H3O+ Line Emission from Starbursts and AGNs

0.26 MB·English

by  S. Aalto

Astronomy&Astrophysicsmanuscriptno.15878 (cid:13)c ESO2011
January5,2011
+
H O Line Emission from Starbursts and AGNs
3
S.Aalto1,F.Costagliola1,F.vanderTak2,3,andR.Meijerink4
1 DepartmentofEarthandSpaceSciences,ChalmersUniversityofTechnology,OnsalaObservatory,SE-43992Onsala,Sweden
e-mail:[email protected]
2 SRON,NetherlandsInstituteforSpaceResearch,Landleven12,NL-9747ADGroningen,TheNetherlands
3 KapteynAstronomicalInstitute,UniversityofGroningen,TheNetherlands
4 LeidenObservatory,LeidenUniversity,P.O.Box9513,NL-2300RA,Leiden,TheNetherlands
Receivedx;acceptedy
1
1 ABSTRACT
0
2 Context.TheH O+moleculeprobesthechemistryandtheionizationrateofdensecircumnucleargasingalaxies.
3
Aims.WeusetheH O+moleculetoinvestigatetheimpactofstarburstandAGNactivityonthechemistryofthemolecularinterstellar
n 3
medium.
a
Methods.UsingtheJCMT,wehaveobservedthe3+−2−364GHzlineofp-H O+towardsthecentersofsevenactivegalaxies.
J 2 2 3
Results.Wehavedetectedp-H3O+towardsIC342,NGC253,NGC1068,NGC4418,andNGC6240.Upperlimitswereobtained
4 forIRAS15250andArp299.WefindlargeH O+abundances(N(H O+)/N(H )>10−8)inalldetectedgalaxiesapartfrominIC342
3 3 2 ∼
whereitisaboutoneorderofmagnitudelower.Wenote,however,thatuncertaintiesinN(H3O+)maybesignificantduetolackof
] definiteinformationonsourcesizeandexcitation.Wefurthermorecomparethederived N(H O+)withN(HCO+)andfindthatthe
O 3
H O+toHCO+columndensityratioislargeinNGC1068(24),moderateinNGC4418andNGC253(4-5),slightlylessthanunityin
3
C NGC6240(0.7)andlowestinIC342(0.2-0.6).WecompareourresultswithmodelsofX-rayandphotondominatedregions(XDRs
. andPDRs).
h Conclusions.ForIC342wefindthatastarburstPDRchemistrycanexplaintheobservedH O+ abundance.Fortheothergalaxies,
3
p thelargeH O+columnsaregenerallyconsistentwithXDRmodels.InparticularforNGC1068theelevatedN(H O+)/N(HCO+)ratio
3 3
- suggestsalowcolumndensityXDR.ForNGC4418however,largeHC NabundancesareinconsistentwiththeXDRinterpretation.
o 3
AnalternativepossibilityisthatH O+ formsthroughH OevaporatingoffdustgrainsandreactingwithHCO+ inwarm,densegas.
r 3 2
t ThisscenariocouldalsopotentiallyfittheresultsforNGC253.FurtherstudiesoftheexcitationanddistributionofH O+-aswellas
s 3
Herschelobservationsofwaterabundances-willhelptofurtherconstrainthemodels.
a
[
Keywords.galaxies:evolution—galaxies:individual:IC342,NGC253,NGC1068,NGC4418,NGC6240,IRAS15250,Arp299

—galaxies:starburst—galaxies:active—radiolines:galaxies—ISM:molecules
1
v
2
8 1. Introduction ded,stageofitsevolution.Thereisstill,however,substantialdi-
6 chotomyintheinterpretationofthelineemissionfromtheabove
0 Molecular line emission is an important tool for probing the
moleculesandthereforenewtracerspeciestocombinewithex-
. highly obscured inner regions of starburst galaxies and buried
1 istinginformationareimportant.
AGNs. Line ratios within and between species help determine
0 The hydronium ion H O+ is a key species in the oxygen
physical conditions and chemistry of the gas, which provide 3
1
chemistry of dense molecular clouds, and useful as a measure
1 essential clues to the type and evolutionary stage of the nu-
of the ionization degree of the gas (Phillipsetal. 1992). Since
: clear activity. Important extragalactic probes of cloud prop-
v erties include molecules such as HCN, HCO+, HNC, CN, H3O+ formation requires H2O to exist in the gas-phase, the
i H O+ moleculeactsasanaturalfiltertoselecthot(T >100K)
X and HC N (e.g. Costagliola&Aalto 2010; Kripsetal. 2008; 3
3 molecular gas, and therefore traces more specific regions than
Gracia´-Carpioetal. 2008; Loenenetal. 2008; Imanishietal.
r molecules usually surveyedtowards other galaxies. The chem-
a 2004; Gao&Solomon 2004; Aaltoetal. 2002) that trace the
dense (n>104 cm−3) star forming phase of the molecular gas. istry of this filtering is the evaporation of icy grain mantles at
HCN, HN∼ C, HCO+ and CN are all species that can be as- T ≈100K(vanderTaketal.2006b),orthatH2Oformsinthe
gasphaseathigh(T >300K)temperatures.
sociated both with photon dominated regions (PDRs) (e.g.
Tielens&Hollenbach1985) in starburstsand X-raydominated Observations of H3O+ emission will therefore help probe
regions(XDRs) (e.g. Maloneyetal. 1996; Meijerink&Spaans the location of dense, warm and active gas in galactic nuclei.
2005) surrounding active galactic nuclei (AGN). In contrast, Combined with information on the H2O abundance, these ob-
HC Nrequiresshieldeddensegastosurviveinsignificantabun- servations may further be used to trace the ionization rate by
3
dance since it is destroyed by UV and particle radiation (e.g. cosmicrays(producedinsupernovae,i.e.,starbursts)and/orX-
reactions with the ions C+ and He+)(Prasad&Huntress 1980; rays(fromanAGN).StudiesbyvanderTaketal.(2006a)have
Rodriguez-Francoetal. 1998, e.g.). These ions are expected to demonstratedthisuseofH3O+forSgrB2intheGalacticCentre.
be abundant in for example XDRs. Thus HC N line emission Furthermore, recent Herschel observations of Galactic sources
3
mayidentifygalaxieswherethestarburstisintheearly,embed- show 984 GHz 0−0 −1+0 H3O+ absorptionin the diffuse gas to-
wards G10.6-0.4 (W31C) (Gerinetal. 2010) and 1.03 - 1.63
Sendoffprintrequeststo:S.Aalto THz H3O+ emission from the high mass star forming region
2 S.Aaltoetal.:ExtragalacticH O+
3
Table1. Samplegalaxiesa. previousexperiencewith extragalacticH O+ we knew that the
3
line is weaker than the standard high density gas tracers such
as HCN and HCO+ so we restricted ourselves to a relatively
small sample of seven objects (coordinates, FIR luminosities
Galaxy α(J2000) δ(J2000) L D anddistancesarepresentedinTable1):
FIR
hhmmss ◦′′′ L Mpc
⊙
IC342 03:46:48.00 +68:05:46.0 6×108L 1.8 IC342isanearbybarredScdgalaxyofmoderateluminosity
⊙ (central400pchasL of6×108 L )withacentralstarburst.
FIR ⊙
NGC253 00:47:33.12 -25:17:17.59 2.1×1010 2.6 Within its central 300 pc (30′′) two molecular arms end in
a clumpy central ring of dense gas (e.g. Downesetal. 1992)
NGC1068d 02:42:40.71 -00:00:47.8 2×1011 14.4 which surrounds a young star cluster. The ring is suggested
to outline the X2 orbits in a larger-scale bar. The chemistry
NGC4418 12:26:54.63 -00:52:39.6 8×1010 27.3 of IC 342 has been investigated in detail in a high-resolution
study by Meier&Turner (2005). They find that the chemistry
NGC6240 16:52:58.89 +02:24:03.4 3.5×1011 107
in the ringis a mixtureof PDR-dominatedregionsandregions
of younger star-forming clouds. The chemistry in the bar/arms
IRAS15250 15:26:59.40 +35:58:38.0 1.12×1012 244
is dominated by shocks as shown by CH OH (Meier&Turner
3
2005) and SiO (Useroetal. 2006). Five (A-E) giant molecular
Arp299 11:28:33.13 +58:33:58.0 8×1011 42
clouds(GMCs)arefoundintheringandarms.The13′′ JCMT
beam of our H O+ observations is pointed towards the region
3
a)References:IC342:Downesetal.(1992);NGC253:Stricklandetal. of GMC B - but also includes GMCs A and E. GMCs B and
(2004); NGC 1068: Telescoetal. (1984); NGC 4418: Ridgwayetal. C are the sites of young ( a few Myr) star formation and are
(1994); NGC 6240: Yun&Carilli (2002); IRAS15250: Veilleuxetal. also the regions where incoming molecular clouds meet the
(1999);Arp299:Aaltoetal.(1997) ring. GMC A has more PDR characteristics. The dust tem-
peratureofIC342isestimatedto42K(e.g.Becklinetal.1980).
W3IRS5(Benzetal.2010).IngeneralH O+ abundancesagree
3
well with expectations from PDR models and - together with NGC 253 is also a nearby barred galaxy located in the
H O+andOH+-provideimportantfurtherinsightintogasphase Sculptor group with a compact nuclear starburst and a IR
2
oxygenchemistry. luminosity that appears to originate in regions of intense
Recently we have detected 3+ − 2− H O+ in the nearby massive star formation within its central few hundred par-
2 2 3
starburst M 82 and the ultraluminous galaxy Arp 220 secs (Stricklandetal. 2004). From their 2 mm spectral scan
(vanderTaketal.2008).Derivedcolumndensities,abundances, Mart´ınetal. (2006) suggest that the chemistry of NGC 253
andH O+/H Oratiosindicateionizationratessimilartooreven shows strong similarities to that of the Galactic Center molec-
3 2
exceeding that in the Galactic Center. In M 82 the extended ular region, which is thought to be dominated by low-velocity
evolvedstarburst(PDR)isalikelysourceofthisionizationrate shocks.HighresolutionSiOobservationsshowbrightemission
while, for the ULIRG Arp 220, an AGN-origin (XDR) is sug- resulting from large scale shocks as well as gas entrained in
gested. a nuclear outflow (Garc´ıa-Burilloetal. 2000). Note also that
In the XDR and PDR modelsgrain-processingis nottaken it is suggested that NGC 253 is a galaxy in which a strong
intoaccountsincethechemistryistakingplaceinthegasphase. starburst and a weak AGN coexist (e.g. Weaveretal. 2002;
However, a formation route for H O+ involving the evapora- Mu¨ller-Sa´nchezetal. 2010). High resolution observations of
3
tion (or removal by shocks) of H O from grain surfaces need HCN and HCO+ 1–0 (Knudsenetal. 2007) show strongly
2
also to be considered. For example, evaporating H O reacting centrally concentrated emission. The 13′′ JCMT beam covers
2
with HCO+ may provide an important source of H O+ (e.g. the bulk of the nuclear emission from HCN and HCO+. The
3
Phillipsetal.1992;vanderTak&vanDishoeck2000). central dust temperature of NGC 253 is estimated to 50 K
We haveused the JamesClerkMaxwellTelescope(JCMT) (Meloetal.2002).
in Hawaii to observe the J = 3+ −2− 364 H O+ line (upper
K 2 2 3
level energy E = 139 K) in seven starburst and active galaxies
u NGC1068isthenearestexampleofatype2Seyfertgalaxy
whichcovera rangeof environments.Our goalis to use H3O+ luminousintheinfrared.SurroundingtheAGNthereisa4′′cir-
asatracerofgaspropertiesingalacticnucleiandtoseeifH O+
3 cumnuclearmolecularringor-disk(CND)andonalargerscale
canserveasadiagnostictooltodistinguishAGNfromstarburst
thereisaNIRstellarbar2.3kpclong.Thisbarisconnectedtoa
activity.Insections2,3and4thesample,observationsandtheir
large-scale, molecular starburst ring that contributes abouthalf
resultsarepresented.H O+lineparametersarepresentedinsec-
3 thebolometricluminosityofthegalaxy(e.g.Telescoetal.1984;
tion4.1andinsection4.2H O+columndensitiesandfractional
3 Scovilleetal. 1988; Tacconietal. 1994; Helfer&Blitz 1995;
abundancesare calculated.In sections 5.1and 5.2 H O+ abun-
3 Tacconietal.1997).BrightHCN1–0lineemissionisobserved
dances in the context of X-ray and UV irradiated models are towards the CND while the HCO+ 1–0 emission is relatively
discussedandin5.3thepotentialimportanceofgrainchemistry.
fainterbyafactorof1.5(e.g.Kohnoetal.2001).OnlytheCND
Inthelastsection,5.5,wepresentabrieffutureoutlook.
andafractionofthebariscoveredbyourJCMT beamandwe
adopt a source size of 2′′ for the CND. The dust of the inner
4′′ appearsto show a strongtemperaturegradient- fromabout
2. Thesample
800 K in the very inner region to 150 - 275 K at a distance of
Wehaveselectedasampleconsistingofsevennearbyluminous 0.′′8 or greater (Tomonoetal. 2006). Alloinetal. (2000) find
starburst and AGNs - and one distant ULIRG. The galaxies temperaturesofabout150K200pcfromthenucleus.Thereisa
are selected from their bright HCN line emission. From our alsoradiojetfromthenucleuswhichfallsintoourJCMTbeam
S.Aaltoetal.:ExtragalacticH O+ 3
3
(e.g.Wilson&Ulvestad1983). ofthereceptorsofthearrayhasabeamsizeof14′′at345GHz
and the sources were placed in the receptor labeled H052. The
NGC 4418 is a luminous, edge-on, Sa-type galaxy with a back end was the Auto-Correlation Spectrometer and Imaging
deeplydust-enshroudednucleus(Spoonetal.2001).NGC4418 System (ACSIS), providing 1.0 GHz bandwidth in 2048 chan-
isaFIR-excessgalaxywithalogarithmicIR-to-radiocontinuum nelsand a maximumresolutionof0.4 kms−1. Inorderto opti-
ratio (q) of 3 (Rousseletal. 2003). This excess may be caused mizebaselinestability,doublebeamswitchingatarateof1Hz
by either a young pre-supernova starburst or a buried AGN was used with an offset of 120′′. Weather conditions were op-
(Aaltoetal. 2007; Rousseletal. 2003; Imanishietal. 2004). timal, with about 1 mm of precipitable water vapor,leading to
Unusually luminous HC N line emission (Aaltoetal. 2002, a typical noise rms of 2 mK at 10 km/s resolution. Telescope
3
2007; Costagliola&Aalto 2010) has been interpreted as a pointing was checked every hour on the CO line emission of
signature of young starburst activity. Mid-IR intensities are nearbyAGBstarsandalwaysfoundtobewithin2′′.Datawere
indicative of dust temperatures of 85 K (Evansetal. 2003) extractedintofitsfileswiththestarlinksoftwareandanalyzedin
insidearadiusof50pc(0.′′5).TheIRluminosity-to-molecular CLASS.Linearbaselinesweresubtracted,lineparameterswere
gas mass ratio is high for a non-ULIRG galaxy indicating that calculatedbyfittingGaussianprofilestothespectra.Theresult-
intense, compact activity is hidden behind the dust. Recent ingspectraareshowninFig.1.Throughoutthispaper,velocities
highresolutionimagingof CO 2–1(Costagliola etal. in prep.) areintheheliocentricframeandredshiftiscomputedusingthe
indicateamolecularsourcesizeof0.′′5. radioconvention.
NGC 6240 is an infrared luminous merger of two massive
spiral galaxies. Two AGN/LINER nuclei are separated by 1′′ 4. Results
andthebulkofthemoleculargashaspiledupbetweenthetwo
nuclei(Ionoetal. 2007; Tacconietal. 1999). LuminousHCO+ 4.1.Lineintensitiesandlinewidths
4–3 emission is also emergingfrom in between the two nuclei
Integrated line intensities, line widths and fitted velocities (he-
wheretheH emissionisalsolocated.Themid-IRsources(24.5
2 liocentric) for the seven observed galaxies can be found in
µm)areassociatedwith thetwoX-raynuclei- thebrightestby
Table 2. p-H O+ line emission was detected towards IC 342,
farbeingthesouthernnucleus(T=55-60K)(Egamietal.2006). 3
NGC4418,NGC253,NGC1068andNGC6240,whileArp299
Ithasbeensuggestedthatthemediumbetweenthetwonucleiis
andIRAS15250werenotdetected.
dominated by starburst superwinds from the southern nucleus
In general it is found that line widths and shapes (Fig. 1)
(Ohyamaetal. 2000) and the molecular gas is indeed highly
agree well with those found for HCN and HCO+ in the cen-
turbulent (Ionoetal. 2007; Tacconietal. 1999). The JCMT
tral regions of the systems (Nguyenetal. 1992; Aaltoetal.
beamencompassestheentiremolecularstructureofthisgalaxy
andweadoptasourcesizeof1′′fortheH O+ emission. 2007; Knudsenetal. 2007). The most noteworthy deviation
3 fromthisisNGC6240wherethelinewidthofHCNandHCO+
(Greveetal.2009)ismorethanafactoroftwogreaterthanwhat
IRAS 15250+3609 is a relatively distant ultraluminous
we find for H O+. A possible explanationfor this could be the
galaxy - probably a major merger with a dominant bright nu- 3
weak H O+ signal resulting in a moderate signal-to-noise pro-
cleus.Atidalfeatureappearstoemergefromthesouthwestside 3
file.ForNGC1068thelinewidthagreeswellwiththatofHCO+
ofthenucleusandturnsaroundontheeasternsidetocreatean
4–3butissomewhatnarrowerthanwhatisobservedforHCN4–
enormousclosedring,27kpcindiameter.Theopticalspectrum
3(202kms−1)(Pe´rez-Beaupuitsetal.2009).
is a composite of H II and LINER features (Veilleuxetal.
ForIC342anunidentified(U)linewasdetectedinFebruary
1999). IRAS 15250 exhibits deep silicate absorption features
2008-butitdidnotappearagaininAugustthesameyear.The
(Spoonetal.2006)indicatingadeeplyenshroudednucleus.Our
lineissomewhatnarrowerthantheH O+lineandappearatave-
JCMTbeamcoversthewholeopticalgalaxy. 3
locityof263kms−1(restfrequency364.477GHz).Itispossible
thatitisanatmosphericO line.
Arp 299 is an IR-luminous merging system of two galax- 3
ies,IC694andNGC3690.Strong12COemissionhasbeende-
tectedfromthenucleiofIC694andNGC3690andfromthein-
4.2.Columndensities,excitationandabundances
terfacebetweenthetwogalaxies(e.g.Sargent&Scoville1991;
Aaltoetal. 1997). Thetwo nuclei,as wellas the westernover- Since we have no direct information on the excitation of the
lapregion,currentlyundergointensestarformationactivity(e.g. H O+ molecule we make some assumptions that will have to
3
Gehrzetal.1983).BrightHCN1–0emissionisemergingfrom betestedinfutureobservations.ThecriticaldensityoftheH O+
3
bothnucleias wellas fromthe overlapregion(e.g. Aaltoetal. transitionis high(aboutn = 106 cm−3 (Phillipsetal. 1992))
crit
1997).WateremissionfromNGC3690suggeststhatitalsohar- andthelowreducedmassoftheH O+ moleculemakesitsexci-
3
boursanAGN (Tarchietal. 2007). To coverall threeregionsa tationverysensitivetoradiativepumpingbydust.Wetherefore
mapwascarriedout. assumethatthecouplingbetweenthecolourtemperatureofthe
IR emission and the excitation temperature of H O+ holds as
3
long as the dust colour temperatureis above 30 K (see discus-
3. Observations sioninvanderTaketal.(2008)).
The 364.7974 GHz J = 3+ −2− line of H O+ was observed ForeachgalaxyweranaRADEXmodel(vanderTaketal.
K 2 2 3 2007)withtemperaturesrangingfromT=10-500K.Inthesecal-
towardsthesamplegalaxiesinFebruaryandAugust2008,with
the James Clerk Maxwell Telescope (JCMT1) on Mauna Kea, culations,thevolumedensityissettoanarbitrarylargevalue,so
Hawaii.Weusedthe16elementsheterodynearrayHARP.Each
theNetherlandsOrganizationforScientificResearch,andtheNational
1 TheJCMTisoperatedbytheJointAstronomyCenteronbehalfof ResearchCouncilofCanada.
theScienceandTechnologyFacilitiesCounciloftheUnitedKingdom, 2 Formoreinformationsee:www.jach.hawaii.edu/JCMT/spectral_line/I
4 S.Aaltoetal.:ExtragalacticH O+
3
Table2. H O+ LineResultsa.
3
0.01 IC342
T*
A
Galaxy R TA∗ Vline δV TA∗
[Kkm/s] [km/s] [km/s] [mK]
IC342
08-2008:b
0 km/s 200 H3O+ 0.30±0.07 39±10 75±18 3.7
02-2008:b
0.06 H O+ 0.22±0.06 40±5 39±13 5.4
3
NGC253 (U-line 0.38±0.05 263±2 30±5 11.6)
NGC253 9.35±0.18 221±2 180±4 48.8
NGC1068d 0.52±0.05 1120±8 160±20 3.0
NGC4418 1.02±0.06 2129±5 171±11 5.6
0 200 400 NGC6240 0.32±0.05 7179±15 177±27 1.7
IRAS15250 <0.012c ... ... <1.0
NGC1068
0.02 Arp299 <0.020d ... ... <2.0
a)Integrated lineintensitiesandpeaklineintensitiesaregiveninT∗,
A
η is 0.65-0.7 and we have adopted 0.7 here for conservative val-
rmmb
uesofN(H O+).b)DatareductionofH O+andU-linetakenatJCMT
3 3
02-2008and08-2008.c)1σupperlimitforaFWHMlinewidthof500
1000 1200
km/s.d)1σupperlimitforaFWHMlinewidthof300km/s.Thepoint-
ingcenterofHARPwasinbetweenthetwonuclei.Weaddedthespec-
trainthethreeemissionregionsofArp299coveredbyHARPtopro-
0.05 NGC4418
duceaspectrumwitha1σupperlimitof2mK.
H O+ abundancesrelativetoH (X(H O+))arelisted inta-
3 2 3
ble3.WehaveusedCO2–1spectrafromtheliteratureandused
a Galactic conversion factor from CO luminosity to H mass
2000 2200 2
(2.5×1020 cm−2 K−1 km−1 s) to estimate N(H ). Note that for
2
Galacticnuclei(andstarbursts)itisarguedthatthisconversion
factor overestimates the H column density by factors of 5-10
NGC6240 2
0.01 (seee.g.Mart´ınetal.(2010)).Sincewewishtoreducetherisk
of overestimating the H O+ abundances we have however still
3
adoptedthestandardconversionfactor.ForIC342andNGC253
wehaveestimatedandaverageN(H )forthe13′′beamsize.For
2
NGC 1068,NGC 4418andNGC 6240we have used the same
adoptedsourcesizeasforH O+.
3
7000 7200
Fig.1. ObservedspectraofH3O+.TheintensityscaleisinT∗A, 4.2.1. ErrorsinH3O+ columndensityandabundances.
notcorrectedfortheJCMTbeamefficiency,thatatthisfrequen-
OurassumptionsontheexcitationandspatialextentofofH O+
cies is 0.7. The parametersof the detected lines were obtained 3
introduceerrorsinthecolumndensityestimates. ForIC342in
byGaussianfits,shownontheplotsasdashedlines.
particularthesourcesizeerrorcanbesignificant.Weassumethat
thesourcefillsthebeamwhichgivesusanaveragecolumnden-
sityforGMCsA,BandE,buttherealcolumndensitytowards
that the excitation of H3O+ is thermalized. The adopted exci- a particular cloud should be higher. For NGC 253 the double
tationtemperatureisessentiallytheradiationtemperatureanda peakedlineshapesuggeststhattheH O+ emissionisemerging
3
radiativeexcitationwithoneexcitationtemperatureissimulated. frombothdense-gaspeaksinthecenterandisfillingtheJCMT
Wethereforedonotusethefullnon-LTEcapacityofRADEX. beam. For NGC 1068 the H O+ source size in the CND could
3
The results are shown in Fig. 2. Each plot has temperature be anything from <1′′ to 4′′. We have assumed a source size
(kinetic or radiation) on the x axis and H O+ column on the y of 2′′. For NGC 4418and NGC 6240the source sizes are <1′′
3 ∼
axis. The contour line corresponds to the observed brightness whichmeansthatthebeamdilutionissignificant,butnoH O+
3
temperature.Thisdependsonourassumptionforthesourcesize. emissionismissed.
S.Aaltoetal.:ExtragalacticH O+ 5
3
Table 3. Parametersa for the RADEX Modeling of the H O+ 5. Discussion
3
Line.
5.1.HO+abundancesandPDR/XDRmodels
3
We compare the column densities of H O+ and H in Table 3
3 2
tochemicalmodelsofcloudsirradiatedbyeitherfar-UV,photon
Galaxy SourceSizeb T N(H O+)cfit X(H O+)d
[′′] [KIR] [c3m−2] 3 dominatedregions(PDRs)orX-rayphotons(XDRs).Ingeneral,
we expectthe XDRs to dominatein molecularISMs surround-
IC342 13 40 1.25×1014 4×10−9 inganAGN.TheXDRandPDRmodelsweusearepresentedin
NGC253 13 50 2.25×1015 1.5×10−8 Meijerink&Spaans(2005)andMeijerinketal.(2007).Ingen-
NGC1068 2 150 1×1015 1×10−8 eral,thethermalandchemicalstructureofXDRsandPDRsare
NGC4418 0.5 80 2.5×1016 3×10−8 different.LargerpartsofanXDRcanbemaintainedathightem-
NGC6240 1 50 5×1015 2.5×10−8 peraureandtheionizationfractionofanXDRcanbeuptotwo
ordersof magnitudehigher (x = 10−2−10−1) than in a PDR.
e
InanXDRH Omayforminthegasphaseathightemperatures
2
a) The temperature range investigated for all galaxies was 10-500 K, (200-300 K) and then react with either H+ or HCO+ to form
butsolutionswerefixedwhereTex=TIR.TheN(H3O+)rangesearched H O+. 3
was1012−1017 cm−2.b)Foradiscussionofadoptedsourcesizessee 3
Source averaged HCO+ abundances are typically ≈ 10−8
section2.c)Thetotal beam-averaged column densityof H O+ is2-3
timeshigher than that of p-H O+ inFig. 2, because theort3ho to para for all detected galaxies except for IC 342. A general result
3
ratioofH O+dropsfromthehigh-temperaturelimitofo/p=1forT > of the models presented in vanderTaketal. (2008) is that the
100Ktoo3/p=2whenT <50K.Herewehavecorrectedcolumnden- X(H3O+) does not exceed 3 × 10−9 in PDRs - even in the
sitiesforano/pratioof1.5.d)IC342: N(H )isestimatedto3×1022 models with an extremely high cosmic-ray ionization rate of
2
cm−2 (from the CO data of Eckartetal. (1990)). NGC 253: N(H ) is ζ = 5.0× 10−15 s−1. H O+ abundances approaching 10−8 and
2 3
estimatedto1.3×1023 cm−2 (usingCOdatafromMauersbergeretal. beyondaremorelikelytooccurinX-raydominatedregionsand
(is19e9s6ti)m.NatGedCt1o0b6e8:2F−rom10T×ac1co0n22iectma−l.2.(1I9f9w4)ethtaekeN(tHhe2)CfoOrt2h–e1CdNatDa we conclude that the XDR scenario fits the X(H3O+) values of
NGC 253,NGC 1068,NGC 4418andNGC 6240.For IC 342,
from Planesasetal. (1989) and assume a source size of 2′′we obtain
theresultsareconsistentwithaPDR,butamodelwithahighζ
N(H )=1.35×1023cm−2.NGC4418:ColumndensityforH fora0.′′5
2 2 of5.0×10−15s−1.
CO2–1sourcesizeisN(H )=7.4×1023cm−2(Costagliolaetalinprep.).
2
NGC6240:Ionoetal.(2007)estimateN(H )to1×1023cm−2fromtheir
2
CO3–2data.
5.1.1. Waterabundances
The H O+/H O ratio is an even better probe of the ionization
3 2
rate,andpotentialnatureoftheemittingsource.Itisstraightfor-
ward to obtain H O+/H O abundanceratios as largeas 10−2 in
3 2
XDRmodels,whileforPDRmodelsratiosaregenerally10−3or
less.HerschelhasalreadymeasuredwaterabundancesinM82
InFig.2.theimpactoftheassumptionoftheexcitationtem-
perature on the resulting H O+ column density is illustrated. (Weißetal. 2010) (see also section 5.4) and results for other
3
Note the strong dependence of N(H O+) for T <60 K. For galaxieswillfollowsoon.
3 ex∼
NGC253(forexample)N(p-H O+)changesfrom9×1014cm−2
3
when Tex=50 K to 7× 1015 for Tex=25 K. When Tex exceeds 5.2.RelativeH3O+andHCO+abundancesandXDR/PDR
60 K, however,the temperaturedependenceon N(H3O+) is al- models
mostgone.Thusforallgalaxies,exceptIC342,wearenotlikely
to overestimate N(H O+) by more than a factor of 2 since the Kohnoetal.(2001)andImanishietal.(2004)findHCN/HCO+
3
adoptedT > 50K.ItispossiblethatH O+ couldbecollision- 1–0 line intensity ratios greater than unity in several
ex∼ 3
allyexcited.Inthiscasetheexcitationtemperatureislikelysig- Seyfert nuclei - where also the HCN/CO 1–0 line ratio is
nificantly lower than the ones we assume here (beacuse of the high. Furthermore, Gracia´-Carpioetal. (2006) find elevated
high critical density). In Fig. 2 it is evident that the N(H O+) HCN/HCO+1–0lineratiosinULIRGs.Thisisofteninterpreted
3
wouldgoupconsiderablywithdecreasingtemperature.Withthe as a sign of an underabundance of HCO+ compared to HCN
assumedT theresultingN(H O+)isalreadyqutelargeforall due to X-ray chemistry.(Althoughit is importantto remember
ex 3
galaxies-evenlargerH O+columnswouldbeaninterestingre- thatanabundancedifferencecannotbeunambiguouslydeduced
3
sultindeed,butverydifficulttoexplainwithcurrentmodels.The from a single-transition line ratio.) Underabundant HCO+ in
excitationcanbeconstrainedthroughobservingmultipletransi- XDRs hasbeenproposedin theoreticalworkbyMaloneyetal.
tionsandRequena-Torresetal(inprep.)arecurrentlystudying (1996). However, more recent models by Meijerink&Spaans
the307and364GHzlineofH O+inasampleofstarburstgalax- (2005) suggest that HCO+ may be underabundant in mod-
3
ies. erate column density (NH < 1022.5 cm−2) XDRs - but for
larger columns the reverse is true and X(HCO+) generally ex-
Weconcludethatthesourcesizeuncertaintiesdominatethe ceeds X(HCN). Here we compare our derived N(H O+) with
errors in the column density calculations. To improve future N(HCO+) for the sample galaxies to see if large H3O+ abun-
N(H3O+) calculationsit is necessary to determine source sizes dancesarepairedwithaparticularlylowHCO+ abund3ance,and
throughhigh resolution observationsand to obtaininformation tocompareN(H O+)/N(HCO+)withtheexpectationsfromcur-
onT (H O+)throughmulti-transitionobservations. 3
ex 3 rentmodels.
InadditionitshouldbenotedthattheestimatesoftheH O+ In the XDR/PDR models, the formation of both H O+ as
3 3
relativeabundancesaredependentonthe reliabilityofthe con- well as HCO+ is mainly driven by reactions with H , H+,
2 2
versionfactorfromCOluminositytoH mass. and H+, and destruction by electrons, unless the electron frac-
2 3
6 S.Aaltoetal.:ExtragalacticH O+
3
Table4. HCO+/H O+Ratios. by intense X-ray emission from two directions on only 370 pc
3
distance.Thisscenariocouldpossiblyexplainboththerelatively
highH O+ andHCO+ abundances.
3
Galaxy N(HCO+)a N(H O+)/N(HCO+)
3
[cm−2] 5.3.GrainchemistryandtheformationofH3O+.
IC342 2−6×1014 0.2-0.6 AbovewehavefoundthattheXDR/PDRmodelscanpotentially
NGC253 5×1014 4 explaintheH3O+ andHCO+ columndensitiesand abundances
NGC1068 4×1013 24 we found for the observed galaxies. However, for NGC 4418
NGC4418 4.7×1015 5 theproblemwithanXDRexplanationisthelargecolumnden-
NGC6240 3−4×1015 0.7 sities of gas and dust observedtowardsits center (see footnote
of Tab. 3) where Costagliola&Aalto (2010) find globalHC N
3
abundancessimilartothosefoundtowardshotcoresinSgrB2.
a) IC 342: Estimated from Nguyenetal. (1992)(Note that a low Intensevibrationalline emissionsuggestthatthe HC N indeed
3
HCO+/H13CO+ 1–0 line ratio of 7 suggests that HCO+ columns may exists in warm-to-hot environments in the very center of the
behigher.NGC253:FromMart´ınetal.(2006)(6.5×1013 forT =12
ex galaxy. The HC N abundances are not consistent with either
Kandfor35′′sourcesize.RescaledN(HCO+)forthe10′′by14′′H O+ 3
3 XDRsorPDRs.
source size of (Knudsenetal. 2007)). NGC 1068: We used the data
provided in Pe´rez-Beaupuitsetal. (2009) and made a rotational dia- Thus, we have searched for alternative explanations to the
gram(assuminga2′′sourcesize)findingTex(HCO+)=36K.NGC4418: H3O+ line emission outside of the XDR and PDR models.
ColumndensityforHCO+(basedondatainAaltoetal.(2007)andun- Elevated HC3N line emission is consistent with the conditions
published JCMT HCO+ 4–3 data) for a 0.′′5source size. NGC 6240: inwarm,denseshieldedgasassociatedwithembeddedstarfor-
WeestimatetheHCO+columndensityfromtheexcitationinformation mation.IsitthenpossibletoobtainrelativeH O+abundancesof
3
giveninIonoetal.(2007)andforasourcesizeof1′′. ≈10−8withouttheX-raychemistry?
Inascenariowheregrainsareimportant,theH Ocanevap-
2
tional abundance is very low x <10−7 − 10−8. However, the orateoffthegrainsattemperatures>100KandinPhillipsetal.
interpretation of an N(H O+)/N(eH∼CO+) abundance ratio in an (1992) the destruction of HCO+ through the reaction of H2O
3 is foundto be importantformationprocessfor H O+ when the
XDR/PDRscenarioisnotentirelystraightforward,butthemod- 3
gasisdenseandwarm.ForawaterabundanceX(H O)of10−5
els provide some useful limits. For example, in the PDR mod- 2
els H O+/HCO+ ratios are unlikely to become larger than 3 and a density n(H2) of 105 cm−3 Phillipsetal. (1992) find an
3 H O+ abundance, X(H O+), of ≈ 10−8 and X(HCO+) a fac-
(Meijerinketal. 2010), while for the XDR models it is quite 3 3
straightforwardtoobtainN(H O+)> N(HCO+),buttheobtained tor 2–3 lower. It is unclear however, if the N(H3O+)/N(H2O)
3 can be greater than 10−3 within the context of their model.
ratioisverycolumndensitydependent.Whenthecolumnden-
sities are small (e.g N(H) ≈ 1022 cm−2) ratios as large 20 can vanderTak&vanDishoeck(2000)findthesameeffectintheir
study of the of Galactic protostar GL 2136. In their Fig. 1 the
beobtained,butabundances(andassociatedbrightnesstemper-
atures) are small, and in order to obtain significant HCO+ and temperatureand density structure is presented including calcu-
H O+columndensitieslargercloudsareneeded.However,when latedconcentrationsofHCO+bothwithandwithoutdestruction
3
increasing the column, the H O+/HCO+ column density ratio byH2O.
slowly decreases, but modera3te ratios around 4–5 are easy to We suggest that for NGC 4418it is possible thatthe H3O+
reproducewiththeXDRmodel. abundanceand the N(H3O+)/N(HCO+) ratio can be explained
byamodelwherethegasiswarmanddenseandwateriscom-
ing offthegrainsto reactwith HCO+ to form H O+. Thispro-
3
5.2.1. N(H3O+)/N(HCO+)inthesamplegalaxies cesscouldcontributeto areductionoftheHCO+ abundancein
Intable4welistN(HCO+)andN(H O+)/N(HCO+)forthesam- thegasphasewhichmayresultinalowerHCO+lineintensityin
ple galaxies. The largest N(H O+)/N3 (HCO+) value is found in anytransition.Notethatshocksmayberesponsibleforremoving
3 thewaterfromthegrainsmakingitavailableforfurtherreactions
NGC 1068 which has a Seyfert nucleusand its inner few hun-
includingH O+ formation.Flower&PineauDesForeˆts(2010)
dred pc has been suggested to be an XDR (e.g. Useroetal. 3
presentsmodelswherewaterabundancesarerelatedtothetype
2004; Pe´rez-Beaupuitsetal. 2009; Garc´ıa-Burilloetal. 2010).
andstrengthofshocksandthereareamultitideofGalacticob-
The large abundance ratio of ≈24 suggested for NGC 1068 is
servational studies showing enhancement of water emission in
consistentwith(alowcolumndensity)XDR.
shocks (Nisinietal. 2010; Leflochetal. 2010; Wampfleretal.
ForNGC 4418andNGC253XDRmodelscanexplainthe
H O+/HCO+ ratios(butwe notethatthe valueswith errorsare 2010;Melnicketal.2008).ApartfromcausingH2Otocomeoff
3 thegrains,shockscanresultinefficientgas-phaseformationof
also within the range of PDR models). For NGC 253 there is
H OfromOHintheshockedhigh-temperatureregime.
evidence that both an AGN and a young starburst is present 2
whileforNGC4418thenuclearactivityissoobscuredthatthe
nature of the activity cannot be discerned (see section 2). The 5.4.HO+,HO+,andHO
H O+/HCO+ratioforIC342isconsistentwithaPDRmodelas 2 3 2
3
istherelativeH O+ abundance. Weißetal. (2010) find a (potentially) surprisingly low
3
For NGC 6240 the H O+/HCO+ ratio is consistent with N(H O)/N(H O+) ratio of only a few which they attribute
3 2 2
bothXDRandPDRmodelswhiletheH O+ relativeabundance to H O evaporating off the grains through shocks, the H O
3 2 2
favoursanXDR.Itisinterestingtonotethatevenifthemolec- then becomes photodissociated into O and OH. Ion-molecule
ular gas of NGC 6240 is collected in-between the two nuclei, reactions then make H O+ while H O abundancesremain low.
2 2
theirradiationcouldstillimpactthegas.ThetwoAGNsaresep- ItisfurthermorenoteworthythatX(H O+) > X(H O+)inM82
2 3
aratedby1′′.5whichmeansthatthemoleculargasisirradiated (vanderTaketal. 2008; Weißetal. 2010). This is consistent
S.Aaltoetal.:ExtragalacticH O+ 7
3
with the low density (n = 103 cm−3), high cosmic ray rate of Starbursts and AGN: The La Palma Connection, ed. J. H. Knapen,
(> 5 ×10−15 s−1) model (Model 2) of Meijerinketal. (2010). J.E.Beckman,I.Shlosman,&T.J.Mahoney,672–+
Thismodeldoeshowevernotincludeanygrainprocessing. Krips,M.,Neri,R.,Garc´ıa-Burillo,S.,etal.2008,ApJ,677,262
Lefloch,B.,Cabrit,S.,Codella,C.,etal.2010,A&A,518,L113+
InvanderTak(2010)aphotoevaporation-ionizationhypoth-
Loenen,A.F.,Spaans,M.,Baan,W.A.,&Meijerink,R.2008,A&A,488,L5
esisfortheH3O+/H2O+/H2OratioisdiscussedforM82noting Maloney,P.R.,Hollenbach,D.J.,&Tielens,A.G.G.M.1996,ApJ,466,561
thattestingthistheoryrequirescalculationofthephotodissoci- Mart´ın,S.,George,M.R.,Wilner,D.J.,&Espada,D.2010,AJ,139,2241
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3
othergalaxies,however,remainstobeseen:the H O/H O+ ra- S.2006,ApJS,164,450
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tioinM82maywellbeunusuallylow.IntheULIRGMrk231
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8 S.Aaltoetal.:ExtragalacticH O+
3
IC 342 NGC 253
15 2 17
1 3
14.5 16.5
1] 1]
0 −s 2 −s
m m
14 k 16 k
−2m] N=5x1013 cm−2 −1 y [K −2m] 1 y [K
Log(N) [c10131.35 −−32 ated Intensit Log(N) [c10151.55 N=9x1014 cm−2 −01 ated Intensit
gr gr
e e
−4 nt −2 nt
12.5 TIR=40 K I 14.5 I
−5 T =50 K −3
IR
12 14
1 1.5 2 2.5 1 1.5 2 2.5
Log (T) [K] Log (T) [K]
10 10
NGC 1068 NGC 4418
17 17
16.8
3
3
16.5
1] 16.6 1]
2 −s −s
m 16.4 2 m
16 k k
−2og(N) [cm]1015.5 01 d Intensity [K −2og(N) [cm]1011561..682 N=1x1016 cm−2 01 d Intensity [K
L 15 −1 ate L ate
gr 15.6 gr
N=4x1014 cm−2 −2 Inte 15.4 −1 Inte
14.5
T =150 K −3 15.2 TIR=80 K −2
IR
14 15
1 1.5 2 2.5 1 1.5 2 2.5
Log (T) [K] Log (T) [K]
10 10
NGC 6240
17
3
16.5
1]
2 −s
m
16 k
−2m] 1 y [K
og(N) [c1015.5 N=2x1015 cm−2 0 d Intensit
L 15 −1 ate
gr
e
−2 nt
I
14.5
T =60 K −3
IR
14
1 1.5 2 2.5
Log (T) [K]
10
Fig.2. OnthexaxiswefindthetemperatureoftheexcitingbackgroundradiationandontheyaxisthecolumndensityofH O+.
3
Foreachgalaxy,thebrightnesstemperatureoftheH O+transitionisshowninagrayscalemap.Thethickblackcontourmarksthe
3
observedvalue,withonesigmaerrorsaremarkedbydashedlines.BackgroundtemperaturesestimatedbyIRobservationsarealso
reportedandusedtoderiveacolumndensityconsistentwiththeobservedbrightnesstemperature.Notethatthecolumndensityfits
includeonlytheparacolumn.