Global Physical Conditions of the Interstellar Medium in Nearby Galaxies

Far-infrared spectra (43-197um) of 34 nearby galaxies obtained by the Long Wavelength Spectrometer (LWS) aboard the Infrared Space Observatory (ISO) were analyzed to investigate the general properties of interstellar matter in galaxies. The line fluxes of

Astronomy&Astrophysicsmanuscriptno.(willbeinsertedbyhandlater)

Globalphysicalconditionsoftheinterstellarmedium

innearbygalaxies

T.Negishi1

,

T.Onaka1,K.-W.Chan1,andT.L.Roellig2

arXiv:astro-ph/0107019v1 2 Jul 2001

1

2

DepartmentofAstronomy,SchoolofScience,UniversityofTokyo,7-3-1Hongo,Bunkyo-ku,Tokyo113-0033,Japan

MS245-6,NASAAmesResearchCenter,Mo ettField,CA94035-1000,U.S.A.

thedateofreceiptandacceptanceshouldbeinsertedlater

Abstract.Far-infraredspectra(43-197µm)of34nearbygalaxiesobtainedbytheLongWavelengthSpectrometer(LWS)aboardtheInfraredSpaceObservatory(ISO)wereanalyzedtoinvestigatethegeneralpropertiesofin-terstellarmatteringalaxies.Thepresentsampleincludesnotonlynormalgalaxiesbutalsostarburstsandactivegalacticnuclei(AGNs).Far-infraredforbiddenlines,suchas[Cii]158µm,[Oi]63µm,[Nii]122µm,and[Oiii]88µm,weredetectedinmostofthesamplegalaxies.[Oi]145µmlinewasdetectedin13galaxies.Theline uxesof[Cii]158µmand[Nii]122µmrelativetothetotalfar-infrared ux(FIR)decreaseasthefar-infraredcolorbecomesbluer,whiletheratioofthe[Oi]63µm uxtoFIRdoesnotshowasystematictrendwiththecolor.The[Oiii]88µmtoFIRratioshowsalargescatterwithaweaktrendofincreasewiththecolor.AGNsdonotshowanydistinguishabletrendfromnormalandstarburstgalaxiesinthefar-infraredspectra,suggestingthatthefar-infraredemissionismainlydrivenbystar-formationactivitieseveninAGNs.Weestimatethephys-icalconditionsofphotodissociationregions(PDRs)inthesamplegalaxies,suchasthefar-ultravioletradiation eldintensityG0andthegasdensitynbyassumingthatalltheobserved[Oi]63µmandfar-infraredcontin-uumemissionscomefromPDRs.ComparisonwithPDRmodelsindicatesthatG0rangesfrom102 104andn～102 104cm 3.ThepresentresultsalsosuggestthatnvariesproportionallywithG0.Theratioof[Cii]158µmtoCO(J=1 0)lineemissionsupportsthelinearincreaseinnwithG0.Weestimatethataboutahalfof[Cii]158µmemissionoriginatesfromPDRsandattributetheresttotheemissionascomingfromlow-densitydi useionizedgas.Theestimatedintensityof[Cii]158µmfromtheionizedgasiscompatiblewiththeobservedintensityof[Nii]122µmifbothlinescomefromthesamedi useionizedgas.Thepresentanalysissuggeststhatthedecreasein[Cii]158µm/FIRwiththefar-infraredcolormaynotbeaccountedforbythedecreaseinthephotoelectricheatinge ciencyowingtotheincreaseinpositivechargesofdustgrainsbecauseameasureofthee ciency,G0/n,isfoundtostayconstantwiththefar-infraredcolor.Insteadthedecreasecanbeinterpretedintermsofeithertheincreaseinthecollisionalde-excitationofthe[Cii]transitionduetotheincreaseinthegasdensityorthedecreaseintheionizedcomponentrelativetothefar-infraredintensitysuggestedbythedecreasein[Nii]122µm/FIR.Basedonthepresentanalysis,wederiveaveragerelationsofthefar-infraredcolorwithG0andningalaxies,whichcanbeappliedtotheinvestigationofinterstellarmatterindistantgalaxies.Keywords.galaxies:ISM–infrared:ISM–linesandbands:ISM–radiationmechanisms:thermal

1.Introduction

Mechanismsforheatinginterstellargasincludecolli-sions,radiationfromstars,shocks,andcosmicrays.

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