3 multi-speciesanalysisfeasible,andthedetectionsandmeasure- has been detected in many star formation regions in the Milky ments of a variety of species, are helping us to reveal the gas Way with severaltransitions from centimeter to sub-millimeter components of galaxies, and how their abundances, densities, (e.g., Suzukietal. 1992). Due to the small rotational constant ratiosreflectthe radiativepropertiesofgalaxies.Multi-species, ( 1/13 of CO), there are many closely spaced rotational tran- multi-transition molecular lines can be combined to diagnose ∼ sitionsofHC N(separatedbyonly9.1GHz)atcentimeterand theevolutionstageofgalaxies(Baanetal.2014),becausediffer- 3 millimeter wavelengths, and its levels are very sensitive to the entspeciesaresensitivetodifferentphysicalenvironments,such changesinexcitation(Meier&Turner2012).Thismakesiteas- ⋆ Based on observations with the 100-m telescope of the MPIfR ierto conductmulti-transitionobservationsofHC3N linesthan (Max-Planck-InstitutfürRadioastronomie)atEffelsberg,andtheSub- other dense molecular gas tracers, and can help better under- millimeterTelescope(SMT).TheSMTisoperatedbytheArizonaRa- stand the excitationconditionsof star forming regions.In con- dioObservatory(ARO),StewardObservatory,UniversityofArizona. trast,thehigh-J linesofotherdensemoleculargastracerssuch Articlenumber,page1of8 A&Aproofs:manuscriptno.jiang_hc3n_v11 asHCNandHCO+ areatveryhighfrequencies,thusitisdiffi- 2.2.HCN24-23observationswiththeSMT10-m 3 culttoobservethemwithground-basedtelescopes.Anotherad- HC N (J = 24 23) (ν = 218.324 GHz) of nine galaxies vantageofusingHC NlinesisthatHC Nisverylikelyoptical 3 rest 3 3 − wasobservedin2009withtheSMT10-mtelescope.TheHPBW thin even in low-J transitions, due to the relatively low abun- is about33 at 218 GHz for SMT, and a single pointingwas dance (Irvineetal. 1987; Lindbergetal. 2011). And low opac- ′′ ∼ usedforeachgalaxytowardtheircentralpositions.Weusedthe ity is important for accurate estimate of dense molecular gas ALMASidebandSeparatingReceiverandtheAcousto-Optical- massforthestudyofrelationshipbetweendensemoleculargas Spectrometers (AOS), which have dual polarization, 970 MHz andstarformation(Gao&Solomon2004a,b;Wangetal.2011; ( 1300 km s 1) bandwidth and 934kHz channelspacing. Ob- Zhangetal.2014). − ∼ servationswerecarriedoutwiththebeam-switchingmodewith Therehave beeneffortsto detectHC N in nearbygalaxies, 3 achopthrowof2 inazimuth(AZ)andachoppingfrequencyof mainly in millimeter band. Observations suggest that HC N is ′ 3 2.2Hz.Pointingandfocuswerecheckedabouteverytwohours related to the warm, dense star forming gas, and is easily dis- by measuring nearby QSOs with strong millimeter continuum sociated by UV radiation(Henkeletal. 1988; Costagliolaetal. emission. The typicalsystem temperatureat 218GHz was less 2011;Costagliolaetal.2011;Lindbergetal.2011;Aladroetal. than300K, and the on-sourcetime foreach galaxywas 60– 2011, Aladroetal. 2015). HC3N was found to be unusually 168minutes. ∼ luminous in NGC 4418, and it is attributed to its high abun- dance (10 7) as well as the intense radiation field in the − dense and warm gas at the center of NGC 4418 (Aaltoetal. 2.3.DataReduction 2007;Costagliola&Aalto2010).Meier&Turner(2005,2012); Meieretal.(2011,2014)presentedhighresolutionobservations The basic parameters of our sample galaxies are listed in Ta- of HC N (J=5-4,10-9,12-11and16-15)ofa few verynearby ble1. The data were reduced with the CLASS program of the 3 galaxies, and gave detailed analysis of the galactic structures GILDAS1 package. First, we checked each spectrum and dis- andmorphologythattracedbyHC Nandotherdensegastrac- cardedthespectrawithunstablebaseline.MostoftheEffelsberg 3 ers(HNC, HCN, CS, etc.).However,these resultsare still lim- spectrado nothave flatbaselines, butoverseveralhundredkm ited bytheir samplesize, andthe chemicalprocessof HC3N is s−1 near the line the baselines can still be fixed. In the SMT stillunclear.Largersamplesarestillnecessaryforanalyzingthe spectra the image signal of strong CO 2-1 in the upper side- propertiesof HC N and how it relate to othergalactic parame- bandaffectthebaselineofthelowersidebandandforM82and 3 ters.InthispaperwepresentthefirstsystematicsurveyofHC3N Arp220theHC3N24-23iscontaminated.Butforothergalaxies (J =2 1)andHC3N(J =24 23)inarelativelylargesample theimageCOlinedoesnotaffecttheHC3Nline.Thenwecom- of near−by galaxies. And the re−sults are compared with the ob- binedspectrawithbothpolarizationsofthesamesourceintoone servationsofHC Ninothertransitions.Thecriticaldensitiesof spectrum.Dependingon the qualityof the spectralbaselines, a 3 HC NJ =2 1andHC NJ =24 23areabout3 103cm 3 first-orderorsecond-orderfittingwasusedtosubtractbaselines 3 3 − and 4 106−cm 3, respectively. A−nd the upper sta×te energies from all averaged spectra. The identifications of the transition − (Eu) of×the two transitions are 1.3 K and 131 K, respectively frequenciesofHC3NhavemadeuseoftheNISTdatabaseRec- (Costagliola&Aalto2010). ommendedRest Frequenciesfor Observed Interstellar Molecu- larMicrowaveTransitions2. To reduce the noise level, the spectra are smoothed to ve- locity resolutions 20 40 km s 1. The velocity-integrated − 2. Observations&Datareduction intensities of the H∼C N l−ine are derived from the Gaussian fit 3 of the spectra, or integrated over a defined window if the line We select nearby infrared bright galaxies (Sandersetal. 2003) profiles significantly deviate from a Gaussian. The intensities withIRAS60µmfluxgreaterthan30Jyanddeclinationgreater than 21 todothissurvey.Itisnotacompletebutrepresenta- arecalculatedusing I = R Tmbdv, whereTmb isthe main beam − ◦ brightnesstemperature.Molecularlineintensityinantennatem- tive sample of infrared bright galaxies. The sample consists of perature (T ) is converted to main beam temperature T via 21galaxies.Notethatduetothe differentbeamsize ofthetwo A∗ mb T = T /MBE,withthemainbeamefficiencyMBE=53%at telescopesweused,themergerArp299(IC694andNGC3690) mb A∗ 18GHzforEffelsbergtelescope,and70%at218GHzforSMT were observedas a single pointing by Effelsberg100-m, while during the observations. The flux density is then derived from thetwogalaxieswereobservedseparatelybytheSMT10-m. T ,usingS/T =0.59Jy/KfortheEffelsbergtelescope,and mb mb 24.6Jy/KfortheSMT. 2.1.HCN2-1observationswiththeEffelsberg100-m 3 HC N(J = 2 1)(ν =18.196GHz)of20galaxieswasob- 3. Results&Discussion 3 rest − servedwithEffelsberg100-mtelescopein2010.TheHalfPower Beam Width (HPBW) is 46.5 at 18 GHz for the 100-m tele- The spectral measurements and estimated intensities of the ′′ scope. We used the 1.9 cm band receiver,500 MHz bandwidth HC3Nlines,includingRMSnoiseandon-sourcetime,arelisted with 16384 channels correlator setup, which provided 8300 inTable2(HC3N2-1)andTable3(HC3N24-23). km s 1 velocity coverageand 0.5 km s 1 velocity res∼olution − − ∼ during the observations. Position-switching mode with beam- 3.1.HCN2-1 throwsofabout 2 wasused.Pointingandfocuswerechecked 3 ′ ± about every two hours. The typical system temperature of the Amongthe20galaxiesobservedbyEffelsberg100-mtelescope, Effelsbergobservationswasabout46K.Theon-sourcetimefor HC N 2-1 is detected in three galaxies: IC 342, NGC 660 and 3 eachgalaxyisabout14–47minutes.Theweatherduringtheob- servationsisnotideal,andthebaselinesofmanysourcesareaf- 1 http://iram.fr/IRAMFR/GILDAS/ fectedandinducedartificialfeatureswhicharehardtoremove. 2 http://www.nist.gov/pml/data/micro/index.cfm Articlenumber,page2of8 Jiang,Wang,Gao&Gu:HC N observationsofNearbyGalaxies(RN) 3 Table1.Thesourcelistofthe21galaxiesobservedonHC N emissionusingtheEffelsberg100mandtheSMTtelescope.Thecolumnsare:(1) 3 galaxyname,(2)and(3)coordinates,(4)Heliocentricvelocities,(5)distances,(6)totalinfraredluminosities(fromSandersetal.2003),(7)CO 1-0linewidth(FWHM)ofthegalaxies,and(8)thetelescopethesegalaxieswereobservedwith. Galaxy RA(J2000)Dec V Distance logL ∆V Telescope Helio IR CO h m s (kms 1) (Mpc) (L ) (kms 1) ◦ ′ ′′ − − (1) (2) (3) (4) (5) (6⊙) (7) (8) NGC520 012434.9 +034730.0 2281 30.22 10.91 270 Effelsberg NGC660 014302.4 +133842.0 850 12.33 10.49 280 Effelsberg NGC891 022233.4 +422057.0 528 8.57 10.27 110 Effelsberg NGC972 023413.4 +291841.0 1543 20.65 10.67 220 Effelsberg NGC1068 024241.4 000045.0 1137 13.7 11.27 280 Effelsberg&SMT IC342 034648.5 −+680546.0 31 4.60 10.17 72.8a Effelsberg&SMT UGC2855 034820.7 +700758.0 1200 19.46 10.75 ... Effelsberg UGC2866 035014.9 +700540.9 1232 20.06 10.68 ... Effelsberg NGC1569 043049.0 +645053.0 104 4.60 9.49 90 Effelsberg − NGC2146 061839.8 +782125.0 882 16.47 11.07 320 Effelsberg&SMT NGC2403 073651.3 +653629.9 161 3.22 9.19 90 Effelsberg M82 095553.1 +694041.0 187 3.63 10.77 150 Effelsberg&SMT NGC3079 100157.8 +554047.0 1116 18.19 10.73 380 Effelsberg NGC3310 103845.9 +533012.0 993 19.81 10.61 140 Effelsberg M66 112015.0 +125930.0 727 10.04 10.38 180 Effelsberg IC694 112833.8 +583345.0 3120 47.74 11.63 250 Effelsbergb&SMT NGC3690 112830.8 +583343.0 3120 47.74 11.32 260 Effelsbergb&SMT Mrk231 125614.2 +565225.0 12600 171.84 12.51 167 Effelsberg Arp220 153457.1 +233010.0 5352 79.90 12.21 360 Effelsberg&SMT NGC6240 165258.9 +022403.0 7160 103.86 11.85 420 SMT NGC6946 203452.6 +600912.0 53 5.32 10.16 130 Effelsberg&SMT Notes.(a) forIC342,the∆V isofCO2-1fromGao&Solomon(2004a);forothergalaxies∆V areofCO1-0fromYoungetal.(1995).(b)The Arp299system(IC694andNGC3690)wasobservedasasinglepointingbytheEffelsberg. M66(SeeFigure1).ThisisthefirstreportofHC N2-1detec- rive upper limits of the integrated intensity for each galaxy (2 3 tionsin externalgalaxies,althoughlimited bythe SNR (Signal σ,whereσ =RMS √δV ∆V)andshowtheminTable2.Note toNoiseRatio)thedetectionrateislow. that the linewidth of HC N· is likely narrower than that of CO, 3 andsuchassumptionmightovereistimatetheupperlimitsofin- tegratedintensity,thusisonlyaroughestimate.Theupperlimits IC342: IC342hasthestrongestpeakintensity(Tmb 14mK) areintherangeof 0.3–1.2Kkms 1.Forthosenon-detection ∼ − ofHC N2-1inthesample,whichisabouttwicethestrengthas ∼ 3 galaxies, We also stack their spectra together, weighted by the theHC N(9-8)lineofIC342detectedbyIRAM30-mtelescope 3 RMSlevelofeachgalaxy,toexamifacumulatedsignalcanbe (Aladroetal.2011),whilethelinewidth(FWHM 60kms 1) − obtained(seeFigurer1).AlthoughtheRMSofthethestacking ∼ issimilartotheirresult. HC N2-1spectrumisreduceddownto0.66mK,wedonotsee 3 anysignsofemission(ataresolutionof30kms 1).Sincethese − galaxies have similar linewidth (100–400 km s 1), we can es- NGC660: The detected HC N 2-1 in NGC 660 hasa similar − 3 timate the stacked upperlimit assuming a linewidth of 200km linewidth(FWHM 294.7kms 1)to CO 1-0( 280kms 1). ∼ − ∼ − s 1,basedontheRMS(0.66mK)ofthestackedspectrum.Thus While the HC N survey by Lindbergetal. (2011) did not ob- − 3 the 2 σ upper limit of these galaxies is about 0.26 K km s 1. serveNGC660,its10-9and12-11transitionswerenotdetected − To eliminatethe possible effectinducedby differentlinewidths by Costagliolaetal. (2011). This difference in the detection of of galaxies, we also tried to group the non-detection galaxies HC N lines may imply that there is little warm and dense gas 3 basedon their CO linewidth. Galaxieswith CO FWHM (Table contentinNGC660,thusthehigh-JHC Nlinescannotbeex- 3 1) wider than 200 km s 1 are stacked as one group, and other cited. − galaxiesarestackedasanothergroup.Neithergroupshowsany signsofemission. M66: InM66HC N2-1isonlydetectedonabout2σlevel, 3 butthisisthefirsttentativedetectionofHC NinM66.Itwasnot 3 observedbyCostagliolaetal.(2011)norLindbergetal.(2011). 3.2.HCN24-23 3 non-detections: Duetothepoorquality(andprobablyinsuffi- cientintegrationtime)oftheHC N2-1data,16outof19galax- AmongtheninegalaxiesobservedbySMT,HC N(J=24-23)is 3 3 ieswerenotdetected.Assumingtheirlinewidthisapproximate detectedinthreegalaxies:IC342,NGC1068andIC694(Fig- to CO 1-0 linewidth (FWHM, from Youngetal. 1995), we de- ure2). Articlenumber,page3of8 A&Aproofs:manuscriptno.jiang_hc3n_v11 IC342: HC N 24-23 of IC 342 was previously detected and 24-23,andcannoteasilyestimatethefillingfactors.Alongwith 3 measured by Aladroetal. (2011), and our observation obtains thelargeuncertaintyoftheemissionintensitiesmeasurements,it consistentresult,althoughcomparingtotheirobservationwedo isdifficulttoestimatethebrightnesstemperatureofthesample. notdetectH COsimultaneously.Inourobservations,IC342is To better understand the excitation environment of HC N, 2 3 theonlygalaxydetectedinboth2-1and24-23transitions.The the effect of free-free and synchrotron emission near 18 GHz line center andwidth ofthe two transitionsare similar, consid- should be also taken into account, as they are more prominent eringobservationaluncertainties.Thismightimplythatthetwo than that in millimeter band that is dominated by dust thermal transitionshavesimilaremittingarea.Andtheratiobetweenthe emission.We detectHC N2-1linesinemissionandnotin ab- 3 integratedintensitiesofHC N24-23/HC N2-1isabout0.6. sorption, and this may be due to the fact that the beam filling 3 3 factoroftheHC Ngasishigherthantheradiocontinuum.Inthe 3 highresolutionradioobservationstowardsa few nearbygalax- NGC1068: In NGC 1068, the integrated intensity of HC N 3 ies(Tsaietal.2006),itisfoundthatcompactradiosourcescon- 24-23isabout2.0Kkms 1(inT ),whichisstrongerthanthat − mb tribute20%–30%ofthetotal2cm(15GHz)emissionfromthe of HC N 10-9 ( 1.1 K km s 1) reported by Costagliolaetal. 3 ∼ − centralkiloparsecofthesegalaxies.Incontrast,thedistribution (2011).Itmayimplythatthereissufficientwarmanddensegas, ofgaswithmoderatecriticaldensitysuchasHC N2-1islikely whichisabletoexcitethehightransitionHC N24-23line.Be- 3 3 morediffuse. sides, it could also be affected by the strong AGN signature of Comparingtootherdensemoleculargastracerssuchasthe thisgalaxy(Wangetal.2014;Tsaietal.2012). popular HCN and HCO+, HC N is generally optically thin in 3 galaxiesowingtoitsrelativelylowabundance,whichmakesitan IC694: Previous observation only obtained upper limits of idealdensegastracerforcalculatingthecolumndensityand/or HC N12-11ForIC694(Lindbergetal.2011).Inourobserva- mass of molecular hydrogen content of galaxies. In the obser- 3 tions,atentativedetectioninIC694(>2σ)isobtained.Theline vationsbyLindbergetal.(2011)andCostagliolaetal.(2011)a profileofIC694obviouslydeviatesfromaGaussian,sowede- lowdetectionrate ofHC3N was reportedandwas explainedas rivetheHC3Nintensitybyintegratingthelinewithinawindow theintrinsicallyfaintemissionofHC3N,andourstackedresult of400kms−1width(Table3). also implies that the HC3N is quite weak in the non-detected Wenotethat,inNGC1068andIC694,HC3N24-23ispossi- galaxies(2σupperlimit=0.14Kkms−1),whichisalsoinfavor blyblendedwithH CO3(0,3)–2(0,2)emission(f =218.22219 ofthisexplanation.Thenon-detectioninM82isconsistentwith 2 ν GHz).Theupperstateenergyofthisispara-H2COlineisabout thelowabundanceofHC3NinM82suggestedbyAladroetal. 10.5K, whichislikely to beexcitedin these cases. The H2CO (2011), that HC3N traces a nascent starburst of galaxy, and it lineisshiftedby141.1kms 1or-102MHzfromtheHC N24- canbe easily destroyedbythe UV radiationin PDRs, whichis − 3 23 line, andit is unclearthathow muchintensityof HC N 24- ubiquitousinactivegalaxies. 3 23 in NGC 1068and IC 694 is contributedby H CO (see Fig- In very recent line surveys of a few local active galaxies 2 ure2).Westilllackenoughdatatodisentanglethisissue,andcan (AGN and/or Starbursts, Aladroetal. 2015; Costagliolaetal. only compare with other observations. For example, in the ob- 2015), several HC3N transitions in 3mm band (HC3N J=10-9, servationofM82byGinardetal.(2015),theyshowedthatnear J=11-10andJ=12-11)weredetected.TheALMAobservations the frequency of 145 GHz, H2CO 2(0,1) – 1(0,1) is as strong byCostagliolaetal.(2015)evenreportedtheHC3NJ=32-31ro- as HC3N 16-15. H2CO is not detected in M 82 in 3mm band tationaltransition,and some of the vibrationallyexcitedHC3N (Aladroetal. 2015). In the observations toward NGC 4418 by lines.Thelatesthighresolutionlinesurveysinafewverynearby Aaltoetal. (2007), they showed that HC N 16-15 is blended galaxies (Meier&Turner 2005, 2012) and Meieretal. (2014, 3 wgriathtedHl2iCneOin,taenndsiHty2.CO may contribute 20% of the total inte- r2o0u1g5h)lyshGowMCthastc,atlhees)daerreivaebdoHutCs3eNvearbauln1d0a−n1c0e(sre(loantiv∼e1to00Hp2c),, whichisaboutanorderofmagnitudelowerthantheabundance ofHCNandsomeothermolecules. non-detections: ThespectraofM82andArp220areseriously TheresultsinAladroetal.(2015)showthat,theHC Nfrac- 3 contaminatedbytheimagesignalofCO2-1fromtheupperside- tional abundance is generally several times lower than that of band(ν=230GHz),whichisstrongandwidehencedifficultto HCN,HCO+ andCS.AndcomparingtootherAGNorstarburst remove.Asaconsequencewecouldnotextractthespectrumof galaxiesintheirsample,HC Nabundanceissignificantlyhigher 3 HC3Nproperly.WetreattheHC3N24-23inM82andArp220 inthetwoULIRGsArp220andMrk231,implyingitissuited asnon-detections,andtheir2σupperlimitsarealsoonlyindica- forstudyingtheactivityofULIRGs.Besides,therewasnoobvi- tive. Although not contaminated by adjacent CO image signal, ousevidenceoftheaffectionbyAGNontheintensityofHC N. 3 HC3N24-23wasnotdetectedinNGC2146,NGC6946,NGC Four galaxies in our sample (NGC 1068, M 82, Mrk 231 and 3690andNGC6240.Forthesenon-detectionwepresent2σup- Arp220)werealsostudiedinAladroetal.(2015).Wecompare per limit of the integrated intensity of HC3N 24-23 in Table3. our data with their results, and the HC3N spectra of Mrk 231 Only four galaxies are not contaminated by CO image signal, andNGC1068fromAladroetal.(2015)areshowninFigure1 thusnostackingisimplementedfortheirHC3N24-23spectra. and 2, to be compared with the non-detection of HC3N 2-1 in Mrk 231, and the detection of HC N 24-23 in NGC 1068, re- 3 spectively.Theirresultsshowthat,in3mmband,theintensities 3.3.Discussion:HCNingalaxies 3 betweenthethreetransitionsofHC N (10-9,11-10and12-11) 3 The HPBW of SMT and Effelsberg observations are 33 and differ not too much, and the peak temperature (T ) of HC N ′′ mb 3 46 ,respectively,whichshouldbeabletocoverthebulkofthe are 4mKforNGC1068, 11mKforM82,and 1.1–1.7 ′′ ∼ ∼ ∼ sample galaxies, especially the galaxy center. Thus our obser- mKforMrk231,and 10mKforArp220.Inourresults,the ∼ vations should be able to cover the region where the majority detectionof HC N 24-23in NGC 1068shows a peak T 7 3 mb ∼ ofdensegasresides.However,withsingle-dishobservationswe mK,whilethenon-detectionofHC N 2-1inMrk231andArp 3 cannotconstraintheemissionsizeofeitherHC N2-1orHC N 220showthat,theRMSwehave( 4–6mK)mightnotbelow 3 3 ∼ Articlenumber,page4of8 Jiang,Wang,Gao&Gu:HC N observationsofNearbyGalaxies(RN) 3 Fig.1.SpectraofthedetectedHC N(J=2-1)inNGC660,M66,IC342bytheEffelsberg.Atthebottomrowalsoshowsthestackedspectraand 3 thespectraofMrk231fromAladroetal.(2015).BluedashedlinesaretheGaussianfitoftheHC N2-1lines.TemperaturescaleisT inmK.A 3 mb colourversionofthisfigureisavailableintheonlinejournal. Fig.2.SpectraofdetectedHC N(J=24-23)inNGC1068,IC342andIC694bytheSMT.AtthebottomleftalsoshowsthespectraofNGC1068 3 fromAladroetal.(2015)forcomparison.BluedashedlineistheGaussianfit.NotethatinIC694itisdifficulttodistinguishtheHC Nemission 3 fromthepossiblyblendedH COlines.TemperaturescaleisT inmK.Acolourversionofthisfigureisavailableintheonlinejournal. 2 mb enoughtodetectiontheHC Nlines.Hereweconcludethat,be- HCO+. We list the ratio betweenthe flux densityof HC N and 3 3 sides the low abundanceof HC N, insufficientintegrationtime HCN1-0inTable2and3,respectively.Becausethedataquaity 3 andnotidealobservingconditionsarethemaincauseforthelow of this work is not good enough for us to present an accurate detectionrateofHC N. estimateontheHC Nfluxdensity,theratiosareonlytentative. 3 3 We see a large variation in the ratios, which could be an ev- It would be interesting to compare the intensity ratios be- idence of the essentially large variation of HC N luminosities tween HC N and other dense gas tracers, such as HCN and 3 3 Articlenumber,page5of8 A&Aproofs:manuscriptno.jiang_hc3n_v11 Table2.HC N2-1spectralmeasurements.AllthetemperaturescalesareT .Thecolumnsare:(1)galaxyname,(2)on-source timeforeach 3 mb galaxy,(3)RMSnoiseofthesmoothedspectrum,(4)velocityresolutionofthesmoothedspectrum,(5)linewidth(FWHM)oftheGaussianfitof theline(ifavailable),(6)HC Nemissionlinecenter,(7)Integratedintensity(anderrors)ofHC N2-1emission.Forthosenon-detections,2σ 3 3 upperlimitsarepresented(seetextinSection3.1),(8)Integratedfluxdensity,and(9)fluxdensityratiobetweenHC N2-1andHCN1-0. 3 Source On-time RMS δV ∆V V0 I(HC3N) S(HC3N) HHCC3NN12−01 (min) (mK) (kms 1) (kms 1) (kms 1) (Kkms 1) (Jykms 1) − − − − − − (1) (2) (3) (4) (5) (6) (7) (8) (9) NGC660 28 4.2 32.2 260.7(90.6) 825(36) 1.47(0.40) 0.86(0.24) 0.034a ∼ IC342 20 6.5 20.1 52.0(12.9) 45(6) 0.77(0.18) 0.45(0.11) 0.005b M66 18 7.0 20.1 44.8(17.7) 775(7) 0.68(0.21) 0.4(0.12) ∼0.114c ∼ NGC520 14 5.7 ... ... ... <1.02 <0.6 <0.087d NGC891 30 2.3 ... ... ... <0.26 <0.15 <0.033e NGC972 33 3.8 ... ... ... <0.61 <0.36 ... NGC1068 25 4.0 ... ... ... <0.73 <0.43 <0.013f UGC2855 23 4.9 ... ... ... <0.76 <0.45 ... ∗ UGC2866 22 3.6 ... ... ... <0.56 <0.33 ... ∗ NGC1569 33 3.0 ... ... ... <0.31 <0.18 ... NGC2146 24 3.4 ... ... ... <0.67 <0.39 <0.021e NGC2403 30 3.2 ... ... ... <0.33 <0.19 ... M82 26 5.5 ... ... ... <0.73 <0.43 <0.011e NGC3079 19 5.5 ... ... ... <1.17 <0.69 <0.225e NGC3310 36 2.3 ... ... ... <0.29 <0.17 ... IC694+NGC3690 47 4.2 ... ... ... <0.72 <0.42 <0.158g Mrk231 44 6.4 ... ... ... <0.91 <0.54 <0.191g Arp220 26 4.5 ... ... ... <0.94 <0.55 <0.069h NGC6946 28 5.1 ... ... ... <0.64 <0.38 <0.018e Notes.(*)Theupperlimitsofthetwogalaxiesarederivedassuminga200kms 1linewidth. − HCN1-0datafrom:(a)Baanetal.(2008); (b)Nguyenetal.(1992);(c)Kripsetal.(2008);(d)Solomonetal.(1992); (e)Gao&Solomon(2004a); (f)Aladroetal.(2015);(g)Jiangetal.(2011); (h)Wangetal.(2016). Table3.HC N24-23spectralmeasurements.ColumnsarethesameasTable2. 3 Source On-time RMS δV ∆V V0 I(HC3N) S(HC3N) HHC3CNN214−023 (min) (mK) (kms 1) (kms 1) (kms 1) (Kkms 1) (Jykms 1) − − − − − − (1) (2) (3) (4) (5) (6) (7) (8) (9) NGC1068 121 1.2 39.1 257.3(24.0)a 1102(13) 2.03(0.18) 49.9(4.4) 0.59 ∼ IC342 132 2.7 20.9 100b 43 0.47(0.12) 11.6(3.0) 0.12 ∼ IC694 127 0.87 41.7 400b 3095 0.90(0.11) 22.1(2.7) 2.76 NGC2146 115 1.01 20.9 320c ... <0.17 4.2 ∼<0.26 M82 60 3.0 20.9 150c ... <0.34 8.4 <0.26 NGC3690 139 0.73 42.0 260c ... <0.15 3.7 <1.27 ARP220 162 0.75 39.1 420c ... <0.19 4.7 <0.48 NGC6240 97 0.92 20.1 420c ... <0.17 4.2 <1.15 NGC6946 168 1.76 21.0 130c ... <0.18 4.4 <0.26 Notes. (a) FWHMfrom the Gaussian fittingand error. (b) line window (full width) used to derived the integrated intensity. (c) CO width from Youngetal.(1995)thatareusedtoderivedtheupperlimits. amonggalaxies.Inthe HC N surveybyLindbergetal. (2011), HC N/HCO+also show large variance.In NGC 253 and M 82, 3 3 ratios like HC N/HCN were used to compare HC N between HC N10-9isonly 1/20asstrongasHCO+1-0,whileinArp 3 3 3 galaxies. Based on that ratio, IC 342 and M 82 were classi- 220HC N 10-9is n∼earlyas strongas HCO+1-0. In our results 3 fied as HC N-luminous galaxies. In our observation we detect suchlineratiosalsoshowlargediversity.Itisnotyetclearhow 3 both HC N 2-1 and HC N 24-23 in IC 342, but neither HC N to interpretthe ratio between HC N and other molecular lines, 3 3 3 3 transition is detected in M 82. On the other hand, we obtained andmoredataofHC Nindifferenttransitionswouldbehelpful 3 HC N 24-23 detections in NGC 1068 which were classified todisentangleitspropertiesindifferenttypesofgalaxies. 3 as a HC3N-poor galaxy in Lindbergetal. (2011). In the sam- Our observationsand other works have presented detection ple of some nearby galaxies observed by Aladroetal. (2015), of HC N emission lines from near 18 GHz up to 292 GHz. 3 theratiobetweenthepeaktemperature(Tmb)ofHC3N/HCNor The newly commissioned Tianma 65 m telescope i∼n Shanghai, Articlenumber,page6of8 Jiang,Wang,Gao&Gu:HC N observationsofNearbyGalaxies(RN) 3 China,isabletoobservelowtransitionHC Nemission,andhas 3 greatpotentialforfurtherHC N2-1surveysforlargesampleof 3 galaxies. 4. Summary We carry out single-dish observations towards a sample of nearby gas-rich galaxies with the Effelsberg telescope and the Submillimeter Telescope. This is the first measurements of HC N2-1inarelativelylargesampleofexternalgalaxies.Our 3 resultsinclude: 1. HC N(J=2-1)(ν=18.196GHz)wasobservedwiththe100- 3 m telescope in 20 galaxies and only three galaxies are de- tected(> 3σ):IC342,M66andNGC660.Thisisthefirst measurements of HC N 2-1 reported in external galaxies, 3 and the first HC N detection in M 66. We stack the spec- 3 traofthosenon-detectionsyetthereisstillnosignofHC N 3 emission. The 2σ upper limit of HC N intensity from the 3 stackedspectrumisabout0.12Kkms 1. − 2. HC N(J=24-23) (ν = 218.324 GHz) was observed in nine 3 galaxieswith the SMT, and it is detected in three galaxies: IC342,IC694andNGC1068. 3. IC 342 is the only galaxiesdetected in both HC N 2-1 and 3 HC N 24-23 transitions in our observations, and the two 3 transitions have similar line center and width, suggesting a similar emitting area. The ratio of integrated intensity of HC N24-23/HC N2-1isabout0.82.Duetothecontamina- 3 3 tionofCO2-1imagesignalintheuppersideband,M82and Arp220aretreatedasnon-detectionofHC N24-23. 3 4. The ratios between HC N and HCN, HCO+ show a large 3 variance among the galaxieswith HC N detections, imply- 3 ing different behavior of the molecular lines in galaxies. Moresampleareneededtobetterunderstandtherelationship betweenHC Nandothermolecules. 3 Acknowledgements. WethankthestaffoftheEffelsbergtelescopeandtheSMT fortheirkindhelpandsupportduringourobservations.Thisprojectisfundedby ChinaPostdoctoralScienceFoundation(grant2015M580438),NationalNatural ScienceFoundationofChina(grant11420101002,11311130491,11590783and 11603075),andtheCASKeyResearchProgramofFrontierSciences.Thisre- searchhasmadeuseofNASA’sAstrophysicsDataSystem,andtheNASA/IPAC ExtragalacticDatabase(NED),whichisoperatedbytheJetPropulsionLabora- tory,CaliforniaInstituteofTechnology,undercontractwiththeNationalAero- nauticsandSpaceAdministration. 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