Cold gas properties of the Herschel Reference Survey. I. 12CO(1-0) and HI data

14.2 MB·

by  A. Boselli

Astronomy&Astrophysicsmanuscriptno.HRS˙CO˙rev2˙editor˙short (cid:13)c ESO2015
June15,2015
Cold gas properties of the Herschel Reference Survey. I. 12CO(1-0)
and Hi data (cid:63)
A.Boselli1,L.Cortese2,3,M.Boquien1,4
1 AixMarseilleUniversite´,CNRS,LAM(Laboratoired’AstrophysiquedeMarseille)UMR7326,13388,Marseille,Francee-mail:
[email protected]
2 CentreforAstrophysics&Supercomputing,SwinburneUniversityofTechnology,MailH30,POBox218,Hawthorn,VIC3122,
Australiae-mail:[email protected]
3 EuropeanSouthernObservatory,Karl-SchwarzschildStr.2,85748GarchingbeiMuenchen,Germany
4 InstituteofAstronomy,UniversityofCambridge,MadingleyRoad,CambridgeCB30HA,UKe-mail:[email protected]
5
1
ABSTRACT
0
2
We present new 12CO(1-0) observations of 59 late-type galaxies belonging to the Herschel Reference Survey (HRS), a complete
n K-band-selected,volume-limited(15(cid:46) D(cid:46)25Mpc)sampleofnearbygalaxiesspanningawiderangeinmorphologicaltypeand
u luminosity. We studied different recipes to correct single-beam observations of nearby galaxies of different sizes and inclinations
J for aperture effects. This was done by comparing single-beam and multiple-beam observations along the major axis, which were
corrected for aperture effects using different empirical or analytical prescriptions, to integrated maps of several nearby galaxies,
2
includingedge-onsystemsobservedbydifferentsurveys.Theresultingrecipeisananalyticalfunctiondeterminedbyassumingthat
1
late-typegalaxiesare3Dexponentiallydecliningdiscswithacharacteristicscalelengthr =0.2r ,wherer istheoptical,
CO 24.5 24.5
g- (orB-) band isophotalradius at the24.5 mag arcsec−2 (25 magarcsec−2), aswell as ascale heightz = 1/100 r . Ournew
] CO 24.5
A COdataarethencombinedwiththoseavailableintheliteraturetoproducethemostupdatedcatalogueofCOobservationsforthe
HRS,nowincluding225outofthe322galaxiesofthecompletesample.The3Dexponentialdiscintegrationisappliedtoallthe
G
galaxiesofthesampletomeasuretheirtotalCOfluxes,whicharelatertransformedintomoleculargasmassesusingaconstantand
. aluminosity-dependentX conversionfactor.WealsocollectHidatafor315HRSgalaxiesfromtheliteratureandpresentitina
h CO
homogenisedform.
p
- Keywords.Galaxies:ISM;Galaxies:general;Galaxies:spiral;Radiolines:galaxies
o
r
t
s
a 1. Introduction observations. The new CO data, combined with those already
[ available in the literature for the rest of the sample, are ho-
TheHerschelReferenceSurvey(HRS)isacompletesampleof
mogenisedtoproduceacompletecatalogueofCOfluxesandre-
2 nearby galaxies defined to study the physical properties of the
3v interstellarmedium(ISM)ingalaxiesofdifferentmorphological aHliisdtiactaunfcroermtaitnhteieslitfeorrataullrethaenodbsheormveodgeHnRisSe.dWitefaolrsonecaorlllyectthede
7 type and luminosity (Boselli et al. 2010a). Composed of 322 wholesample.TheseCOandHidataarecrucialforanydetailed
galaxies, it has recently been observed by Herschel with the
7 studyofthephysicalpropertiesoftheISM.
SPIRE (Ciesla et al. 2012) and PACS (Cortese et al. in prep.)
7
. instruments.Toprovidethecommunitywiththelargestpossible
1 The cold gas is the dominant phase of the ISM in late-type
set of homogeneous data, our team has undertaken several
0 galaxies. It exceeds in mass the dust component by ∼ a factor
observational campaigns or collected data from the literature
4 of 100-200 (Sodroski et al. 1994). This cold gas component
to cover the widest possible range in wavelengths. These data
1 can be observed easily in local galaxies. The atomic hydrogen
include deep UV imaging with GALEX (Cortese et al. 2012a;
: (Hi) can be observed directly through the emission of the spin
v Bosellietal.2011),mid-infraredimagingwithIRAConSpitzer
Xi and WISE (Ciesla et al. in prep.), MIPS Spitzer photometry inversion line at 21 cm (1420 Mhz). Because of its symmetric
structure, the molecular hydrogen (H ) has no permanent
(Bendo et al. 2012), Hα imaging (Boselli et al. in prep.), and 2
r electric dipole moment. Dipole rotational transitions are thus
a medium-resolution integrated optical spectroscopy (Boselli et
strongly forbidden, making it very hard to directly observe
al.2013a).
the cold phase of this molecule, which in late-type galaxies
has generally a temperature of ∼ 10 K. For this reason, the
ThispaperpresentsnewCOobservationsof59HRSgalax-
molecular hydrogen mass is generally determined through
ies obtained at the Kitt Peak 12m radiotelescope. These new
observing of the second most aboundant molecule in the cold
COdata,combinedwiththoseavailableforboththeotherHRS
ISM,carbonmonoxide,undertheassumptionthatCOisagood
objects and for other nearby galaxies recently mapped in the
tracerofH (Young&Scoville1991).Indeedithasbeenshown
12CO(1-0) line with various instruments, are used to compare 2
differentprescriptionstocorrectforaperture-effectssingle-beam thatthedynamicalmassofthegiantmolecularcloudsistightly
relatedtotheintensityoftheCOline.Themostcommonlyused
(cid:63) Tables 1, 2, 10, 11, 12 are available in electronic form at the methodofdeterminingthemolecularhydrogenmassisbasedon
CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via observingofthe12CO(1-0)rotationallineat2.6mm(115GHz)
http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ andassumingagivenconversionfactor XCO betweentheinten-
2 Bosellietal.:12CO(1-0)andHidataoftheHRS
sityoftheCOlineandtheH columndensity(seeBoselli2011). et al. 2010; Eales et al. 2010; Gomez et al. 2010; Pohlen et al.
2
2010;Sauvageetal.2010).
Inrecentyears,severalworkshavequestionedtheexistence
of a standard conversion factor, but rather indicated that X
CO 2. Thesample
might change with the different physical conditions charac-
terising the properties of the ISM, such as the hardness of The Herschel Reference Survey (HRS) is an Herschel/SPIRE
the ionising and non-ionising stellar radiation fields, and the guaranteedtimekeyprojectaimedatobservingacomplete,K-
metallicity and the density of the gas (Boselli et al. 2002, Bell band-selected (K ≤ 8.7 mag for early-types, K ≤ 12 for type ≥
et al. 2007, Bolatto et al. 2008, Liszt et al. 2010, Leroy et al. Sa),volume-limited(15(cid:46)D(cid:46)25Mpc)sampleofnearbygalax-
2011, Shetty et al. 2011a,b, Narayanan et al. 2012, Bolatto et ies.TheHRSsurvey,aswellastheselectedsample,areexten-
al.2013,Sandstrometal.2013).Wethusdeterminedmolecular sively presented in Boselli et al. (2010a). Briefly, the sample is
gas masses using both a constant or a luminosity-dependent composed of 322 galaxies out of which 260 are late-type sys-
X conversion factor. As constant value we used the one tems1.GalaxieswereselectedintheK-bandtakenasaproxyfor
CO
determinedintheMilkyWayusingγ-rayabsorptiondatafrom galaxy stellar mass (Gavazzi et al. 1996). The sample includes
COBE (X = 2.3 1020 cm−2/(K km s−1), or else, if expressed objects in environments of different densities, from the core of
CO
asα =3.6M /(Kkms−1 pc−2),Strongetal.1988),whichis theVirgoclustertoloosegroupsandfairlyisolatedsystems.As
CO (cid:12)
generallyassumedtoberepresentativeofenvironmentssimilar defined, the present sample is ideal for any statistical study of
to those encountered in the solar neighbourhood in terms of themeangalaxypopulationofthenearbyuniverse.
metallicityandradiationfield.Asvariableconversionfactor,we The galaxies observed in this work are late-type systems of
adopt the calibration proposed by Boselli et al. (2002) based K-band magnitude brighter than K< 10, expressely selected
on the H-band luminosity, log X [cm−2/(K km s−1)] = -0.38 to complete the sample down to this K-band magnitude limit
CO
× log L [L ] + 24.23. We chose this calibration because it (142/151ofthelate-types).Includingournewobservations,225
H H(cid:12)
can be determined easily for all HRS galaxies, for which an outofthe322HRSgalaxies(143detections),168/260forlate-
H-bandmagnitudeisavailablefrom2MASS.Furthermore,this type (134 detected), and 57/62 for early-type systems (9 de-
calibrationisameancalibrationcomparedtothoseproposedin tected)have12CO(1-0)data.
theliterature(Bolattoetal.2013). Wealsocompileandhomogenise21cmHidatafromthelitera-
tureforallgalaxiesintheHRS.Hidataareavailablefor315out
ofthe322galaxiesofthesample.ThewholeHRSsamplewith
The paper is structured as follows. Section 2 describes the
itsmaincharacteristicsisgiveninTable1,arrangedasfollows:
HRS sample, while Sects. 3 and 4 present the new CO obser-
vations. In Sect. 5 we discuss and apply different aperture cor-
– Column 1: Herschel Reference Sample (HRS) name, from
rection techniques to the whole set of CO data available in the
Bosellietal.(2010a).
literaturefortheentireHRSsample,andwethusderivetotalCO
– Column 2: Zwicky name, from the Catalogue of Galaxies
fluxes and molecular gas masses. Section 6 gives homogenised
and of Cluster of Galaxies (CGCG; Zwicky et al. 1961-
Hidataforthewholesample.AbriefconclusionisgiveninSect.
1968).
7. Despite the presence of several complete samples of nearby
– Column 3: Virgo Cluster Catalogue (VCC) name, from
galaxies with CO data (Sage 1993; FCRAO survey, Young et
Binggelietal.(1985).
al. 1995; Boselli et al. 1997; Sauty et al. 2003; Lisenfeld et al.
– Column 4: Uppsala General Catalog (UGC) name (Nilson
2011, Saintonge et al. 2011), the HRS is the only one with the
1973).
complete set of far-infrared and sub-millimetric Herschel data
– Column 5: New General Catalogue (NGC) name (Dreyer
necessary for a detalied study of the properties of the ISM of
1888).
normalgalaxies.Atthesametime,itisastatisticallysignificant,
– Column6:IndexCatalogue(IC)name(Dreyer1908).
complete sample of nearby galaxies spanning a wide range in
– Columns 7 and 8: J2000 right ascension and declinations,
morphologicaltypeandstellarmass,makingitideallysuitedto
fromNED.
studythecoldgaspropertiesofnormalgalaxies.Itcanbeused,
– Column9:Morphologicaltype,fromNED,orfromourown
forinstance,toextendtolowerstellarmasses(byafactorof∼
classificationifnotavailable.
10)thereferenceworkofSaintongeetal.(2011)ontheCOLD
– Column 10: Distance, in Mpc. Distances have been deter-
GASS sample, which only includes massive galaxies. We thus
minedfromtherecessionalvelocityassumingaHubblecon-
devotethreeotherspecificworkstotheanalysisofthegasprop- stant H = 70 km s−1 Mpc−1 for galaxies outside the Virgo
ertiesoftheHRSusingthisuniquesetofdata.One,alreadypub- 0
cluster,andassumedtobe23Mpcforgalaxiesbelongingto
lished,analysestheeffectsoftheenvironmentontheHiscaling
theVirgoBcloud,17fortheotherVirgomembers(Gavazzi
relations of the sample (Cortese et al. 2011). A second one is
etal.1999).
focussedondeterminingthemeantotalandmoleculargasscal-
– Column 11: Total K-band magnitude (K ), from 2MASS
ing relations of the HRS (Boselli et al. 2013b), while the last Stot
(Skrutskieetal.2006).
one treats the effects of the environment on the molecular gas
– Column 12: g-band optical isophotal diameter (24.5 mag
content of cluster objects (Boselli et al. 2013c). The interested
arcsec−2),fromCorteseetal.(2012a).FortheHRSgalaxies
readercanfindtheresultsofotherworksbasedonthecombined
withoutSDSSimages,theg-bandisophotaldiameterwasde-
analysis of the Herschel and the other multifrequency data in terminedfromtherelationr (g)=0.871(±0.017)r (B)+
Corteseetal.(2012b;dustscalingrelationsalongtheHubblese- 24.5 25
quence),Smithetal.(2012;dustpropertiesofearly-typegalax-
1 WithrespecttotheoriginalsamplegiveninBosellietal.(2010a),
ies), Boselli et al. (2012; far infrared colours), Boquien et al.
weremovedthegalaxyHRS228whosenewredshiftindicatesitasa
(2012,2013;dustattenuationpropertiesinresolvedgalaxies)or backgroundobject.Wealsorevisedthemorphologicaltypefor6galax-
in the publication of several papers during the science demon- ies:NGC5701,nowclassifiedasSa;NGC4438andNGC4457,now
stration phase of the instrument (Boselli et al. 2010b; Cortese Sb;NGC4179,nowS0;VCC1549,nowdE;andNGC4691nowSa.
Bosellietal.:12CO(1-0)andHidataoftheHRS 3
6.041(±2.101) (Spearman correlation coefficient ρ = 0.92), The observational results are listed in Table 2. Of the 59 ob-
where r (B) is the radius given in NED. This relation was served galaxies, 13 were not detected. Table 2 is arranged as
25
determinedusingtheHRSgalaxieswithbothsetsofdata. follows:
– Column 13: inclination of the galaxy, determined using the
prescription based on the morphological type described in – Column1:HRSname.
Haynes&Giovanelli(1984)andthei-bandellipticitygiven – Columns2and3:R.A.andDec.pointingoffset,inarcsec.
inCorteseetal.(2012a). – Column 4: detection flag, 1 for detected, 0 for undetected
– Column14:Heliocentricradialvelocity(inkms−1),fromHi sources.
datawhenavailable,otherwisefromNED. – Column 5: observing run: 1,2 and 3 are for the three 2008
– Column 15: Cluster or cloud membership, from Gavazzi et runs,4forthe2009run.
al. (1999) for Virgo and Tully (1988) or Nolthenius (1993) – Column6:rmsnoise,inmK,ontheT∗scale,measuredafter
R
wheneveravailable,orfromourownestimateotherwise. thespectraaresmoothedtoavelocityresolutionδVCO=15.7
– Column 16: Code to indicate whether Hi data are available kms−1.
(1)ornot(0). – Column 7: number of scans (ON+OFF). Each scan is six
– Column17:CodetoindicatewhetherCOdataareavailable minuteslong.
(cid:82)
(1)ornot(0). – Column8:Intensityofthe I(CO)line(I(CO)= T∗dv)inK
R
km s−1 (area definition, which corresponds to the area un-
3. Observations dertheprofileofthelinemeasuredinbetweentheupperand
lower limits in velocity within which the line is detected.
COobservationswerecarriedoutduringfourremote-observing Galaxies are considered as detected whenever the signal-
runs from the Laboratoire d’Astrophysique de Marseille in to-noise, defined as S/N = I(CO) is greater than 2, where
spring2008and2009usingtheNRAOKittPeak12mtelescope ∆I(CO)isdefinedineq.(2).F∆oI(rCOu)ndetectedgalaxies,there-
2. One hundred forty-one hours were allocated to this project,
portedvalueisanupperlimitdeterminedasfollows:
outofwhich∼20hourslostforbadweatherconditionsortech-
nical problems. At 115 GHz [12CO(1–0)], the telescope’s half- I(CO)=5σ(W δV )1/2Kkms−1 (1)
HI CO
power beam width (HPBW) is 55”, which corresponds to 5.3
kpc at the average distance of 20 Mpc of the HRS galaxies. where σ is the rms noise of the spectrum, W the Hi
HI
Weatherconditionswerefairlygood,withtypicalzenithopaci- line width, and δV the spectral resolution (here taken at
CO
tiesof0.15-0.25.Thepointingaccuracywascheckedeverynight δV =15.7kms−1).ForgalaxieswithW unavailable,the
CO HI
bybroadbandcontinuumobservationsofnearbyplanetsorthe Hi width has been determined assuming a standard W =
HI
radiogalaxy 3C273, with an average error of 5” rms. We used 300 sin(i) km s−1, where i is the galaxy inclination, with a
a dual-polarisation SIS mixer, with a receiver temperature for minimumvalueofW =50kms−1foralmostedge-onsys-
each polarisation of about Tsys=300-400 K (in TR∗ scale) at the tems. HI
elevationofthesources.Weusedadualbeam-switchingproce- – Column 9: Error on the intensity of the CO line, ∆I(CO),
dure,withtwosymmetricreferencepositionsoffsetby4’inaz- computedas
imuth.Thebackendwasa256channelfilterbankspectrometer
withchannelwidthof2MHz.Eachsix-minutescanbeganbya ∆I(CO)=2σ(W δV )1/2Kkms−1 (2)
CO CO
chopperwheelcalibrationonaloadatambienttemperature,with
an OFF-ON-ON-OFF set of pointings for each scan. Galaxies where σ is the rms noise of the spectrum, W the CO
CO
were observed at their nominal coordinates listed in Table 1. linewidth (given in Col. 11), and δV the spectral resolu-
CO
Fefteenobjectswithanopticaldiameterexceedingthreearcmin- tion.
uteswerealsomappedalongthemajoraxis,withtwoone-beam – Column 10: Heliocentric velocity determined from the
offpositions.Theface-ongalaxyHRS42(NGC3596)wasob- CO line (Gaussian fit), in km s−1 (optical definition
servedalongacrosswithonebeamoffset.Thetotalintegration v=cz=∆λ/λ ).Theestimatederroriscomparabletotheres-
0
timewasonaverage120minutesON+OFF(i.e.60minuteson olution,thus∼15kms−1.
thesource),yieldingrmsnoiselevelsofabout3mK(intheT∗ – Column11:FullwidthatzeroleveloftheCOline,inkms−1,
R
scale) after velocity smoothing to 15.7 km s−1. The baselines withanestimatederrorof∼20kms−1.Forgalaxieswitha
wereflatowingtotheuseofbeam-switching,therebyrequiring suffixathewidthoftheCOlineisthefullwidthhalfmax-
onlylinearbaselinestobesubtracted.Theantennatemperature imum (FWHM) determined from a Gaussian fit. The width
T∗ was corrected for telescope and atmospheric losses. In the oftheCOlinegenerallycorrespondstothewidthoftheHi
R
followinganalysisweusetheT∗ scale.Thisscalecanbetrans- line,indicatedbytheredhorizontallineinFig.1.
formed into the main-beam brigRhtness temperature scale, T , – Column12:Peaktemperature,inT∗ scale(K).
mb R
withT =T∗/0.84(wherethemainbeamefficiencyisη =0.54
mb R mb
and the forward scattering and spillover efficiency η =0.68).
fss 4.2. UncertaintyontheI(CO)data
TheT∗ scalecanbeconvertedintofluxusing39Jy/K.
R
INcolumn9∆I(CO)givestheerrorontheintensityoftheCO
line as determined from the typical rms of the observed galax-
4. Results
ies. A different estimate of this uncertainty can be determined
4.1. Resultsofourobservations by comparing independent measurements of the same galaxy.
AllgalaxiesobservedinthisworkarenewCOobservationsand
The12CO(1–0)spectraofalltheobservedgalaxies,reducedwith
thusdonothaveanysimilardataintheliterature.Theonlyex-
theCLASSpackage(Forveilleetal.1990),areshowninFig.1.
ception is HRS 48 (NGC 3631), observed at the beginning of
2 TheKittPeak12-mtelescopewasoperatedbytheArizonaRadio each run for checking the tuning of the instrument on a CO-
Observatory brightsourceandfortestingtherepeatabilityoftheobservations.
4 Bosellietal.:12CO(1-0)andHidataoftheHRS
WethushavethreedifferentCOobservationsofthesamegalaxy where η is the main beam efficiency. For the definition of a
mb
thatcanbecompared.Thesamegalaxyalsohastwootherinde- completeandconsistentsetofdatafortheHRSgalaxiesitisthus
pendentsingle-beamobservations,onefromtheFCRAOsurvey necessarythatalltheseobservationsarehomogenisedonacom-
(Young et al. 1995) and a second one from the CO survey of munscale.Wethusdecidedtoprovide,forallgalaxies,12CO(1-
nearbygalaxiesdonewiththeOnsalaradiotelescopebyElfhag 0) fluxes in units of Jy km s−1. For a source with a Gaussian
et al. (1996). Table 3 gives the different I(CO) values obtained profileobservedwithaGaussianbeamofsimilarsize,thecon-
within our observing runs and in the literature. Given that the stantnecessarytotransformmainbeamtemperatures(inK)into
dataobtainedbytheFCRAO(Youngetal.1995)andtheOnsala fluxes(inJy)isgivenbytherelation(Wilsonetal.2009):
surveys have been obtained in different beams and on different
temperature scales, we also compare the total extrapolated CO (cid:18) θ (cid:19)2(cid:18) ν (cid:19)2
fluxes obtained as described in the following sections. Table 3 S(Jy)/Tmb(K)=8.18×10−7 , (5)
arcsec GHz
showsthatbyusingexactlythesametelescopeconfigurationwe
have differences from one to another observing run in the CO where θ is the FWHM of the beam and ν is the frequency. For
fluxestimateofHRS48aslargeas(cid:39)30%.Thecomparisonwith sources extended with respect to the beam, as is the case for
other data available in the literature, here done on the extrapo- allHRSgalaxies,thetransformationofmainbeamtemperatures
latedCOfluxesofHRS48toremoveanyfirst-orderdependence intoJyismuchmorecomplexandrequirestheantennapattern
ontheaperturecorrectionandtelescopetemperaturescale,gives andthedistributionoftheemittingsource(Wilsonetal.2009).
differenceswiththemeanvalueobtainedinthisworkoftheor- SinceaprioritheCOdistributionoftheemittinggalaxieswithin
derof(cid:39)8-13%.Thesedifferencesaresignificantlysmallerthan theantennapatternisunknown,astronomersgenerallytransform
those observed within our own independent data for the bright antennatemperaturesintoCOfluxesusingaconstantwhosetyp-
HRS 48. We recall, however, that for this particular object, our icalvalueforeachtelescope,determinedwitheq.5,islistedin
three independent observations were done with a significantly Table4.Forconsistencywithpreviousworks,however,weadopt
smallernumberofscans(∼6scanseach)thanourtypicalobser- the same constants as used in the literature and as given in the
vations of the other HRS galaxies (∼ 20 scans). Given that the originalreferenceswheretheCOdatahavebeenpublished.We
uncertaintyontheCOintensityI(CO)dependsonthermsofthe prefertousethevaluespublishedintheoriginalreferencesjust
spectrum(seeeq.2),wethusexpectthatforHRS48,thequite tomakeiteasyanydirectcomparisonwiththealreadypublished
large dispersion in our data is partly due to the low integration data. These conversion constants might differ ((cid:39) 15%) slightly
time. Table 3 shows, however, that the difference between our fromthosegiveninTable4,whicharemeanvaluesdetermined
ownobservationsofHRS48arelargerthantheexpecteduncer- forgalaxiesatredshiftz=0.
tainties. Given the peaked distribution of the emitting molecu-
lar gas in galaxies, it is conceivable that small pointing offstes
can be the origin of this discrepancy. In other words, the rela- 5.2. Integrateddataofmappedgalaxies
tiongivenintheprevioussectiontoestimatetheuncertaintyon
single-beam observations slightly underestimates ∆I(CO). The 5.2.1. TotalCOfluxes
error on the CO central beam observations done in this work Some of the 225 HRS galaxies with CO data have been
shouldthusbeoftheorderof30%. completely mapped either with a multibeam detector or in
interferometric mode and have thus high-quality integrated CO
fluxes. Of these, 18 objects have been observed by Kuno et al.
5. COdatafromtheliterature
(2007) with the 25 beams BEARS array mounted on the 45 m
Nobeyama Radio Observatory telescope, 19 objects by Chung
5.1. Unittransformation
etal.(2009a)withthe16beamsSEQUOIAarrayonthe13.7m
The12CO(1-0)dataavailableintheliteraturecomefromawide FCRAO radio telescope, and 2 other galaxies with the IRAM
variety ofreferences andfor thisreason areoften givenon dif- 30 m radio telescope by our team (see below). Ten galaxies
ferent scales, as summarised in Table 4. Data coming from the have also been observed during the BIMA SONG survey of
FCRAO telescope are generally presented on the T∗ scale (ob- nearby galaxies with the Owens Valley Radio Observatory
A
servedsourceantennatemperaturecorrectedforatmosphericat- in interferometric mode by Helfer et al. (2003). To these, we
tenuation,radiativeloss,andrearwardscatteringandspillover), can add 71 galaxies observed along the major axis during the
those from the 12 m Kitt Peak telescope on the T∗ scale (ob- FCRAO CO survey of nearby galaxies by Young et al. (1995).
R
servedsourceantennatemperaturecorrectedforatmosphericat- Another object, NGC 4565, has also been partly mapped with
tenuation, radiative loss, and rearward and forward scattering the IRAM 30 m (Neininger et al. 1996) and with the 45 m
and spillover), while those from the IRAM 30, the SEST, and Nobeyama (Sofue & Nakai 1994) radiotelescopes. Because of
the Onsala telescopes on the T scale (source brightness tem- the complete coverage of the stellar disc, in particular for the
mb
perature as measured by the main diffraction beam of the tele- single-dish observations done at the Nobeyama, IRAM, and
scope).Thedifferenttemperaturescalescanbetransformedinto FCRAO radio telescopes, we consider that all these data are of
a common scale following the prescription of Kutner & Ulich much higher quality than other single-beam observations that
(1981).TheT∗ temperaturecanbetransformedintotheantenna require aperture corrections. We thus decided to use mapped
R
temperatureas observationswheneverpossibleinsteadofsinglebeamdata.
T∗ =T∗/η , (3) A few of these objects have integrated data taken by more
R A fss
than one survey. We compared the different sets of data to iso-
where η is the forward spillover and scattering efficiency, late those that we consider of higher quality. To do that, we
fss
whilethemainbeamtemperaturecanbeobtainedas cross-matched the different surveys, including galaxies outside
theHRSsample(22extraobjects)toincreasethestatistics,and
T =T∗/η , (4) comparedthedifferentsetsofdatainFig.1.Alldatawerefirst
mb A mb
Bosellietal.:12CO(1-0)andHidataoftheHRS 5
Fig.1. Comparison between the integrated SCO fluxes determined by Kuno et al. (2007) and those determined by Helfer et al.
(2003; red filled dots), Chung et al. (2009a; blue filled triangles), our own observations of the two galaxies NGC 4548 and NGC
4579withtheIRAMradiotelescope(greenfilledsquares)andtheextrapolatedfluxesoftheFCRAOsurvey(Youngetal.1995;cyan
empty circles) on logarithmic scale for all galaxies in common (left) and on linear scale for those objects with SCO in the range
100-5000Jykms−1 sampledbytheHRS(right).Thelinearbestfittothedata,determinedassumingtheKunoetal.(2007)dataas
independentvariableintherange100-5000Jykms−1,areshownbythereddottedline(Helferetal.2003),thebluelongdashed
line(Chungetal.2009a)andthecyansolidline(Youngetal.1995).Theshortdashedblacklineshowsthe1:1relationships.
transformed into CO fluxes (in Jy km s−1) using the prescrip- effectinthecorrectionduetosize,wefittheHelferetal.(2003)
tions indicated in the original papers for a correct comparison. andYoungetal.(1995)vs.Kunoetal.(2007)COfluxrelations
FortheFCRAOdataofYoungetal.(1995)weusedtheextrap- usingonlyCOdataintherange100<SCO<5000Jykms−1,
olatedSCOfluxesprovidedinthisreferenceinthecomparison. whichistherangeintheCOfluxcoveredbytheHRSgalaxies.
Figure 1 shows that the multibeam observations of Kuno et al. NorestrictionintheChungetal.(2009a)dataisrequiredgiven
(2007) and those obtained by our team using the IRAM radio thatthefluxesofalltheirgalaxiesarewithinthisrange.Thebest
telescope agree within 20% (SCO /SCO =0.83±0.03), linearfitstothedata,showninFig.1,aregiveninTable5.
Kuno IRAM
while a certain scatter is present when the data of Kuno et al.
For the Young et al. (1995) data, however, the proposed
(2007)arecomparedwiththoseobtainedbytheotherteams.All
correctiondoesnotsignificantlydecreasethemeanratioandthe
theotherdatasetsgiveSCOfluxesthataregenerallysmallerthan
scatter in the SCO /SCO (corrected) ratio. Indeed, the
thoseofKunoetal.(2007).ThedifferencebetweentheKunoet Kuno Young
meanratioSCO /SCO determinedusingthefullsetof
Kuno Young
al.(2007)andthoseobtainedbytheothersurveysdoesnotseem
original data published by Young et al. (1995) for all galaxies
todependonthetotalfluxoftheemittingsources,withtheex- incommonbetweenthesetwosurveysisSCO /SCO =
Kuno Young
ception of the BIMA SONG data of Helfer et al. (2003). Here
1.10 ± 0.44, while the one determined using the corrected data
there is a clear deviation from the one-to-one relation at high is SCO /SCO (corrected) = 0.90 ± 0.34. The majority
Kuno Young
SCOfluxes,thatcouldbeeasilyascribedtoanoncompletecov-
of the HRS galaxies mapped by Young et al. (1995) without
erageofthestellardiscinthemoreextendedsourcescombined
any other higher quality data from Kuno et al. (2007), Helfer
with a slightly lower sensitivity to the diffuse emission of in-
etal.(2003),orChungetal.(2009a),however,haveonlythree
terferometricobservationswithrespecttothemultibeamobser-
detected beams along the major axis, and their total CO flux is
vationsdonebyKunoetal.(2007).ThedifferencewithChung SCO <5000Jykms−1.Ifwerestrictthecomparisonofthe
Kuno
et al. (2009a) might also come from a higher sensitivity of the
Youngetal.(1995)andKunoetal.(2007)datatothesubsample
NobeyamaradiotelescopewithrespecttotheFCRAO.Thedif-
of HRS galaxies in common, the ratio determined with the
ference with the SCO fluxes of Young et al. (1995) probably originally published data, SCO /SCO = 1.29 ± 0.51,
Kuno Young
comesfromtheroughextrapolationtechniqueappliedtothema-
can be compared to that determined using the corrected data,
joraxisCOdata.Thedifferencebetweenthesetwosetsofdata SCO /SCO (corrected) = 1.33 ± 0.55. The proposed
Kuno Young
is, however, remarkably small, indicating that overall the total
correction does not introduce any significant improvement in
SCO flux of spiral galaxies can be fairly well deduced using a
the data. We thus decided not to apply any correction to the
verysimpleobservingtechniquecombinedwithanappropriate
FCRAOYoungetal.(1995)dataoftheHRSgalaxies.
aperturecorrection.
ThisobservationalevidencesuggeststhatthesetofdataofKuno
Figure 2 shows the relationship between the different inte-
and collaborators be considered as reference, and the others be
gratedCOfluxescorrectedusingtheprescriptionsgiveninTable
corrected using best fitting relations to minimise the scatter in
5andthoseofKunoetal.(2007)fortheHRSgalaxieswith100
thecomparisonwithKunoetal.(2007).Toavoidanysystematic
<SCO<5000Jykms−1.Giventhedispersioninthecorrected
6 Bosellietal.:12CO(1-0)andHidataoftheHRS
ofdetectedbeamsalongthemajoraxis.Whenonlythreebeam
observations are available, the comparison with the Kuno et al.
(2007)dataindicatesthatthedispersionintherelationishigher
than that we obtained using only the central beam, and we ex-
trapolated it using our own prescriptions (see below). We thus
decidedtousetheextrapolatedCOfluxesofYoungetal.(1995)
only when more than three detections along the major axis are
available;otherwise,weconsiderthegalaxiesasobservedonly
in the central beam and measure their total emission using our
ownprescriptions.
We thus end up with 37 HRS galaxies with integrated data, 17
from Kuno et al. (2007), 0 from Helfer et al. (2003)4, 6 from
Chungetal.(2009a),and13fromYoungetal.(1995).Tothese
we canadd agalaxy, NGC 4565(HRS 213),partly mapped by
Neininger et al. (1996) with the IRAM radiotelescope and by
Sofue & Nakai (1994) with the Nobeyama radiotelescope. By
correctingtheirownindependentsetsofdatatoextendthemea-
sureoftheCOfluxtothewholegalaxy,theyendupwithato-
tal flux of SCO = 1672.2 Jy km s−1 and SCO = 1643 Jy km
s−1, respectively. There is also an earlier estimate of the CO
flux of this galaxy by Richmond & Knapp (1986) from obser-
vations taken with the Bell Laboratories radiotelescope, which
gives SCO = 4487 Jy km s−1. Our own estimate based on the
extrapolationofthecentralsinglebeamasdescribedinsect.5.3,
givesSCO =1550Jykms−1,avaluemuchclosertotheesti-
3D
matesofNeiningeretal.(1996)andSofue&Nakai(1994).We
thusdecidedtotakeforthisobjectSCO=1672.2Jykms−1.
5.2.2. Uncertaintiesonintegrateddata
Table 5 and Fig. 2 can also be used to estimate the typical
uncertainty on the integrated data. The comparison between
Kuno et al. (2007) with our own observations of two HRS
galaxies done with the IRAM radio telescope indicates that
SCO /SCO =0.83±0.03.Despitethelackofstatistics,
Kuno IRAM
this comparison indicates that the two sets of data are consis-
tentwithin∼17%.Thisuncertaintyshouldbesharedbetween
thetwodifferentsetsofdata.TheuncertaintyontheKunoetal.
(2007) CO fluxes is thus assumed to be of the order of 12 %.
TheuncertaintyintheHelferetal.(2003)dataisalsocompara-
ble. The one in the Chung et al. (2009a) data is ∼ 40 %, while
in the Young et al. (1995) extrapolated data slightly larger (∼
45 %). It is hard to estimate the uncertainty on the CO flux of
NGC4565giventhatthethreeindependentmeasurementscome
fromtheextrapolationofobservationsofafractionofthisedge-
Fig.2.ComparisonbetweentheintegratedSCOfluxdetermined
ongalaxy.Twoindependentmeasurementsareconsistentwithin
by Kuno et al. (2007) and those determined by Helfer et al.
2%, and the third one is different by a factor of ∼ 3. We thus
(2003;redfilleddots),Chungetal.(2009a;bluefilledtriangles),
arbitrarilyassumeforthissourceanuncertaintyof∼30%.
correctedasdescribedinthetext,andtheextrapolatedfluxesof
theFCRAOsurvey(Youngetal.1995;cyanemptycircles),for
allHRSgalaxiesincommon,onlogarithmic(upperpanel)and 5.3. Single-beamobservationsofthecentresofgalaxies
onlinearscales(lowerpanel).Theshortdashedblacklineshows
the1:1relationships. 5.3.1. Extrapolationofthecentralbeam
The majority of the 225 HRS galaxies with CO data have only
onesingle-beamobservationintheircentralposition.Forthese
relations seen in Fig. 2 and in Table 5, we decided to take the
objectsthebeamofthetelescopedoesnotnecessarilycoverthe
Kuno et al. (2007), Helfer et al. (2003), Chung et al. (2009a),
whole surface of the stellar disc. These CO data must thus be
andYoungetal.(1995)datainorderofpriority(whenevermul-
tipledatasetsareavailable)3.ThequalityoftheFCRAOdataof corrected for aperture effects to derive total CO fluxes of the
observedgalaxies.Theextrapolationofthecentralbeamobser-
Young et al. (1995), however, strongly depends on the number
3 TheonlyexceptionisthegalaxyHRS91,NGC4192,forwhichwe thepositionthatthisgalaxywouldtakeinthe M(H2)vs. Mstar scaling
adoptthefluxgiveninYoungetal.(1995)sincemuchclosertothree relationgiveninPaperIII.
independentestimatesfromsinglebeamobservations.ThefluxofKuno 4 AllHRSgalaxiesintheBIMASONGsurveyhavebeenobserved
etal.(2007)isindeedunderestimatedbyafactorof∼2,asindicatedby byKunoetal.(2007).
Bosellietal.:12CO(1-0)andHidataoftheHRS 7
vationcanbedoneusingeitherempiricalrelationscalibratedon y' y'
nearby mapped galaxies or analytic functions known to repro-
duce the radial distribution of the CO emission well. The first
z
methodhasbeenproposedbySaintongeetal.(2011),whosim- y
ulated the observation of the galaxies mapped by Kuno et al.
(2007)withGaussianbeamsofdifferentsizes.Usingthistech- y
x' x'
nique,thetotalCOfluxSCOSaintonge(tot)ofagalaxywithacen- x x
z
tralbeamfluxSCO(CB)isgivenbytherelation
SCO(CB) z' z'
SCO (tot)= ,(6)
Saintonge 1.094−0.176∗N.Beam+0.00968∗N.Beam2
whereN.Beamisthesizeofthegalaxyinnumberofbeams: Fig.3.Definitionofthereferentialofthegalaxy(black)andthat
oftheobserver(red)inthecaseofaface–on(left)andanedge–
D (B)
N.beam= 25 , (7) on (left) galaxy. The referential of the observer corresponds to
θ thereferentialofthegalaxyrotatedbyangleiaroundthexaxis.
with D (B) the 25 mag arcsec−2 isophotal B-band diameter5,
25
and θ the FWHM of the beam of the radiotelescope (see Table
where i is the inclination of the disc. The integral given in eq.
4),bothmeasuredinarcseconds.
(10)canbesolvednumerically,andSCO(0)canbedetermined
ThesecondapproachhasrecentlybeenproposedbyLisenfeldet
bycomparingeq.(10)withtheCOfluxobservedinthecentral
al.(2011).ConsideringthattheCOemissionofnearbymapped
beam.Witheq.(9),itcanbeusedtoderivethetotalSCO (tot)
galaxies is represented well by an exponential disc of scale 2D
oftheobservedgalaxies.
lengthr :
CO Oursampleincludesalargenumberofedge-ongalaxiessuch
SCO(r)=SCO(0)e−r/rCO, (8) as NGC 4565. For these objects the integral given in eq. (10)
does not consider that the molecular gas disc also has a given
where SCO(0) is the CO emission in the centre of the galaxy. thickness in the z direction orthogonal to the plane of the disk.
ThetotalCOfluxofanexponentialdiscisgivenbytherelation: WethusmodifytheprescriptionofLisenfeldetal.(2011)totake
into account that the molecular gas disc has a 3D-distribution.
(cid:90) 2π(cid:90) ∞ Consistentlywithwhatisgenerallyassumedforthedustdistri-
SCO (tot) = rSCO(r)drdθ, bution in edge-on galaxies (Xilouris et al. 1999; De Looze et
2D
0 0 al. 2012), we assume an exponential distribution even in the z
(cid:90) ∞ direction:
= 2πrSCO(0)e−r/rCOdr,
0 SCO(r,z)=SCO(0)e−r/rCOe−|z|/zCO, (11)
= 2πr2 SCO(0). (9)
CO
where z is the scale height of the disc. The total CO flux is
AsdiscussedinLisenfeldetal.(2011),thescalelengthofthe CO
then
CO emitting disc, r , is correlated well with the optical scale
CO
lengthofthestellardiscandwithr ,theoptical25magarcsec−2
25
isophotal radius6. The observations of the THINGS sample of (cid:90) ∞(cid:90) ∞(cid:90) 2π
nearby objects done by Leroy et al. (2008) indicates that, on SCO3D(tot) = rSCO(r,z)dθdrdz,
average, r /r = 0.2. Lisenfeld et al. (2011) derive a consis- −∞ 0 0
CO 25 (cid:90) ∞(cid:90) ∞
tent value using different sets of data, from the BIMA SONG = 2πrSCO(0)e−r/rCOe−|z|/zCOdrdz,
survey of Regan et al. (2001), to the sample of Nishiyama &
−∞ 0
Nakai(2001)observedwiththeNobeyamaradiotelescope,tothe = 4πr2 z SCO(0). (12)
FCRAOgalaxiesmappedbyYoungetal.(1995)alongthema- CO CO
jor axis. The CO emission of the observed galaxies within the TocomputethetotalCOfluxofthegalaxy,wefirstcompute
centralbeamcanbedeterminedbyconvolvingtheCOintensity the flux detected in the central beam. We do so in the referen-
profilewithaGaussianbeam: tialoftheobserverdefinedbytheorthogonalaxes x(cid:48),y(cid:48),andz(cid:48),
where the last is the coordinate along the line of sight (Fig. 3).
SCO2D(CB) = 4SCO(0)(cid:90) ∞(cid:90) ∞exp−ln(2)(cid:32)2θx(cid:33)2+(cid:32)2ycθos(i)Tt(cid:33)h2heegraelfaexryenrtoiataltoedftbhyeaonbgsleervi.erWceororbestapionntdhsetfooltlhoewrienfgereeqnutiaatlioonf
0 0 incartesiancoordinates:
 (cid:112) 
exp− xr2C+O y2dxdy, (10) SCO (CB)=4SCO(0)(cid:90) ∞(cid:90) ∞(cid:90) ∞exp(cid:34)−4ln(2)x(cid:48)2+y(cid:48)2(cid:35)
3D θ2
5 Forthesampleofgalaxieswithintegratedvaluesintheliterature −∞ 0 0
 (cid:113) 
iusso6epdFhfoootrratlchodemiasmpaamertipenlrgegtoihvfeegndaiifflnaexNrieeEnsDtwa.pitehrtiunrteegcroartreedctvioanlus,esDi2n5itshtehelitBer-abtaunrde exp− x(cid:48)2+(y(cid:48)corsC(Oi)−z(cid:48)sin(i))2 − |y(cid:48)sin(i)z+COz(cid:48)cos(i)|dx(cid:48)dy(cid:48)dz((cid:48)1.3)
usedforcomparingthedifferentaperturecorrections,r istheisophotal
25
radiusderivedfromtheopticaldiametergiveninNED,whichgenerally
correspondstotheB-bandisophotaldiameterat25magarcsec−2.For MappedCOobservationsofedge-ongalaxiesareneededto
theHRSgalaxiesweusetheg-bandisophotalradiusr (g)givenin estimatethetypicalscaleheightoftheCOdiscinspirals.NGC
24.5
Table1. 891 is the only edge-on galaxy fully mapped in the 12CO(1-0)
8 Bosellietal.:12CO(1-0)andHidataoftheHRS
line (Scoville et al. 1993; Yim et al. 2011). The most recent
BIMA-SONGobservationsofthisobjecthaverevealedthatthe
scaleheightoftheCOdiscisz (cid:39)0.185kpc(Yimetal.20117)
CO
or, equivalently, z /r ∼ 1/101. Previous observations of the
CO 25
samegalaxyhavegivenaz-scalerangingfrom0.160kpcinthe
nucleustoz =0.276kpcattheedge(Scovilleetal.1993).To
CO
checkwhetherthesevaluesofz arerepresentativeofnormal,
CO
spiralgalaxies,wecomparedthemtothetypicalscaleheightof
thedustcomponentofotheredge-ongalaxies.Indeed,giventhe
tight correlation between dust and molecular gas, we can rea-
sonably assume that the tickness of the dusty disc is compara-
ble to that of the molecular gas. The scale height of the dust
disccanbemeasuredusingeitherfar-infrared(dustinemission)
oroptical(dustinabsorption)images.Fromtheanalysisofthe
Herschel/SPIREimagesofthesamegalaxy,Bianchi&Xilouris
(2011)havedeterminedthatz =0.200kpc,consistentlywith
dust
z (cid:39) 0.185 kpc determined by Yim et al. (2011). Using en-
CO
ergytransfermodelsadaptedtoreproducethefar-infraredemis-
sion observed by Herschel of the edge-on galaxy NGC 4565,
De Looze et al. (2012) derive z = 2.5 arcsec, corresponding
dust
toz /r ∼1/194.Theanalysisoftheopticalimagesofseven
dust 25
nearby edge-on galaxies done by Xilouris et al. (1997, 1999) Fig.5. Relationship between the ratio of the 2D (eq. 9) vs. 3D
gives values of zdust/r25 ranging from 1/50 to 1/184, as sum- (eq.12)aperturecorrectionsSCO2D/SCO3Dandtheratioofthe
marised in Table 6. The mean value determined from all these molecular gas scale height of the disc to the beam size zCO/θ
data is z /r (cid:39) 1/99, consistent with z /r ∼ 1/101 deter- for the HRS galaxies of the sample. Black open circles are for
dust 25 CO 25
minedbyYimetal.(2011)inNGC891.Asimilarvalue(z /r galaxies with an inclination ≤ 80 deg, red symbols for edge-
dust 25
=1/108)hasbeenalsoobtainedfromthedirectanalysisofseven on systems (i > 80 deg). The black solid line gives the relation
edge-ongalaxiesobservedbyHerschel(Verstappenetal.2013). determinedformodeledge–ongalaxies.
Since r (g) (cid:39) r (B), we assume z /r (B) = z /r (g) =
24.5 25 CO 25 CO 24.5
1/100.
To understand which among these three different recipes is Jy km s−1, but at the same time systematically overestimates
consideredthemostlikelytoestimatethetotalCOemissionof SCO3D(tot) in the brightest galaxies. We thus decided to adopt
galaxies,weappliedthemtoalltheobjectsobservedbyKunoet the correction given in eq. (11), keeping rCO/r24.5 = 0.2 for all
al.(2007)forwhichsingle-beamobservationsareavailableand galaxieswithonlyonesingle-beamobservation.
compared the extrapolated fluxes to those measured using the Wecanalsocheckwhethertheassumptionofa3Dmolecular
COmaps. gasdiscwithzdust/r24.5 (cid:39)1/100isappropriateforextrapolating
Figure 4 shows the relationship between the CO fluxes ex- single-beamobservationsofedge-ongalaxies.Todothat,weex-
trapolated from central-beam observations as described in this trapolateallsingle-beamobservationsofnearbyedge-ongalax-
textandthetotalCOfluxesgiveninKunoetal.(2007)forthose ies with data available in the literature for which accurate esti-
galaxies with single-beam observations available in the litera- mates of the total CO emission are available from complete or
ture. The comparison uses the extrapolation prescription pro- partialCOmapping(seeTable8).Table8indicatesthat,despite
posedbySaintongeetal.(2011)(eq.6,rightpanels)andtheone the large difference either in the CO fluxes of the central beam
proposedinthisworkbasedona3D-distributionofthemolecu- observations or in those determined from interferometry, par-
largas(eq.11).TheextrapolationrecipeproposedbyLisenfeld tiallymappedormajoraxismappededge-ongalaxies,thetotal
etal.(2011)givesbasicallythesameresultsastheoneproposed COfluxesdeterminedusingtheprescriptionsgiveninthiswork
in this work, with the exception of edge-on galaxies where it ineq.(8;2D)oreq.(11;3D)areclosetothebroadrangeofob-
underestimatesthetotalCOfluxbyafewpercent(seeFigure5). servationaldata.Equation(8)gives,asexpected,slightlylower
values(afewpercent)thaneq.(11)justbecauseitassumesan
Figure 4 and Table 7 show that both the 2D- (eq. 8) and
infinitelythindiscinthezdirection(seeFig.5).Thecomparison
the3D-(eq.11)analyticprescriptionsgivenabovearemoreap-
betweenaperture-correctedandtotalCOfluxesgiveninTable8
propriate than the empirical relation of Saintonge et al. (2011).
suggests that the 2D extrapolations underestimate the total CO
Indeed this last recipe systematically underestimates the total
emissionmorethanthe3Donedoes(butonlybyafew%).We
CO flux whenever the beam size of the telescope is smaller
thusdecidedtopreferandadoptthe3Drecipegivenineq.(11)
than one fifth ofthe optical diameter of the target (empty sym-
bols). We have also tested whether the use of a slightly differ- inthiswork.
enty exponential disc scale lengths in eqs. (8) and (11) gives
better agreement between the CO fluxes extrapolated from the
5.3.2. Uncertaintiesonsinglebeamextrapolateddata
central-beam observations and the integrated values of Kuno
et al. (2007). Using a higher r /r ratio than the canonical By comparing the flux extrapolated from single-beam obser-
CO 24.5
value of 0.2 indicated by Lisenfeld et al. (2011) leads to ratios vations to the integrated value for galaxies in the Kuno et al.
SCO /SCO (cid:39) 1 in those galaxies with SCO < 5000 (2007)sample,wecanquantifythetypicaluncertaintyintheto-
3D Kuno Kuno
talCOemissionduetoacombinedeffectoftheuncertaintyon
7 Theanalyticfunctionadoptedinthisworktoreproducethez-scale theI(CO)measurementandoftheaperturecorrection.Figure6
distributionofthemoleculargasisGaussian,notexponential,thusnot showstherelationshipbetweentheratioofthetotalCOfluxas
directlycomparabletotheoneadoptedinthiswork. determinedfromthe3D-extrapolationofthecentral-beamobser-
Bosellietal.:12CO(1-0)andHidataoftheHRS 9
Fig.4.Comparisonofsingle-beamobservationscorrectedforapertureeffectsusingtheprescriptionbasedona3D-distributionof
the molecular gas (eq. 11; left panels) and that of Saintoinge et al. (2011) (eq. 6; right panels) on logarithmic (upper panels) and
linear (in the range 0-5000 Jy km s−1 sampled by the HRS galaxies) scales (lower panels) for galaxies with integrated data from
Kunoetal.(2007).DifferentsymbolsareusedforgalaxiesobservedintheFCRAOsurveyofYoungetal.(1995;cyan),Starketal.
(1986;magenta),andotherreferences(black).FilleddotsareforgalaxieswithD (B)/θ≤5,emptysymbolsforthoseobjectswith
25
D (B)/θ>5.Theshortdashedblacklineshowsthe1:1relationships.
25
vationtothetotalvaluedeterminedfromthemapsforgalaxies flux estimate is as large as 123 % (SCO /SCO = 1.46 ±
3D Kuno
intheKunoetal.(2007)sampleasafunctionofthesurfaceof 1.14).
the galaxy covered by the telescope beam. Figure 6 shows that
whenever the beam covers more than 10 % of the surface of
5.4. Multiplebeamobservations
thedisc,themeanuncertaintyinthetotal,extrapolatedCOflux
isoftheorderof44%(SCO /SCO =1.01±0.44).Inthe
3D Kuno SomeHRSgalaxieshavemultipleobservationsalongthemajor
remaining galaxies, where the beam covers less than 10 % of
axis.Thesedatacanbecombinedandextrapolatedtodetermine
the surface of the galaxy, the mean uncertainty in the total CO total CO luminosities as done with different techniques found
in the literature. Typical examples are the method of Solomon
10 Bosellietal.:12CO(1-0)andHidataoftheHRS
come mainly from the FCRAO survey of Young et al. (1995),
withafewfromourownobservations.Aspreviouslydiscussed,
the FCRAO gives quite accurate extrapolated fluxes for these
objects,valuesthatweadopthere.Thereare,however,afewob-
jectswithonlytwoorthreedetectedbeamsalongthemajoraxis,
wheretheextrapolationoftheFCRAOisnotveryaccurate.This
is probably because, with such a small sampling, it is hard to
accuratelydeducearadialprofileoftheCOemission.Eighteen
HRSgalaxieshavetwoorthreedetectionsalongthemajoraxis
(fromtheFCRAOsurvey,4,andfromourownobservationspre-
sentedinthiswork,14).Itisthusworthunderstandingwhether
thisradialinformationcanbeusedtoconstrain thedistribution
of the molecular gas along the disc better and thus allow more
accurate determination of the total CO emission. To do this we
appliedanupdatedversionofthetechniqueusedbytheCOLD
GASSsurvey(Saintongeetal.2011)forgalaxiesobservedinthe
centralposition(SCO(CB))andinanadjacentbeam(SCO(adj))
alongthemajoraxis.Followingthistechnique,thetotalCOflux
ofthegalaxyisgivenbytherelation
SCO(CB)
SCO (tot)= ,(14)
SaintongeMB 1.166−3.557× f +3.360× f2
OFF OFF
where f istheratioofthefluxmeasuredinthetwobeams:
OFF
SCO(CB)
f = , (15)
OFF SCO(adj)
where SCO (tot) stands for the total, extrapolated CO
SaintongeMB
flux determined from multiple beam observations. The vari-
able f gives an empirical estimate of the slope of the ra-
OFF
Fig.6.RelationshipbetweentheratioofthetotalCOfluxasde- dial variation of the CO emission. We then compared the to-
terminedfromtheextrapolationofthecentralbeamobservation talCOfluxesdeterminedusingthisrelationwiththeintegrated
to the total value determined from the maps for galaxies with fluxes of Kuno et al. (2007) in Fig. 7. Different and indepen-
integrateddatafromKunoetal.(2007)asafunctionofthefill- dentsetsofdataareavailablefromtheFCRAOsurveyofYoung
ingfactorff,definedasthefractionofthegalaxycoveredbythe et al. (1995), from Stark et al. (1996), from this work, and
central beam (lower panel). Different symbols are used for the fromseveralotherreferencesintheliterature.Table9givesfor
galaxies observed in the FCRAO survey of Young et al. (1995; comparison the mean values of the ratio SCO /SCO and
3D Kuno
cyan),Starketal.(1986;magenta)andotherreferences(black). SCO /SCO for those galaxies in the Kuno et al.
SaintoingeMB Kuno
Filled dots are for galaxies with D (B)/θ ≤ 5, empty symbols (2007) sample with multiple beam observations along the ma-
25
forthoseobjectswithD (B)/θ>5.Thesolidlineshowstheone jor axis. Figure 7 and Table 9 indicate that the CO fluxes ex-
25
ratio,thelongdashedlinesthetypical1σuncertainty(44%)for trapolatedusingeq.(11),hencenottakingtheCOobservations
galaxieswherethebeamsizecoversmorethan10% ofthetotal along the major axis into account, are on average more accu-
opticalsurfaceofthegalaxy,thedottedline1σuncertainty(114 rate than those extrapolated using the multi-beam prescription
%)forgalaxieswherethebeamsizecoverslessthanorequalto of Saintonge et al. (2011). These last, indeed, generally under-
10% oftheirsurface.Theupperpanelshowsthedistributionin estimate the total CO emission of the observed galaxies, while
theareacoveredbythecentralbeamforallHRSgalaxieswith the 3D extrapolation is quite accurate if the size of the galaxy
available CO data (black histogram). The blue histogram gives does not exceed about five times the FWHM of the telescope
the distribution of all galaxies observed by our team, including beam. Most of the galaxies of the HRS, and in particular those
previousdata(Bosellietal.1995;2002,Sautyetal.2003),while observedinthiswork,matchthiscondition.Wethusadopteda
theredoneonlythoseobservedinthiswork. 3D-extrapolation correction also for these objects with two- or
three-beamobservationsalongthemajoraxis.
&Sage(1988)orthatoftheFCRAOteam(Youngetal.1995).
5.5. ThehomogenisedCOdatacatalogue
Whiletheformerusedifferentpolynomialstocombinetheemis-
sioninthedifferentbeams,thelatterusetheradialvariationin Single-beam observations of HRS galaxies are available from
theCOemissiontodeduceaCOprofile(exponential,Gaussian, different references in the literature or from our own observa-
etc.) that is later used to fit and extrapolate the observations. tions, including those presented in this work. Several objects
Morerecently,Saintongeetal.(2011)haveproposedaverysim- have multiple, independent observations. Overall there are 344
pleprescriptionpreviouslydeterminedbysimulatingtheobser- independentpointingson225HRSgalaxies.Wehavecollected
vationthroughtwoadjacentbeams(onecentral,thesecondone allthesedataandlistedtheminTable10,arrangedasfollows
beam off-centre) of the Kuno et al. (2007) sample of mapped
galaxies. The simulations have been done using beams of dif- – Column1:HRSname.
ferent sizes. Multiple beam observations of the HRS galaxies – Column2:TelescopecodedasinTable4.