the molecular gas content of spiral galaxies in the coma...

Astron. Astrophys. 327, 522–538 (1997) ASTRONOMYAND

ASTROPHYSICS

The molecular gas content of spiral galaxiesin the Coma/A1367 supercluster?

A. Boselli1,2,3, G. Gavazzi4,5, J. Lequeux3, V. Buat1,6, F. Casoli3, J. Dickey7, and J. Donas1

1 Laboratoire d’Astronomie Spatiale du CNRS, BP 8, Traverse du Siphon, F-13376 Marseille Cedex 12, France2 Max-Planck-Institut fur Kernphysik, Postfach 103980, D-69117 Heidelberg, Germany3 DEMIRM and URA 336 du CNRS, Observatoire de Paris, 61 Av. de l’Observatoire, F-75014 Paris, France4 Universita degli Studi di Milano, Dipartimento di Fisica, Via Celoria 16, I-20133 Milano, Italy5 Osservatorio Astronomico di Brera, via Brera 28, I-20121 Milano, Italy6 Laboratoire des interactions photons-matieres, faculte des Sciences et Techniques de St. Jerome, F-13397 Marseille Cedex 137 Department of Astronomy, University of Minnesota, 116 Church Street S.E., Minneapolis, MN 55455, USA

Received 23 December 1996 / Accepted 7 May 1997

Abstract. We present 12CO(J=1–0) line observations of 73 spi-ral galaxies mostly in the Coma/A1367 supercluster. From thesedata, combined with data available in the literature, we extractthe first complete, optically selected sample (mpg < 15.2) of 37isolated and of 27 cluster galaxies.

Adopting a standard conversion factorX=N(H2)/I(CO), weestimate that the molecular hydrogen content of isolated spiralgalaxies is, on average, 20% of the atomic hydrogen reservoir,significantly lower than previous estimates based on samplesselected by FIR criteria, thus biased towards CO rich objects.

We show that the frequency distributions of the CO defi-ciency parameter, defined as the difference between the expectedand the observed molecular gas content of a galaxy of given lu-minosity (or linear diameter), computed separately for clusterand isolated galaxies, are not significantly different, indicatingthat the environment does not affect the molecular gas contentof spiral discs.

A well defined relationship exists between Mi(H2) and thestar formation activity in bright galaxies, while it is weaker atlower luminosities. We interpret this finding as indicating thatCO emission traces relatively well the H2 mass only in high-mass galaxies, such as the Milky Way. On the other hand, inlow-mass spirals the higher far–UV radiation field produced byyoung O–B stars and the lower metallicity cause the photodis-sociation of the diffuse molecular gas, weakening the expectedrelationship between star formation and the CO emission. Theconversion factor between the CO line intensity and the amountof molecular hydrogen being ill-determined and variable withthe UV flux and abundances, it is difficult to assess the relation-

Send offprint requests to: Alessandro Boselli? based on observations made with the 12-m National Radio Astro-nomical Observatory, Kitt Peak, Arizona

ship between the star formation and the amount of molecularhydrogen.1

Key words: Galaxies: general – spiral – ISM – intergalacticmedium – star formation – radio lines: galaxies

1. Introduction

It is well established that for star formation to take place it isnecessary that the diffuse neutral hydrogen goes through a phasetransformation into molecular hydrogen during the collapse ofmolecular clouds. To understand this process at galactic scales itis necessary to quantify the amount of gas in the various phasesin galaxies with a wide range of star formation activity. The HIcontent is quite strightforward determined from measurementsof the 21 cm line. The star formation rate can be quantifiedto the first order via Hα emission and UV fluxes using some-what uncertain assumptions on the amount of extinction and onthe IMF. Conversely the amount of molecular hydrogen is esti-mated indirectly from observations of the 12CO(1–0) line, usinga CO-H2 conversion factorX=N(H2)/I(CO), where N(H2) is thecolumn density of H2 and I(CO) the integrated intensity of theCO line. In the solar neighbourhood, X was determined to beX=2.3 1020 mol cm−2 (K km s−1)−1 from COS-B data (Stronget al. 1988), but this value has been recently revised to 1.0 1020

mol cm−2 (K km s−1)−1 from GRO/EGRET data (Digel et al.1996). The latter is in agreement with that found by Guelin etal. (1993) in NGC 891 and by Neininger et al. (1996) in NGC

1 Figures 1 are partly available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html

A. Boselli et al.: The molecular gas content of spiral galaxies in the Coma/A1367 supercluster 523

4565. There is no reason why X should be universal (Polk etal. 1988). From the same EGRET data Digel et al. (1996) findX=2.5 1020 mol cm−2 (K km s−1)−1 in the Perseus arm of ourGalaxy. The inner regions of M31 contain significant amountsof cold molecular hydrogen with a CO emission lower than ex-pected, implying X=2.3 1021 mol cm−2 (K km s−1)−1 (Allen& Lequeux 1993). Moreover in the SMC a value of X as highas 1021 mol cm−2 (K km s−1)−1 has been estimated by Rubio etal. (1993a). In the Galaxy X also differs in the diffuse mediumand in molecular clouds: Polk et al. (1988) suggest X=0.5 1020

mol cm−2 (K km s−1)−1 in the diffuse medium andX=4.2 1020

mol cm−2 (K km s−1)−1 in the clouds.Concerning the surveys of nearby galaxies, the available

samples observed in CO generally suffer from selection biases:they are mostly FIR-selected, thus, as we will show in this paper,biased towards CO–rich galaxies. Such is for example the sam-ple extracted from the FCRAO survey of Young et al. (1995).The only available samples not suffering from this bias are theone of Virgo galaxies brighter than 12.0 Bo

T mag observed in COby Kenney & Young (1988a) and that of Sage (1993). Both arehowever predominantly composed by relatively low-luminositynearby galaxies. To determine the molecular gas content of opti-cally selected bright spiral galaxies, we choose to study the COproperties of a complete sample of normal disc galaxies in theComa/A1367 supercluster selected according to optical criteria.The limiting magnitude combined with the average distance of70 Mpc (for H0 = 100 km s−1 Mpc−1) is such that the samplecontains galaxies of intermediate and high luminosity (-19.2<Mpg<-21.5), with apparent sizes of the order of 1–2 arcmin,well matching the 55 arcseconds beam size of the NRAO 12mtelescope. The sample contains two rich clusters as well as rela-tively isolated objects in the bridge between the two clusters, allat the same distance. This allows us to compare the CO proper-ties of galaxies in a variety of local density environments, freefrom distance-dependent biases.

The sample is described in Sect. 2; in Sects. 3 and 4 theobservational results are presented. The molecular gas proper-ties of spiral galaxies in the various environments, the selectioneffects and the relationships with star formation indicators asfunctions of luminosity are given in Sect. 5. The conclusionsare summarized in Sect. 6. The appendix contains an analysisof the relationship between theX conversion factor and the dif-ferent physical conditions of the ISM.

2. The sample

In this paper we report observations of the 12CO(1–0) line emis-sion of 73 CGCG galaxies (Zwicky et al. 1961-1968); 67 of thembelong to the region 11h30m ≤ RA ≤ 13h30m, 18o ≤ dec ≤32o containing the Coma supercluster, which includes the Comaand the Abell 1367 clusters and relatively isolated galaxies inthe bridge between these two clusters. Of the remaining 6, 3members of the Hercules supercluster and 3 of the Cancer clus-ter were observed as filler objects and will not be used in thefollowing analysis.

Target galaxies are listed in Table 1, arranged as follows:

– Column 1: CGCG designation (Zwicky et al. 1961-1968).The first 3 digits indicate the field number, the last 3 thesource number in the field.

– Column 2 and 3: NGC/IC and UGC names.– Columns 4 and 5: 1950 celestial coordinates with a few

arcsec accuracy, from Gavazzi & Boselli (1996).– Column 6: Morphological type, from Gavazzi & Boselli

(1996).– Column 7: A = member of A1367; C = member of Coma

cluster; I = isolated; G = group; P = pair; N = NGC 5056group; I, G, P and N are all included in the Coma superclusterregion (see Gavazzi 1987 for more details); H51 and H97 =member of A2151 and A2197 in the Hercules supercluster,cA and cD= member of the cloud A and D in the Cancercluster (Bothun et al. 1983).

– Column 8: Heliocentric velocity, in km s−1, from Gavazzi(1989) and references therein.

– Column 9: Distance in Mpc, determined using H0 = 100 kms−1 Mpc−1; we assume 69 Mpc for the Coma cluster and65 Mpc for A1367 (Gavazzi 1987). For galaxies belongingto groups, the distances are determined from their averageredshifts. For double systems or isolated supercluster ob-jects, distances are determined from the individual redshifts.Cloud A Cancer galaxies are taken at 47 Mpc, cloud D at36 Mpc. Galaxies in the Hercules supercluster are assumedat 110 Mpc (A2151), and at 92 Mpc (A2197).

– Columns 10 and 11: Galaxy major and minor optical bluediameters, in arcminutes, from Gavazzi & Boselli (1996).These are isophotal diameters at the 25th mag arcsec−2 de-termined from available CCDs.

– Column 12: Photographic magnitude as given in the CGCG.– Column 13: Width of the HI line (in km s−1) averaged be-

tween 20 and 50% of its maximum, from Gavazzi (1989)and references therein.

42/67 galaxies observed in this work, together with galax-ies with previously published data (Casoli et al. 1991; Dickey& Kazes 1992; Boselli et al. 1995a; Casoli et al. 1996a), con-stitute an optically-selected sample complete to mpg < 15.2 inthe Coma and A 1367 clusters (27 objects), and in the Comasupercluster region selected according to strict isolation criteria(37 objects). The remaining 27 objects not matching the com-pleteness criterium were observed for other projects, such asfor example that of studying the CO properties of blue galax-ies in the Coma region in view of ISO follow-up observations(Gavazzi et al., in preparation). The criterium adopted to definecluster and isolated galaxies is extensively described in Gavazzi(1987) and Gavazzi et al. (1995), and here briefly summarized.Galaxies are assumed to belong to Coma when they have a pro-jected distance R from the cluster core (as determined from theX-ray observations) R< 0.5o and are in the velocity range 4500< vel < 9500 km s−1 or 0.5o < R < 2.0o with 5500 < vel <8000 km s−1; A1367 members are all galaxies with R < 0.5o

in the velocity range 4300 < vel < 8600 km s−1 or 0.5o < R< 1.0o with 5500 < vel < 8000 km s−1. The isolated sample

524 A. Boselli et al.: The molecular gas content of spiral galaxies in the Coma/A1367 supercluster

is composed of all galaxies members of the Coma supercluster(6000 < vel < 8000 km s−1) with the closest companion at adistance > 300 kpc.

The galaxies analysed in this work are spirals of morpho-logical type Sa-Sc and peculiar2: given the limiting magnitude(<15.2) of the present survey, Im galaxies at the distance ofComa are not included in the sample. The analysed galaxies arelisted in Table 3, as described in Sect. 5.

3. The observations

The observations were carried out in four remote-observing runsfrom the Paris Observatory in November 1994, January 1995,May 1995 and June 1996 using the NRAO Kitt Peak 12 mtelescope 3.

At 115 GHz [12CO(1–0)], the telescope half-power beamwidth (HPBW) is 55” which corresponds to 19-17 kpc at theassumed distance of 69-65 Mpc for the Coma/A1367 clusters(for H0 = 100 km s−1 Mpc−1). Weather conditions were good,with typical zenith opacities of 0.10-0.20 (1994 and 1995 runs)and 0.30-0.45 (1996 run). The pointing accuracy was checkedevery 5 hours by broad band continuum observations of Marsand 3C273, with an average error of 7” rms. We used a dual-polarization SIS mixer, with a receiver temperature of aboutTrec=80 K and Tsys=180-300 K (1994 and 1995 runs) and 300-450 K (1996 run) (in T∗R scale) at the elevation of the sources.We used a dual beam-switching procedure, with two symmetricreference positions offset by 4’ in azimuth. The backend is a256 channel spectrometer with channel width of 2 MHz. Each6-minute scan began by a chopper wheel calibration on a load atambient temperature, with a chopper wheel calibration on a coldload every two scans. Galaxies were observed at their nominalcoordinates listed in Table 1, with one position per galaxy. Thetotal integration time was on average 120 minutes on+off (i.e.60 minutes on the source), yielding rms noise level of about 1-2mK (in the T∗R scale) after velocity smoothing to 20 km s−1.The baselines were flat owing to the use of beam-switching,thereby requiring that only linear baselines be subtracted. Theantenna temperature T∗R was corrected for telescope and atmo-spheric losses. In the following analysis we use the main-beambrightness temperature scale, Tmb, with Tmb=T∗R/0.82 (whereηmb=0.56 and ηfss=0.68). This scale is appropriate for sourceswith sizes comparable to, or smaller than the beam size. Thesemain-beam temperatures can be converted into flux densitiesusing 29 Jy/K.

2 The 3 galaxies 128089, 160038 and 160068, which were classifiedSa on the POSS plates at the time of our observations, thus selectedto be observed at Kitt Peak, were subsequently classified S0, S0a andS0 respectively thanks to new CCD images now available (Gavazzi &Boselli 1996).3 The Kitt Peak 12-m telescope is operated by Associated Univer-sities, Inc., under cooperative agreement with the National ScienceFoundation.

4. Results

The 12CO(1–0) spectra of all the observed galaxies, reducedwith the CLASS package (Forveille et al. 1990), are shown inFig. 1 (where only the first 12 spectra are shown: the remaining61 are available in electronic form): the observational resultsare listed in Table 2. Of the 73 observed galaxies 24 were notdetected and 3 tentatively detected. Table 2 is arranged as fol-lows:

– Column 1: CGCG galaxy name (Zwicky et al. 1961-1968).– Column 2: Integration time (on+off), in minutes.– Column 3: rms noise, in mK, in the T∗R scale.– Column 4: Intensity of the I(CO) line (I(CO)=

∫T∗Rdv) in

K km s−1 (area definition)). For undetected galaxies, thereported value is an upper limit determined as follows:

I(CO) = 2σ(∆VHIδVCO)1/2Kkms−1 (1)

where σ is the rms noise of the spectrum, ∆ VHI is the HIline width, and δ VCO is the spectral resolution (for galaxieswith ∆ VHI not available, the HI width has been determinedassuming a standard ∆ VHI = 300 sin(i) km s−1, where iis the galaxy inclination or ∆ VHI = 50 km s−1 if i = 0). δVCO=20 km s−1.

– Column 5: error on the intensity of the CO line, ∆I(CO),computed as:

∆I(CO) = 2σ(∆VCOδVCO)1/2Kkms−1 (2)

where σ is the rms noise of the spectrum, ∆ VCO is theCO linewidth (given in Col. 7), and δ VCO is the spectralresolution. Spectra were smoothed to δ VCO = 20 km s−1.

– Column 6: Heliocentric velocity determined from theCO line (gaussian fit), in km s−1 (optical definitionv=cz=∆λ/λ0). The estimated error is comparable with theresolution, thus 20 km s−1.

– Column 7: Width at 50% of the CO line, in km s−1, with anestimated error of ∼ 20 km s−1.

– Column 8: The “indicative” Mi(H2) mass, determined as inSect. 4.1.

4.1. The molecular hydrogen content

CO measurements are often used to estimate the molecular hy-drogen content of galaxies adopting a standard conversion factorfrom CO intensities to H2 column densities (X). In order to al-low comparison to previous work we convert the CO line fluxesinto indicative H2 masses using the conventional Galactic con-version factorX=2.3 1020 mol cm−2 (K km s−1)−1 of Strong etal. (1988) where I(CO) from Column 4 of Table 2 is convertedinto the Tmb scale. The indicative molecular gas mass is givenby:

Mi(H2) = 68.08×(D/Mpc)2I(CO)(Kkms−1)(θ/1”)2 (3)

where θ is the half-power beam width (HPBW) of the telescopeand D the distance to the source (from Table 1).

Since the galaxies were observed only at the central position,our mass determination should be considered as lower limits


Table 1. The target galaxies

Name NGC/IC UGC RA(1950) dec(1950) type agg vel dist a b mpg ∆VHIh m s o ′ ” kms−1 Mpc ′ ′ kms−1

119016 2545 4287 81118.40 213027.0 Sb cD 3384 36.0 2.13 1.23 13.2 443119057 2565 4334 81652.20 221122.0 Sab cD 3584 36.0 1.83 0.86 13.8 428119109 2595 4422 82447.00 213844.0 Sc cA 4332 47.0 3.33 1.96 13.9 354127018 − − 113708.25 225745.6 Sb I 6935 69.3 0.78 0.74 15.0 162157012 − − 113802.06 290827.1 Sbc I 6814 68.1 0.83 0.70 15.1 18497063 − − 113939.75 201932.9 Pec A 6102 65.0 0.58 0.34 15.7 16497062 − − 113938.88 201511.3 Pec A 7809 65.0 1.01 0.40 15.5 27497072 − − 114009.43 201832.6 Sa A 6334 65.0 1.21 0.54 15.0 31097082 2951 6688 114048.93 200138.3 Sa A 6100 65.0 1.30 0.64 15.0 −97092 − − 114122.52 202746.3 Sbc A 6373 65.0 0.76 0.54 15.5 45597093 − − 114126.30 200342.9 Pec A 4857 65.0 0.96 0.39 15.5 41697102N − − 114141.50 203002.1 Sa A 6361 65.0 1.08 0.65 15.1 33697114 − − 114212.24 200302.8 Pec A 8522 65.0 0.54 0.49 15.4 −97121 − 6719 114211.67 202408.1 Sab A 6573 65.0 1.23 0.83 14.6 40497129W 3861 6724 114228.43 201502.7 Sb A 5082 65.0 2.36 1.27 14.0 48997130 3864 − 114240.28 194011.2 Sa A 6697 65.0 0.70 0.54 15.5 −97133E − − 114242.17 201749.8 Pec A 5365 65.0 0.89 0.56 16.5 27697138 − − 114309.38 201831.1 Pec A 5317 65.0 0.75 0.64 15.5 160127052 3884 6746 114336.73 204010.8 Sa A 6968 65.0 2.04 1.44 14.0 534127054 3883 6754 114411.78 205710.7 Sa G 7026 68.2 2.60 2.26 14.2 228157064 3984 6943 115517.50 291903.6 Sbc I 6407 64.1 1.41 0.95 14.8 10298013 − − 115602.25 190827.5 Sc I 6949 69.5 0.89 0.49 15.1 336128003 − − 120053.81 222916.8 Pec I 6435 64.3 0.97 0.72 14.6 331158009N − 7064 120210.10 312720.4 Sb P 7494 74.9 1.01 0.98 14.0 13398058 4110 7102 120430.25 184834.8 Sbc I 7207 72.1 1.36 0.73 14.7 499158036 4146 7163 120745.81 264231.6 Sb I 6532 65.3 1.22 1.19 13.8 210128049 − − 120802.31 261222.1 Sc I 6445 64.4 1.17 0.71 15.0 29098116 − 7263 121253.00 193411.4 Sc I 6229 62.3 1.04 0.79 14.9 200158054 − − 121323.37 265623.7 Pec I 7685 76.8 0.81 0.79 14.6 194128073 3122 7341 121550.19 252940.8 Sb I 6949 69.5 1.53 0.77 14.7 311158081 − 7395 121757.44 312656.7 Pec I 6734 67.3 0.50 0.44 14.5 130128080 − − 121824.75 245645.9 Sb I 7349 73.5 0.68 0.52 15.0 251158105 3330 7527 122326.94 310712.7 Sbc I 6824 68.2 1.20 0.57 15.1 393128089 791 7555 122428.81 225457.5 S0 I 6841 68.4 1.16 1.10 14.2 85159008 4475 7632 122718.00 273110.4 Sb I 7388 73.9 1.70 1.02 14.6 350159033 3598 7791 123452.94 282858.8 Sa I 7673 76.7 1.52 0.49 15.0 530159059 − 7890 124038.31 275916.7 Sbc I 7528 75.3 0.77 0.54 14.5 286159061 − − 124047.69 312133.2 Sbc I 6966 69.7 1.09 1.02 14.8 332129021 − − 124237.94 212632.5 S.. I 6697 67.0 0.33 0.32 15.3 314129022 813 7928 124243.50 231833.1 Sb I 6972 69.7 1.10 1.00 14.4 353159076 821 7957 124500.44 300337.3 Sbc I 6743 67.4 1.34 1.31 14.5 231159095 826 − 124855.00 311950.3 Sbc I 6837 68.3 0.77 0.58 14.9 215159096 − 8004 124913.25 313728.7 Sc I 6187 61.9 1.67 0.62 15.1 313159104 − − 125050.00 272157.0 S0 C 6154 69.0 1.05 0.71 15.0 −160005 − 8025 125137.50 295226.0 Sb I 6316 63.2 1.87 0.43 14.8 528160020 − − 125340.69 275653.6 Pec C 4968 69.0 0.66 0.32 15.5 288160038 − 8069 125446.60 291855.0 S0a C 7472 69.0 1.18 0.53 14.8 −160058 − − 125544.81 285841.0 Sbc C 7609 69.0 1.24 0.42 15.5 295160068 4853 8092 125610.10 275204.0 S0 C 7550 69.0 0.98 0.77 14.2 −160064 − − 125610.19 273202.6 Pec C 7368 69.0 0.64 0.58 15.4 150160067 − − 125612.00 272647.0 Pec C 7653 69.0 0.56 0.52 15.4 217160073 − − 125640.34 275449.2 Pec C 5554 69.0 0.79 0.70 15.1 341160081 4892 8108 125738.69 271001.1 Sa C 5898 69.0 1.56 0.45 14.7 −160086 − − 125808.87 275423.1 Pec C 7476 69.0 0.75 0.54 15.4 279160088 842 8118 125815.47 291719.9 Sb C 7287 69.0 1.39 0.63 14.6 396160257 4907 − 125824.37 282537.8 Sa C 5868 69.0 1.13 1.08 14.6 −160098 − − 125900.87 285712.0 Pec C 8762 69.0 0.70 0.64 15.3 158160095 4921 8134 125901.50 280917.3 Sb C 5450 69.0 2.28 2.23 13.7 231160102 4088 8140 125919.51 291848.0 Sab C 7108 69.0 1.78 0.60 14.8 510160107 − − 125940.30 293120.0 Sa C 7246 69.0 1.13 0.27 14.9 −160128 − − 130158.90 290443.0 Pec C 8066 69.0 0.63 0.62 15.3 −160127 − − 130202.19 273421.1 Sc C 5523 69.0 0.95 0.64 15.5 195160139 − − 130414.62 290700.8 Pec C 4748 69.0 1.22 0.64 15.0 211160148 − 8229 130630.87 282701.1 Sa N 5983 59.8 1.47 1.07 14.3 364160168 5041 8319 131211.25 305812.3 Sb I 7476 74.8 1.50 1.43 14.2 295160182 − 8359 131522.81 274959.7 Sab I 6994 69.9 1.27 0.67 15.0 396160192 5081 8366 131646.56 284606.8 Sab I 6656 66.6 2.38 0.82 14.3 546101043 − 8437 132254.62 184243.3 Sa I 6677 66.8 1.36 0.30 15.0 −101054 5158 8459 132521.31 180213.1 Sb I 6606 66.1 1.24 1.08 13.8 162161073 − 8498 132807.87 315243.2 Pec P 7320 73.2 1.92 0.82 14.2 636108036 − 10121 155731.00 185627.0 Sbc H51 8827 110.0 1.07 0.64 14.6 383108148 6062 10202 160410.30 195445.0 Sc H51 11722 110.0 1.23 0.80 14.4 407224038 − 10407 162648.33 411941.4 Pec H97 8446 92.0 0.77 0.70 14.3 150


Fig. 1. 12CO(1–0) line spectra (smoothed to δVCO=20 km s−1) of the first 12 observed galaxies (in order of increasing Zwicky name) in theT∗R scale. The horizontal line indicates the HI line width listed in Table 1. The ramaining spectra are available in electronic form.


Table 2. Results of the observations, in the T∗R scale.

Name int− time rmsI(CO) ∆I(CO) V (CO) ∆VCO logMi(H2)min(on + off )mKKkms−1Kkms−1kms−1kms−1M

119016 276 1.35 1.08 0.19 3330 391 8.55119057 138 2.12 < 0.39 0.39 − − < 8.11119109 84 2.83 1.71 0.45 4366 310 8.98127018 168 2.07 0.85 0.21 6923 123 9.01157012 330 1.12 0.38 0.09 6862 85 8.6597063 90 1.50 < 0.17 0.17 − − < 8.2697062 120 1.58 < 0.23 0.23 − − < 8.3997072 96 1.58 1.03 0.23 6343 267 9.0497082 162 1.23 0.45 0.20 6057 329 8.6897092 108 1.36 0.63 0.14 6470 126 8.8397093 138 1.12 0.29 0.14 4909 184 8.4997102N 114 1.61 < 0.26 0.26 − − < 8.4597114 216 1.06 0.37 0.13 8475 184 8.5997121 240 0.99 0.61 0.17 6573 375 8.8197129W 366 1.18 1.04 0.25 5159 561 9.0497130 222 1.45 < 0.18 0.18 − − < 8.2897133E 180 1.27 0.38 0.22 5397 392 8.6197138 102 1.33 < 0.15 0.15 − − 8.20127052 168 0.99 0.89 0.18 6973 420 8.98127054 168 1.14 0.55 0.14 7015 195 8.81157064 162 2.00 0.74 0.12 6404 42 8.8898013 198 1.09 0.73 0.16 6887 268 8.95128003 66 1.34 0.84 0.19 6453 255 8.94158009N 24 4.00 2.03 0.37 7492 110 9.4698058 156 1.01 0.99 0.18 7231 420 9.11158036 186 1.16 0.44 0.13 6557 170 8.67128049 276 1.28 0.60 0.18 6473 246 8.8098116 180 1.28 0.61 0.14 6230 144 8.77158054 138 1.17 0.32 0.08 7663 60 8.68128073 144 1.43 0.88 0.23 6900 315 9.03158081 72 1.61 < 0.16 0.16 − − < 8.28128080 138 1.89 0.81 0.24 7331 208 9.04158105 210 1.50 0.44 0.25 − 357 8.71128089 120 1.57 0.46 0.09 6848 40 8.73159008 150 1.13 1.01 0.22 7439 490 9.14159033 168 1.30 < 0.28 0.28 − − < 8.60159059 102 1.34 0.98 0.24 7532 395 9.15159061 240 1.37 < 0.22 0.22 − − < 8.43129021 42 3.38 < 0.54 0.54 − − < 8.78129022 114 1.13 0.78 0.15 6878 230 8.98159076 72 2.03 0.81 0.26 6764 205 8.97159095 282 1.26 < 0.16 0.16 − − < 8.29159096 162 1.67 < 0.26 0.26 − − < 8.40159104 132 1.24 < 0.08 0.08 − − < 7.96160005 180 1.55 < 0.32 0.32 − − < 8.50160020 162 1.24 0.24 0.18 4941 149 8.14160038 114 1.38 < 0.21 0.21 − − < 8.40160058 78 1.19 0.99 0.20 − 267 9.07160068 114 1.28 < 0.08 0.08 − − < 7.99160064 102 1.35 < 0.15 0.15 − − < 8.25160067 144 1.17 0.62 0.15 7653 306 8.87160073 156 1.42 0.47 0.23 5445 82 8.75160081 162 1.38 < 0.22 0.22 − − < 8.41160086 120 1.07 0.06 0.16 − 63 7.86160088 186 1.50 1.05 0.24 7233 315 9.10160257 264 1.16 0.61 0.17 5795 268 8.86160098 102 1.29 0.64 0.14 8762 114 8.88160095 102 1.78 1.63 0.21 5503 170 9.29160102 294 1.02 0.53 0.20 − 497 8.80160107 156 1.51 < 0.24 0.24 − − < 8.45160128 174 0.99 0.33 0.17 − 349 8.60160127 72 1.63 < 0.20 0.20 − − < 8.39160139 114 1.51 < 0.20 0.20 − − < 8.04160148 162 1.64 < 0.28 0.28 − − < 8.41160168 84 1.69 1.47 0.26 7472 290 9.31160182 258 1.78 1.04 0.32 7055 416 9.11160192 132 1.19 1.64 0.27 6692 650 9.26101043 96 1.49 < 0.23 0.23 − − < 8.41101054 114 2.18 0.49 0.10 6652 110 8.73161073 174 1.09 1.42 0.24 7312 625 9.28108036 42 2.44 0.90 0.40 8778 335 9.44108148 72 2.10 < 0.38 0.38 − − < 9.07224038 60 1.56 0.78 0.18 8431 170 9.22

of the total Mi(H2). However since the angular sizes of theobserved galaxies generally do not exceed the adopted beamby more than a factor of two and since the CO distribution inspiral galaxies is centrally peaked, generally with an exponentialdistribution with a scale length ≈ 1.5 smaller than the opticalone (Young et al. 1995; Xu et al. 1997), the present data shouldgive accurate measurements of the total CO emission.

4.2. Comparison with other observations

Four target galaxies were previously observed using the Onsala20m radiotelescope (HPBW of 34” at 115 GHz; Boselli et al.1995a) and 3 using the IRAM 30m telescope (HPBW of 22”).In spite of the different beam sizes of the telescopes, the two setsof CO intensity determinations are consistent within a factor of2.

CGCG 97129W: detected at IRAM with I(CO)=4.77 K kms−1 (Dickey & Kazes 1992; Tmb scale) and at Kitt Peak withI(CO)=1.04 K km s−1, yielding Mi(H2)=6.64 108 M at IRAMand Mi(H2)=1.10 109 M at Kitt Peak.

CGCG 158009N: detected at Onsala with I(CO)=5.6 K km s−1

(Tmb scale) and at Kitt Peak with I(CO)=2.03 K km s−1, yieldingMi(H2)=2.47 109 M at Onsala and Mi(H2)=2.88 109 M atKitt Peak.

CGCG 158054: undetected at Onsala with I(CO)<1.2 K km s−1

(Tmb scale) and detected at Kitt Peak with I(CO)=0.32 K kms−1, yielding Mi(H2)<5.57 108 M and Mi(H2)=4.79 108 Mrespectively.

CGCG 160102: Casoli et al. (1991) did not detected this galaxywith the IRAM 30m telescope due to a pointing error of 1 arcminto the north. The object was detected at Kitt Peak.

CGCG 160148: undetected at Onsala (I(CO)<2.4 K km s−1;Tmb scale) and at Kitt Peak (I(CO)<0.28 K km s−1). The lowerlimits to the molecular hydrogen mass are Mi(H2)<6.75 108

M and Mi(H2)<2.57 108 M respectively.

CGCG 160257: detected at IRAM with I(CO)=2.03 K kms−1 (Tmb scale; Casoli et al. 1996) and at Kitt Peak withI(CO)=0.61 K km s−1, yielding Mi(H2)=3.18 108 M at IRAMand Mi(H2)=7.24 108 M at Kitt Peak.

CGCG 224038: detected at Onsala with I(CO)=3.1 K km s−1

(Tmb scale) and at Kitt Peak I(CO)=0.78 K km s−1, yieldingMi(H2)=2.04 109 M and Mi(H2)=1.66 109 M respectively.

When multiple observations are available, the Kitt Peak val-ues are used in the following analysis.

5. Discussion

The sample in Table 3 is ideal for analysing the general molecu-lar gas properties of spiral galaxies since it is not biased towardsFIR bright galaxies, and is composed of cluster and isolated spi-ral galaxies all at the same distance (62 < dist < 76 Mpc, seeTable 3), thus avoiding possible distance-dependent biases.

In order to remove the first order dependence on the galaxysize or luminosity, we normalize Mi(H2) and, when appropriate,


the SFR indicators using the near–infrared (H band) luminosityLH taken from Gavazzi & Boselli (1996), or using the opticalsurface (hybrid surface densities). The H luminosity LH , in unitsof LH, is determined adopting the relation log LH=11.36–0.4H+2logD, and assuming MH ()=3.39 (Wamsteker 1981);H is the H magnitude andD is the distance in Mpc. LH is a goodindicator of galaxy total mass as discussed in Gavazzi (1993)and Gavazzi et al. (1996), who found a linear proportionalitybetween LH and the dynamical mass within the optical radiusof an optically selected sample of galaxies. When possible, weprefer to normalize to the mass rather than to the optical surfaceof galaxies because this second normalization would determinehybrid surface densities.

The parameters of the Coma/A1367 optically–selected (<15.2 mpg) sample used in the following analysis are listed inTable 3, as follows:

– Column 1: galaxy CGCG name.– Column 2: morphological type, from Gavazzi & Boselli

(1996).– Column 3: cluster membership, according to Gavazzi

(1987), referred to “aggregation” criterium, as in Table 1(see Sect. 2).

– Column 4: cluster membership assigned according to thestatistical method described in Gavazzi et al. (1995), basedon dynamical considerations (hereafter referred as the“caustic” criterium). If W is the weighting function definedin Gavazzi et al. (1995) for Ω0 = 0.5, indicating the proba-bility to a galaxy to reside within the caustic (thus the prob-ability to be dynamically related to the cluster), we assumecluster members those objects with W > 0.80 and isolatedgalaxies those with W < 0.80. As discussed in Gavazzi etal. (1995), the “caustic” criterium is less restrictive than the“aggregation” criterium since it includes in the cluster sam-ple some objects at larger angular distances.

– Columns 5 and 6: galaxy major and minor optical blue diam-eters, in arcminutes, from Gavazzi & Boselli (1996). Theseare isophotal diameters at the 25th mag arcsec−2 isophote.

– Column 7: assumed distance of the galaxy in Mpc.– Column 8: corrected H magnitude, from Gavazzi & Boselli

(1996).– Column 9: corrected B magnitude, from Gavazzi & Boselli

(1996).– Column 10: uncorrected UV 2000 A magnitude, from Donas

et al. (1990; 1995).– Column 11: logarithm of the atomic hydrogen mass in M,

determined as in Gavazzi (1987).– Column 12: HI deficiency, determined as in Haynes & Gio-

vanelli (1984).– Column 13: logarithm of the indicative molecular hydrogen

content in M, determined as described in Sect. 4.1.– Column 14: references to the CO data: 1) this work, 1994

run; 2) this work, 1995 run; 3) this work, 1996 run; 4) Boselliet al. (1994); 5) Boselli et al. (1995a); 6) Casoli et al. (1991);7) Dickey & Kazes (1992); 8) Casoli et al. (1996a).

– Column 15: ratio r of the optical diameter to the HPBWof the telescope used in the CO observations; m indicates

Fig. 2. the logarithm of Mi(H2)/LH in different morphological classes.Empty symbols are for CO undetected galaxies; the mean value in eachmorphological class is marked with + for cluster objects and with ×for the isolated galaxies.

mapped objects; galaxies marked ∗ have been observed alsowith the 12m Kitt Peak telescope (HPBW = 55”; Casoli etal. 1996a), with consistent results. r gives a raw estimate ofthe CO sampling of the target galaxy. Given the peaked COdistribution compared with the optical one (see Sect. 4.1),the observations should give accurate measurement of thetotal CO emission of galaxies with r < 2, while they shouldunderestimate by a factor of < 2 the total CO emission ofobjects with r = 4 (Sauty et al., in preparation).

– Column 16: Hα E.W., in A, from Gavazzi et al. (1991)and references therein. An ∗ indicates galaxies recently ob-served, for which only preliminary data (used in the follow-ing analysis) are available; these Hα data will be presentedin a forecoming paper (Gavazzi et al., 1997).

– Column 17: (U-B)c corrected colour, from Gavazzi &Boselli (1996).

– Column 18: the ratio FIR/H, in logarithmic scale, deter-mined from the 60 µm FIR flux (in mJy) and the H flux (inmJy), defined as H flux=1.030 106 10−H/2.5 mJy.

5.1. The molecular gas content of spiral galaxies

The Coma supercluster galaxies have indicative molecular gasmasses of the order of 109 solar masses, ranging from 108 to afew 109 M. In order to remove the first order dependence ofMi(H2) on the galaxy size or luminosity, we plot in Fig. 2 thelogarithm of Mi(H2)/LH versus the morphological type; a weaktrend is observed, indicating that early-type spirals have, on theaverage, a lower indicative molecular gas content (per unit mass)than late-type galaxies. For isolated galaxies, however, no trendwith morphological type is observed.

Fig. 3 shows the correlation between the logarithm ofMi(H2)/LH and the mass of galaxies, as traced by the H lu-minosity (Gavazzi et al. 1996) for isolated galaxies (“caustic”


Table 3. Parameters used in the analysis of the optically selected sample galaxies.

Name Type agg W 05 a b dist Hc Bc UV M (HI) HIdef Mi(H2) Ref r Hα U − B FIR/H′ ′ Mpc M M A

97026 Pec G 1.00 0.79 0.49 71.3 11.40 14.09 − 9.76 −0.50 9.10 8 0.86 88 −0.26 1.91127018 Sb I 0.25 0.78 0.74 69.3 12.06 15.11 − − − 9.01 3 0.85 16 0.07 < 1.23157012 Sbc I 0.00 0.83 0.70 68.1 12.58 14.81 − 9.48 −0.23 8.65 3 0.91 28 −0.14 −97068 Sbc A 1.00 1.23 0.76 65.0 11.19 14.36 14.2 9.60 −0.07 9.57 7 m 44 0.12 1.7997072 Sa A 1.00 1.21 0.54 65.0 11.21 14.48 15.1 8.70 0.70 9.04 2 1.32 5 0.21 1.1097082 Sa A 1.00 1.30 0.64 65.0 10.18 14.57 − < 8.38 > 1.06 8.68 2 1.54 ∗ 0.38 < 0.53127038 Sc I 0.47 1.91 1.55 69.1 10.37 13.62 − 10.11 −0.20 9.46 8 2.08 16 0.12 1.1197087 Pec A 1.00 2.00 0.50 65.0 10.71 12.90 12.7 9.54 0.36 9.53 5 m 74 −0.48 1.5897091 Sbc A 1.00 1.12 0.81 65.0 11.13 14.53 14.5 9.51 −0.06 8.83 6 3.05 23 0.05 1.4297102N Sa A 1.00 1.08 0.65 65.0 11.03 14.75 15.3 8.91 0.41 < 8.45 2 1.20 ∗ − < 0.8597121 Sab A 1.00 1.23 0.83 65.0 10.56 14.24 15.0 9.15 0.26 8.81 1 1.34 −1 0.28 < 0.6697120 Sa A 1.00 1.32 0.85 65.0 10.60 14.04 15.6 8.50 0.96 9.21 8 1.45 4 0.39 1.1697122 Pec A 1.00 1.45 0.47 65.0 11.26 13.78 15.1 9.30 0.35 8.92 8 2.56∗ 46 −0.06 1.5797129W Sb A 1.00 2.36 1.27 65.0 9.87 13.24 13.6 9.79 0.08 9.04 3 2.57 ∗ 0.22 0.66127052 Sa A 1.00 1.70 1.26 65.0 9.71 13.48 14.4 9.46 0.18 8.98 1 2.22 ∗ 0.52 < 0.31127054 Sb G 1.00 3.10 2.80 68.2 10.31 13.52 13.4 10.11 −0.01 8.81 1 3.38 ∗ 0.44 0.51157035 Sb I 0.00 2.12 1.57 62.8 10.15 13.31 − 9.93 −0.16 9.77 7 m 19 0.05 < 1.21127095 Sc G 0.99 1.69 1.23 68.2 10.32 13.57 − 9.76 0.04 9.30 8 1.84 15 0.15 1.02127099 Sc P 0.99 1.34 0.67 64.5 10.73 13.83 − 9.41 0.17 9.17 7 3.65 0 0.09 1.37157064 Sbc I 0.00 1.41 0.95 64.1 11.60 14.22 − 9.59 0.03 8.88 3 1.54 ∗ − 1.2098013 Sc I 0.03 0.89 0.49 69.5 11.76 14.48 − 9.40 −0.07 8.95 3 0.92 0 − < 1.43128003 Pec I 0.00 0.97 0.72 64.3 11.63 14.35 − 9.44 −0.10 8.94 1 1.06 41 −0.23 1.5198058 Sbc I 0.00 1.36 0.73 72.1 10.57 14.15 − 9.70 −0.03 9.11 1 1.48 ∗ 0.31 < 0.68158036 Sb I 0.00 1.22 1.19 65.3 10.34 14.04 − 9.71 −0.31 8.67 1 1.33 ∗ 0.31 < 0.67128049 Sc I 0.00 1.17 0.71 64.4 11.35 14.54 − 8.96 0.53 8.80 3 1.28 20 0.26 1.1198116 Sc I 0.00 1.04 0.79 62.3 11.92 14.51 13.4 9.66 −0.30 8.77 3 1.13 ∗ −0.12 1.47158054 Pec I 0.00 0.90 0.60 76.8 12.07 14.29 − 9.51 −0.10 8.68 1 0.88 72 −0.60 1.99128073 Sb I 0.00 1.53 0.77 69.5 11.16 13.99 − 9.76 −0.15 9.03 1 1.67 ∗ 0.05 0.86158081 Pec I 0.00 0.50 0.44 67.3 12.09 14.94 − 9.01 −0.14 < 8.27 1 0.55 24 −0.22 1.48128080 Sb I 0.00 0.68 0.52 73.5 11.75 14.79 − 9.13 −0.05 9.04 3 0.74 24 −0.04 1.36158105 Sbc I 0.00 1.20 0.57 68.2 11.34 14.34 − 9.73 −0.19 8.71 3 1.31 ∗ 0.11 1.01128089 Sa I 0.00 1.10 1.10 68.4 10.85 14.53 − 8.98 0.39 8.73 1 1.20 ∗ 0.48 < 0.74159008 Sb I 0.00 1.70 1.02 73.9 11.04 14.08 − 9.69 0.04 9.14 1 1.85 ∗ 0.13 < 0.86159033 Sa I 0.00 1.52 0.49 76.7 10.75 14.30 − 9.38 0.30 < 8.60 3 1.66 ∗ 0.62 < 0.79129020 Sb I 0.00 1.03 0.62 65.8 10.87 14.51 − 9.25 0.04 < 8.54 7 2.81 ∗ 0.52 < 0.79159059 Sab I 0.28 0.85 0.62 75.3 12.30 14.58 − 9.51 −0.26 9.14 1 0.93 ∗ −0.10 1.54159061 Sbc I 1.00 1.09 1.02 69.7 11.31 14.96 − 9.24 0.24 < 8.43 3 1.19 ∗ 0.53 < 0.93100005 Pec I 0.00 1.34 0.54 66.1 10.59 13.72 − 9.12 0.48 9.19 7 3.65 19 0.02 1.58129022 Sab I 0.00 1.35 1.00 69.7 10.62 14.10 − 9.73 −0.20 8.98 1 1.20 5 0.27 < 0.67159072N Pec P 0.95 2.20 0.40 66.3 10.42 12.93 − 9.84 0.14 9.25 6 6.00 18 0.08 1.75159072S Pec P 0.88 1.70 0.70 65.9 10.69 13.54 − 9.78 0.00 9.05 6 4.64 12 0.29 −159076 Sbc I 1.00 1.34 1.31 67.4 11.12 14.32 − 9.26 0.36 8.97 1 1.46 15 0.03 0.98159091 S.. P 1.00 0.97 0.69 64.6 11.62 14.81 − 8.95 0.39 9.09 6 2.64 6 0.15 1.40159095 Sbc I 1.00 0.77 0.58 68.4 11.26 15.02 − 9.39 −0.18 < 8.29 3 0.84 ∗ 0.33 < 0.93159096 Sc I 0.11 1.67 0.62 61.9 11.85 14.08 − 9.60 0.12 < 8.40 3 1.82 ∗ − < 1.22159102 Sab C 1.00 1.28 0.51 69.0 10.45 13.84 − 9.57 −0.09 9.53 8 1.48 ∗ 0.13 1.55159104 S.. C 0.98 1.19 0.80 69.0 10.94 14.70 − < 8.63 > 0.92 < 8.30 2 1.30 − 0.57 < 0.81160005 Sb I 0.99 1.87 0.43 63.2 10.24 13.86 − 9.78 −0.09 < 8.50 3 2.04 ∗ − 0.37160025 Sa C 1.00 1.19 0.86 69.0 10.32 13.99 − < 8.46 > 0.97 < 8.02 6 3.24 ∗ 0.43 0.39160032 Sb C 0.97 0.87 0.83 69.0 11.71 14.89 − < 8.54 > 0.66 8.58 6 2.64 ∗ 0.44 1.02160055 Pec C 1.00 1.51 0.58 69.0 10.78 13.52 13.8 9.05 0.67 9.19 6 4.11 33 −0.11 1.52160212 Sa C 1.00 1.22 0.37 69.0 11.08 14.40 15.7 < 8.42 > 1.03 9.06 6 4.06 −1 0.22 0.99160073 Pec C 1.00 0.79 0.70 69.0 12.59 15.09 15.0 8.58 0.65 8.75 2 0.89 ∗ −0.06 1.58160081 Sb C 0.99 1.90 0.32 69.0 10.60 13.62 − < 8.57 > 1.19 < 8.41 1 1.70 ∗ 0.27 < 0.78160252 Pec C 1.00 0.85 0.36 69.0 12.08 14.52 14.8 8.72 0.57 9.13 6 2.32∗ ∗ −0.36 2.03160088 Sb C 1.00 1.12 0.64 69.0 11.06 14.33 15.2 9.07 0.32 9.10 1 1.52 ∗ 0.21 0.85160257 Sa C 1.00 1.13 1.08 69.0 10.72 14.39 15.4 < 8.52 > 0.87 8.86 3 1.23 6 0.52 < 0.69160260 Sa C 1.00 1.89 1.50 69.0 10.00 13.48 14.7 8.86 0.90 9.44 8 1.49 ∗ 0.35 0.90160095 Sb C 1.00 2.28 2.23 69.0 9.45 13.42 14.9 9.03 0.86 9.29 1 2.49 ∗ 0.52 0.13160102 Sab C 1.00 1.78 0.60 69.0 10.78 13.99 15.2 9.67 0.05 8.80 3 1.94 6 0.16 0.92160106 Pec C 1.00 0.89 0.54 69.0 12.04 14.83 15.6 < 8.50 > 0.82 9.03 6 1.23 20 −0.01 1.69130006 Sbc I 1.00 0.68 0.61 65.2 12.09 14.87 − 9.08 −0.01 8.73 8 0.74 ∗ − 1.37130008 Sc I 0.99 0.57 0.49 72.7 11.94 14.91 − 9.48 −0.46 < 8.74 8 0.62 49 −0.18 1.77160137 Sa C 1.00 1.33 0.68 69.0 10.42 13.56 − 9.51 0.01 9.29 8 1.45 11 0.06 1.07160139 Pec C 0.09 1.22 0.64 47.0 13.07 14.61 14.0 9.35 −0.08 < 8.04 2 1.33 ∗ −0.45 1.82130014 Sbc I 0.99 0.92 0.70 71.0 11.48 14.66 − 9.37 0.00 < 8.81 7 2.51 20 0.17 1.63160168 Sc I 0.10 1.39 1.18 74.8 11.05 14.07 − 10.11 −0.39 9.31 1 1.64 ∗ 0.12 0.97160182 Sab I 1.00 1.27 0.67 69.9 11.24 14.40 − 9.37 0.12 9.11 3 1.38 12 0.13 1.12160192 Sb I 0.93 2.06 0.90 66.6 10.23 13.47 − 9.88 −0.08 9.26 1 2.60 ∗ 0.55 0.44101043 Sa I 0.00 1.36 0.30 66.8 10.63 14.17 − < 8.49 > 1.01 < 8.41 3 1.48 ∗ 0.64 < 0.77161052 Pec I 0.00 0.30 0.30 70.7 12.40 15.22 − 9.21 −0.70 < 8.81 4 0.53 48 −0.24 1.77101054 Sab I 0.00 1.40 1.30 66.1 10.63 14.02 − 9.67 −0.16 8.73 1 1.35 ∗ 0.19 0.83


Fig. 3. The relationship between the Mi(H2)/LH ratio (in logarithmicscale, solar units) and the mass of galaxies as traced by the H lumi-nosity (in solar units) for isolated spirals. Empty symbols are for COundetected objects.

definition). The figure shows a weak trend between the fractionof molecular gas and the mass of the galaxy, with low mass ob-jects richer in H2 than massive galaxies. The linear regressionanalysis, done using only detected galaxies, gives the followingresult:

logMi(H2)/LH = 3.89(±0.26)− 0.52(±0.21)× logLH (4)

In Fig. 4 we plot the distribution of the Mi(H2)/M(HI) ratiofor the subsample of isolated galaxies, in order to exclude the HIdeficient cluster objects. In spite of the poor statistics, it is clearthat, as already found by Casoli et al. (1996b), the moleculargas fraction in spiral galaxies is only about 20% of the atomicgas (if the assumed value of X is correct), independent on themorphological type and mass. This fraction would drop to lessthan 10% if we were to use the revised value of X from Digelet al. (1996). This result differs from that of Young & Scov-ille (1991) based on a FIR-selected sample, who found that forSa–Sc galaxies M(HI)∼Mi(H2) (assuming a conversion factorX=2.8 1020 mol cm−2 (K km s−1)−1). The small range in mor-phological type among the isolated sample (Sa-Sc) prevents usto see whether the Mi(H2)/M(HI) ratio is morphological typedependent.

Using the survival analysis in order to take into account theupper limits (Isobe et al. 1986), we obtain for isolated galaxies amean value of logMi(H2)/LH = -1.64± 0.35 (M/LH), where± 0.35 is the standard deviation, and Mi(H2)/LB = -1.26± 0.28(M/LB), whereLB=12.19–0.4Bc+2logD,Bc being the cor-rected B magnitude (see Table 5). These values should be com-pared with the one obtained for the FCRAO survey, Mi(H2)/LB= -1.00 (M/LB), by Young et al. (1989) (also determinedassuming X=2.3 1020 mol cm−2 (K km s−1)−1).

Since the CO emission of only 4 galaxies out of 27 in-cluded in the reference sample might be underestimated (op-tical diameter over beam ratio r >2.2; see Table 3), the differ-

Fig. 4. The relationship between the Mi(H2)/M(HI) ratio (in loga-rithmic scale) and the morphological type for isolated spiral galaxies.Empty symbols are for CO undetected objects

ent Mi(H2)/M(HI) and Mi(H2)/LB ratios observed between oursample and that of Young & Scoville (1991) are real, and areprobably due to two different sample selection criteria (see nextsection).

5.2. The molecular gas content of optically and FIR selectedsamples of spiral galaxies

Most CO surveys of spiral galaxies are based on FIR (Ca-soli et al. 1996a), optical (Stark et al. 1986; Kenney & Young1988a; Braine et al. 1993) or mixed FIR-optical selection cri-teria (Young et al. 1989; 1995; Solomon & Sage 1988), an ex-ception being the volume limited sample of Sage (1993). Thesample analysed in this work is purely optically selected and itincludes galaxies at the same distance. Thus a selection on flux(or magnitude) corresponds to a selection on luminosity.

No correlation is observed between the Mi(H2)/LH ratio andthe optical B magnitude or luminosity. Optically bright galaxies(B magnitude< 14.30) have on average a CO emission per unitmass, B luminosity or area comparable or slightly weaker thanoptically weak sources (B magnitude > 14.30).

Conversely there is a clear correlation between the molecu-lar gas content per unit mass of galaxies and their FIR flux, withthe FIR bright objects generally richer in molecular gas than theFIR weak sources, as shown in Fig. 5.

Dividing the sample in two subsamples of FIR bright andFIR weak sources (using an arbitrary threshold of S60µm =0.35Jy to include a comparable number of objects in the 2 subsam-ples), the mean values and the distributions of the logarithm ofthe ratios Mi(H2)/LH , Mi(H2)/LB and Mi(H2)/πr2 (determinedusing the survival analysis to take into account upper limits) aresignificantly different (see Table 4).

We conclude that samples selected according to FIR fluxesfavour CO bright galaxies. Thus previous works aimed at de-termining the average molecular gas content of “normal” spiral


Table 4. Mean values of the normalized molecular gas content (in logarithmic scale) for FIR bright (flux at 60 µm >0.35 Jy), FIR weak (flux at60 µm <0.35 Jy), optically bright (Bc <14.30 mag) and weak (Bc > 14.30 mag) galaxies determined using a survival analysis in order to takeinto account upper limits.

sample logMi(H2)/LH logMi(H2)/LB logMi(H2)/πr2

units M /LH M /LB M /kpc2

FIR bright mean −1.49 −1.14 6.43std(median) 0.27 0.25 0.27

FIR weak mean −2.00 −1.48 5.98std(median) 0.55 0.47 0.44

optical bright mean −1.78 −1.35 6.14std(median) 0.41 0.38 0.40

optical weak mean −1.65 −1.21 6.30std(median) 0.44 0.29 0.38

Table 5. Mean values of the normalized molecular gas content (in logarithmic scale) for isolated and cluster galaxies determined using a survivalanalysis in order to take into account upper limits.

sample logMi(H2)/LH logMi(H2)/LB logMi(H2)/πr2

units M /LH M /LB M /kpc2

isolated mean −1.64 −1.26 6.25std(median) 0.35 0.28 0.34

cluster mean −1.80 −1.32 6.19std(median) 0.55 0.45 0.43

Fig. 5. The relationship between the Mi(H2)/LH ratio and the IRAS60 micron flux (both in logarithmic scale) for the Coma superclustergalaxies. Empty symbols are for CO undetected objects.

galaxies, based on FIR and/or FIR-optically selected samples(Young & Scoville 1991 and references therein) tend to favorstrong CO-emitters.

5.3. The effects of environment on the molecular gas content ofspiral galaxies

The Coma/A1367 supercluster sample is ideal for the purposeof this section because:

a) it contains relatively isolated objects and galaxies in denseregions (Coma, A1367), where the environmental conditionsare known to be more extreme than in clusters with lower X-rayemission such as Virgo and Fornax (Magri et al. 1988).

b) the sample is optically complete thus, contrary to FIR selectedones, it is not biased towards CO rich galaxies.

c) cluster and isolated galaxies are all at the same distance,making the analysis distance independent.

d) the angular size of the galaxies is comparable with the beamsize of the telescope: no extrapolation is thus needed to infer thetotal CO flux.

The frequency distribution of the normalized Mi(H2)/LHratio and of the Mi(H2)/πr2 (r being the linear optical radius)hybrid surface density (both in logarithmic scales) for clusterand isolated galaxies are plotted in Fig. 6a and b (upper limitsare marked with arrows; the membership is assigned using the“caustic” criterium described in Table 3).

Cluster galaxies have a similar molecular gas content thanisolated objects: nonparametric two-sample statistical tests(generalized Wilcoxon tests; logrank test) indicate that the twofrequency distributions are traced by the same parent populationwith a probability between 22 and 97%. The mean values for


Fig. 6a and b. The frequency distribution of the indicative molec-ular hydrogen mass a: normalized to the H band luminosity andb: normalized to the optical surface (in logarithmic scale) for clus-ter (full line, empty histogram) and isolated (dashed line, shaded his-togram) galaxies. Lower limits are marked with empty and full trian-gles respectively for cluster and isolated galaxies. The membership isassigned using the “caustic” criterium described in Table 3.

isolated and cluster objects computed taking into account upperlimits using the Kaplan-Meier estimator are given in Table 5.

This result, however, is conclusive only if the cluster and theisolated samples are composed of galaxies of similar morphol-ogy and/or mass, i.e. if the morphology and/or mass segregationare not important. The paucity of our data prevents us to com-pare the molecular gas content of cluster and isolated galaxiesin separate morphological classes. The analysis presented inSect. 5.1, however, shows that the dependence of the molec-ular gas content of (isolated) galaxies on their morphologicaltype is very weak, if present(see Fig. 2). Thus the effects of themorphological segregation can be disregarded.

In Sect. 5.1 we have also shown that there is a correlationbetween the normalized molecular gas content of galaxies andtheir mass or linear dimension. In order to remove this depen-dence, we define, following Haynes & Giovanelli (1984) the COdeficiency parameter. Given the relationships between Mi(H2)and LH or (2r)2, determined using detected and undetected iso-lated galaxies selected according to the “caustic” criterium:

logMi(H2) = 3.58(±0.30) + 0.51(±0.21)× logLH (5)

and:

logMi(H2) = 7.85(±0.30) + 0.39(±0.16)× log(2r)2 (6)

(where mass and luminosity are in solar units and r in kpc), theCO deficiency parameter is given by the relation:

COdef = logMi(H2)e − logMi(H2)o (7)

where Mi(H2)e is the expected molecular gas mass of a galaxyof a given H luminosity or linear diameter, as determined fromEqs. 5 and 6, and Mi(H2)o is its observed molecular gas mass.The suffixes “L” and “r” to the CO deficiency will be used toindicate when Mi(H2)e is determined from Eqs. 5 or 6. TheCO deficiency parameter does not depend on the size/mass ofthe target galaxy, and is distance independent. As expected, theaverage CO deficiency of isolated galaxies is 0.0, with a standarddeviation of ∼0.3.

We plot in Fig. 7a and b the frequency distribution of theCO deficiency (L) and (r) parameters for cluster and isolatedgalaxies (lower limits are marked with arrows).

Fig. 7 and the non parametric two-sample statistical testsindicate that no systematic differences are observable betweenthe two populations. Only three galaxies have a CO deficiency (Land r)>0.5: 160025 (COdef(L)=1.12, COdef(r)=0.90), 160081(COdef(L)=0.68, COdef(r)=0.67) and 160005 (COdef(L)=0.62,COdef(r)=0.55). These are all Coma cluster objects, and the firsttwo are also extremely HI deficient galaxies. No correlationis observed in general between the CO and the HI deficiencyparameter.

The relationship between the CO deficiency parameter (Land r) and the angular distance from the cluster centre is shownin Fig. 8a and b. The average dispersion in the CO deficiency pa-rameter (∼0.3) can be observed in the reference sample (angulardistance from the cluster centre > 2 degrees). The dispersion inthe CO deficiency of cluster galaxies is slightly larger than in thereference sample, with few extreme objects close to the clustercore. The galaxies at an angular distance smaller than 2 degreesfrom the cluster centre, generally HI-deficient (Gavazzi 1987),have in general a normal molecular gas content if compared toisolated objects.

The lack of molecular gas deficient spiral galaxies, alreadyobserved in Virgo by Kenney & Young (1989) and Boselli(1994), is not unexpected even in rich clusters such as Coma. Theram pressure responsible for the HI removal in cluster galaxiesis not sufficient to perturb the molecular gas which is more cen-trally concentrated than HI, thus shielded inside the potential


Fig. 7a and b. The frequency distribution of the CO deficiency param-eter determined from a: the H band luminosity and b the optical lineardiameter for cluster (full line, empty histogram) and isolated (dashedline, shaded histogram) galaxies. Lower limits are marked with emptyand full triangles respectively for cluster and isolated galaxies. Themembership is assigned using the “caustic” criterium described in Ta-ble 3.

well of the galaxy ( Kenney & Young 1989; Young & Scoville1991; Boselli et al. 1994).

The lack of a suitable reference sample and the adopted FIRselection criteria (which favoured the observation of CO richgalaxies) prevented previous studies to reach a firm conclusionon the effect of the environment on the molecular gas content ofthe Coma cluster galaxies (Casoli et al. 1991; 1996b). Rengara-jan & Iyengar (1992) and Kenney & Young (1988b) have shownthe existence of CO deficient galaxies in the Virgo cluster. How-ever several criticisms can be addressed to these works: Ren-garajan & Iyengar (1992) used as reference sample the one ofYoung et al. (1989) which, being FIR selected, is biased towardsCO strong emitters. Kenney & Young (1988b) found moleculargas-deficient low-mass spirals by comparing their normalized

Fig. 8a and b. The relationship between the angular distance from thecluster centre (in degrees) and the CO deficiency parameter determinedfrom a the H luminosity and b the optical linear diameter as describedin the text. Filled symbols represent detected galaxies, empty circlesundetected objects (lower limits).

CO emission with that of the brighter Virgo galaxies. The anal-ysis presented in the next section suggests that the moleculargas content of low mass galaxies, when determined assumingthe solar neighborhood CO–H2 conversion factor, is probablysignificantly underestimated. Furthermore Boselli (1994) hasshown using a large sample of Virgo galaxies that, contrary towhat happens for the HI gas, the CO emission of Virgo spiralsdoes not change with the angular distance to the cluster centre.Boselli et al. (in preparation) have also shown that the molecu-lar gas content of two anemic and extremely HI deficient Virgogalaxies, accurately determined with a complete map of theiroptical discs, is normal and comparable with the one of isolatedobjects of similar mass and luminosity.

We thus conclude that cluster galaxies are not deficient inmolecular gas.


Fig. 9. Relation between the normalized star formation index HαE.W.,and the indicative molecular gas mass normalized to the H luminosity.Empty symbols are for CO undetected galaxies.

5.4. The relationship between the molecular gas content and theactivity of star formation

The relationship between the activity of star formation and theindicative molecular gas content of galaxies was discussed byBoselli et al. (1995b). Here we summarize the main results ofthat work extended to an almost doubled sample by using newCO data.

Many indicators of the star formation activity (SFA) ofgalaxies have been proposed in the literature (see Kennicutt(1990), Boselli (1994), and references therein). We will use theHα equivalent width (E.W.), which is a luminosity-independentindex of the recent (< 107 years), high mass (M > 10 M)star formation activity (Kennicutt 1983; 1990); the (m2000–H)colour (where m2000 is the uncorrected far–UV magnitude at2000 A), which is an indicator of a less recent (∼ 108 years)star formation activity normalized to the average past star for-mation rate (Lequeux 1988); the (U-B)c corrected colour, whichis an indicator of the integrated star formation history (Larson& Tinsley 1978; Lequeux 1988) and the FIR/H flux ratio (Ken-nicutt 1990). The FIR at 60 µm flux as measured by IRAS,which has the advantage of not being affected by the extinction,is however an ambiguous star formation tracer since it is notonly due to dust heated by young stars, but also to heating bythe general interstellar field (Sauvage & Thuan 1992).

Since the pronounced change in the photometric propertiesof spiral galaxies along the Hubble sequence is predominantlydue to changes in the star formation histories of discs, and onlysecondarily to changes in the bulge/disc ratio (Kennicutt et al.1994), the adopted normalization should not introduce a second-order dependence with morphological type. The adopted nor-malized indicators thus trace the effective star formation activ-ity of discs and are only secondarily affected by the presence ofbulges.

In Fig. 9 we plot the ratio Mi(H2)/LH versus the Hα E.W.There is a good correlation between the star formation rate(SFR) of the observed galaxies and their indicative moleculargas content. The same result is obtained using the optical andUV colours and normalized FIR fluxes as tracer of star forma-tion.

This correlation was not observed by Kennicutt (1989), Buat(1992) and Boselli (1994) using nearby samples of galaxies,generally dominated by low luminosity objects. As discussedin Boselli et al. (1995b) a good correlation between the tracersof star formation and the molecular gas content can be observedonly in massive, bright galaxies such as the one included in theComa supercluster sample. In Fig. 10 we plot the relationshipbetween the Hα E.W. and Mi(H2)/LH separately for two rangesof H luminosity: galaxies with log LH>10.6 LH (Fig. 10a),and with log LH<10.6 LH (Fig. 10b). The sample plotted inFig. 10b (log LH<10.6 LH) corresponds to the intermediate-mass sample used by Boselli et al. (1995b) (10.0 < log LH <10.7 LH), all galaxies but two being in the Coma superclusterwith log LH > 10.0 LH.

The two ranges are arbitrarily chosen to contain a similarnumber of objects. The main result of Boselli et al. (1995b)can be seen in Fig. 10. The relationship between the SFR andMi(H2) is clearer for high-mass galaxies than for intermediate-mass objects. Notice that assuming a distance modulus of 3.75magnitudes between Coma and Virgo, the adopted magnitudelimit of 15.2 mpg for Coma corresponds to mpg ≤ 11.4 at Virgo.If compared to the Virgo cluster sample analysed by Boselli(1994), the average H luminosity of the Coma supercluster sam-ple is <log LH>Coma = 10.57 LH with a standard deviationof 0.32, while the Virgo sample has <log LH>V irgo = 10.02±0.64 LH, thus slightly lower.

As discussed in Boselli et al. (1995b), this property doesnot depend on the adopted normalization: a similar result isobtained using FIR fluxes and molecular hydrogen masses nor-malized to the square of the optical diameter, and dividing thesample in two classes of increasing optical linear diameter. An-other possible criticism to the previous conclusion is that thecorrelation shown in Fig. 10 is a morphological type effect, thegalaxies of different types lying in different parts of the dia-gram. The morphological types are however well mixed in thediagram and the same correlation can be seen when galaxies aredivided in two subsamples, one including Sa-Sab-Sbs and theother Sbc-Sc-Pecs.

We propose that the lack of correlation between Mi(H2)and star formation for the fainter galaxies is due to scatter inthe conversion factor X and not necessarily to a real lack ofcorrelation between the true mass of molecular gas and starformation; in other words, we suggest that Mi(H2) is a poorindicator of M(H2) for low luminosity galaxies. This will bediscussed in Appendix A.

6. Summary

The 12CO(1–0) line observations presented in this paper, com-bined with data available in the literature, constitute the first


Fig. 10a and b. Relation between the normalized star forma-tion index Hα E.W., Mi(H2)/LH separately for two ranges of Hluminosity: a high-mass galaxies, with log LH>10.6 LH, andb intermediate-mass galaxies, with log LH<10.6 LH. Empty sym-bols are for CO undetected galaxies.

optically selected complete samples of cluster galaxies (Comaand A1367) and of relatively isolated galaxies, located in theridge between the two clusters. The adopted selection criteria,the completeness of the sample and the presence of cluster andisolated galaxies (both at the same distance) make this sam-ple ideal for comparing the molecular gas properties of spiralgalaxies in different environments.

Adopting a standard CO line intensity-H2 column densityconversion factor, we determine the frequency distribution of themolecular gas content of spiral galaxies. The estimated molec-ular hydrogen content of optically selected and isolated spiralgalaxies is, on the average, 20% of the atomic hydrogen, inde-pendent of the morphological type. This value, which is char-acteristic of “normal” optically selected spiral discs, is signifi-

cantly lower than previous estimates based on samples heavilybiased by FIR selection criteria.

Comparing different subsamples of late type objects in theComa supercluster, we show that FIR selected samples are bi-ased towards CO bright galaxies. Thus the previous estimatesof the average molecular gas fraction of spiral galaxies basedon FIR selected samples are biased towards molecular gas richobjects, and are not representatives of optically selected galax-ies.

The effects of the environment on the molecular gas contentof spiral galaxies are minor. Following Haynes & Giovanelli(1984), we define a CO deficiency parameter as the differencebetween the expected molecular gas mass of a galaxy of a givenH luminosity or linear diameter (as determined from the analy-sis of the reference sample) and its real observed Mi(H2) value.Cluster and isolated objects have, on average, similar CO “de-ficiencies” and Mi(H2)/LH distributions.

In the Coma/A1367 supercluster sample, dominated by mas-sive objects, there is a well-defined relationship between themolecular gas content of galaxies and their star formation ac-tivity, as traced by the Hα equivalent width, the optical colourindices and the far infrared emission. The above relationship isabsent in Virgo cluster galaxies (Boselli 1994) as well as in othernearby samples (Kennicutt 1989; Buat 1992) all dominated bylow luminosity galaxies.

The complex structure of the ISM, its different physical con-ditions, such as the far-UV radiation field, the dust content andthe clumpy distribution of the atomic and molecular gas, suggestthat the molecular hydrogen content is not simply related to theglobal CO emission of spiral galaxies. While CO is probably agood tracer of the molecular gas in high-mass galaxies, such asthe Milky Way, in low-mass spirals CO traces less efficientlythe molecular hydrogen content, and a value ofX=N(H2)/I(CO)different from the solar neighbourhood value might have to beused. Furthermore in these objects, the high far-UV radiationfield produced by young O-B stars and the low metallicity (Zarit-sky et al. 1994), which corresponds to a lower dust content andconsequently to a lower extinction, facilitates the photodisso-ciation of the diffuse molecular gas, weakening the expectedcorrelation between star formation and molecular gas content.

Acknowledgements. We wish to thank the KPNO telescope operatorswho assisted us during remote observing from Paris. A.B. was partlysupported by a post-doctoral fellowship of the Italian C.N.R., throughthe Verbundforschung Astronomie/Astrophysik of DARA under grantn. 50 OR 9501, and by a post-doctoral MPIG-CNRS fellowship. G.G.wishes to thank the director of DEMIRM for hospitality during his stayin Paris. We wish to thank C. Balkowski for precious suggestions. A.B.wishes to thank H. Facques for hospitality during his stay in Paris.

Appendix A: the I(CO)-N(H2) conversion

A discussion on this subjects has recently been published byArimoto et al. (1996) but it concerns only individual molecularclouds and complexes assuming that they are in virial equilib-rium.


The assumption underlying a universal ratio X =N(H2)/I(CO) between the intensity I(CO) of the 12CO(1–0) lineand the column density of molecular hydrogen N(H2) is thatCO and H2 are contained in optically thick molecular cloudswith a similar brightness in the 12CO(1–0) line (Dickman et al.1986). The beam of the radiotelescope looking at an externalgalaxy encompasses a large number of such clouds: thus theintensity of the CO line is supposed to be a measure of the num-ber of clouds, hence of the amount of H2 seen in the beam. Thisis clearly a very rough approximation, although it might wellexist a universal spectrum of molecular clouds, with perhaps afractal structure at all observable scales (e.g. Falgarone et al.1992). Their CO line emission properties depend on the far-UVradiation density and on other parameters, resulting in a largerange of variation in X . When the UV radiation field increases,the radiation photodissociates both CO and H2 more efficientlyin the low column-density (not self-shielded) regions such as thediffuse medium and the external parts of molecular clouds. Theincreased UV field makes the CO at the interfaces of the molec-ular clouds hotter, so that the 12CO(1–0) line, which is opticallythick, has a higher brightness temperature. The two effects actin opposite directions as far as the global CO emission of themedium is concerned: an enhanced UV flux reduces the fillingfactor of the CO-emitting regions, but increasing their bright-ness. These phenomena are discussed in details by Maloney &Black (1988), Rubio et al. (1993b), Lequeux et al. (1994) andAllen et al. (1995). They also discuss the role of the low-energycosmic-ray flux which contributes to the heating and presum-ably varies spatially together with UV flux, as they are producedby the same sources (massive stars evolving as supernovae). Theabundances of heavy-elements and of dust, which are also re-lated, affect the photoionization and the heating: less dust meansless absorption of UV radiation, hence more photodissociation,and less formation of H2, resulting in smaller abundances ofboth CO and H2 and in more heating of CO (Verter & Hodge1995).

In the solar neighbourhood, the UV radiation field is suchthat diffuse (or “translucent”) clouds and the low-density partsof the molecular clouds still contain CO and H2, and contributeto the global CO emission (Lucas & Liszt 1996). This is indi-cated by the low observed 12CO(2–1)/12CO(1–0) line intensityratio of≈ 0.5 (Sakamoto et al. 1995) of the low-density gas, ob-viously due to sub-thermal excitation of CO. Diffuse clouds areefficient in emitting in the CO lines, consequentlyX is small: inthe diffuse medium, de Vries et al. (1988) and Mayerdierks &Heithansen (1996) estimateX20 = 0.5± 0.3 in units of 1020 mol.cm −2 (K km s−1)−1. The global value of X for the whole localISM (diffuse medium and clouds) is X20 = 1.0± 0.1 as derivedfrom EGRET gamma-ray observations (Digel et al. 1996), sig-nificantly smaller than the old COS-B value ofX20 = 2.3 (Stronget al. 1988). The value of X in dense molecular clouds is ill-determined but is undoubtedly larger, because N(H2) is muchlarger than in the diffuse medium while the CO-line is onlyslightly brighter.

Even in the “molecular” ring at 5 kpc from the centre ofour Galaxy the UV flux is not sufficient to photodissociate the

diffuse gas which may contribute to the CO emission as muchas the dense clouds (Polk et al. 1988; Chiar et al. 1994). The12CO(2–1)/12CO(1–0) line intensity ratio is≈ 0.7 in this region(Sakamoto et al. 1995), still indicating a strong contributionfrom subthermal gas. In the Perseus arm at 3–4 kpc from theSun towards the anticenter, X20 = 2.5± 0.9 (Digel et al. 1996),presumably due to a lower far-UV flux. There seems to be asimilar trend in the values of X for molecular clouds, but theabsolute values are poorly known as the H2 masses are gener-ally uncertain virial masses, and the parameters of the cloudshave not been defined homogeneusly for the different surveys.The global value for our Galaxy from EGRET is tentativelyestimated as X20 ≈ 1.5 (Hunter et al. in preparation).

There are not many reliable estimates ofX in external galax-ies. In NGC 891, an edge-on galaxy often considered as a twinof our own Galaxy, Guelin et al. (1993) determine X20 ≈ 1.0,in good agreement with the EGRET local and global valuesfor our Galaxy. Neininger et al. (1996) find a similar value forNGC 4565. Guelin et al. (1995) findX20 ≈ 0.6 for the inner discof M 51, a galaxy which forms stars more actively than NGC891 and our own Galaxy and where the far-UV flux is higher.For NGC 891 and M51 the 12CO(2–1)/12CO(1–0) line intensityratio is significantly smaller than 1 (respectively 0.75 and 0.8:Garcia-Burillo et al. 1992, 1993) showing that the low-densitygas contributes strongly to the emission. In the SMC the far-UVflux is 10 times larger and the heavy-element abundances anddust-to-gas ratio 10 times smaller than in the solar neighbour-hood, so that the diffuse H2 and CO are photodissociated andsurvive only in the dense parts of the molecular clouds as indeedshown by the 12CO(2–1)/12CO(1–0) line intensity ratio ≥ 1.0(Rubio et al. 1993a, 1993b; Lequeux et al. 1994). In these cloudsX20 ≈ 10. Even with a large uncertainty, this value is larger thanfor similar clouds in our own Galaxy, even those of the Perseusarm (Rubio et al. 1993b). The value of X determined for theSMC clouds correspond to the average for the whole SMC asthere is no H2 and CO left outside the densest regions. At theother extreme, the gas in the inner disk of M 31 seems verycold and almost entirely molecular due to the lack of UV ra-diation and low-energy cosmic rays; the 12CO(2–1)/12CO(1–0)line intensity ratio is about 0.4, indicating that the CO emissionis dominated by low-density gas (Allen et al. 1995; Loinard etal. 1995). The corresponding value ofX20 ≈ 20 (within a factor3) is explained by the weakness of the CO emission and the verylarge column density of H2 (Allen et al. 1995).

It is clear from the preceding discussion that, contrary toa common belief, the I(CO) to N(H2) conversion factor differswidely from galaxy to galaxy, and even between different partsof a single galaxy. There are satisfactory qualitative explana-tions for the behaviour of X , which depends on the UV flux,on the low-energy cosmic-ray flux, on metallicity and on theabundance of dust. However these parameters are often poorlyknown and it is difficult to predict the value of X in a givengalaxy. It seems however that X does not vary by large factorsin luminous (log LH > 10.7 LH) spiral galaxies similar tothe Milky Way, in which there is emission from both the denseand less dense parts of the ISM. Large changes are expected for


Table A1. Values of X20, guesses on the physical parameters and CO line ratios in a variety of situations; I21/I10 and I10/J10 mean respectivelythe 12CO(2–1)/12CO(1–0) and 12CO(1–0)/13CO(1–0) line intensity ratios.

galaxy Xa)20 Far − UV b) Cosmic raysb) Z/Zb)

I21/I10 I10/J10

Dust

The GalaxySolar neighb. 1.01 118 118 11 0.52 63

Gal. centre << 1.04,5 ? ? 2− 3? ? ≈ 66,7,8

Molec. ring. ? 3? 3? 1.5? 0.72 63,7,8,9

Perseus arm 2.51 0.5? 0.51 ≈ 1 ? ?Whole Gal. 1.510

NGC 891 1.011 ≈ 1 ≈ 1 ≈ 1 0.7512 8.512

NGC 4569 1.026 − − ≈ 1 − −M 51 0.613 2× (NGC891)14 ? 313 0.815 8− 1015

SMC 1016,18 1017 10? 0.117 ≥ 1.018 1518

M 31 (inner disk) 2019 0.119 0.1? 2− 3 0.419,20 1019

Starburst FIR − bright ? >> 1 >> 1? ≈ 1 0.3− 1.921−25 9− 4021−25

a) in units of 1020 mol cm−2(K km s−1)−1; b) relative to solar neighbourhood.Notes and references for Table A1: 1Digel et al. 1996; 2Sakamoto et al. 1995; 3Sanders D.B., Scoville N.Z., Tilanus R.P.J., Wang Z., Zhou S.1993, in Back to the Galaxy, ed.Holt S.S. & Verter F., American institute of Physics, p. 311; 4Audouze J., Lequeux J., Masnou J.-L., PugetJ.-L. 1979, A&A 80, 276 (this reference discussed the problem of the surprisingly low gamma-ray flux from the Galactic center, which is bestexplained by a low gas content); 5Sodroski T.J., Odegard N., Dwek E. et al. 1995, ApJ, 452, 262 (from FIR-submm COBE observations); 6BaniaT.M. 1986, ApJ 308, 868; 7Liszt H., Burton W., Xiang D., 1984, A&A, 140, 303; 8Liszt H., 1993, ApJ, 411, 720; 9Polk et al. 1988; 10Provisionalvalue from EGRET by Hunter D.S. et al., in preparation; 11Guelin et al. 1993; 12Garcia-Burillo et al. 1992; 13 Guelin et al. 1995; 14Garcia-Burilloet al. 1993; 15From ratio of 60 micron face-on surfaces brightnesses: fluxes from Young J.S., Xie S., Kenney J.D.P., Rics W.L. 1988, ApJS 70,699; 16Rubio et al. 1993b; 17Rubio et al. 1993a; 18Lequeux et al. 1994; 19Allen & Lequeux 1993; 20Loinard et al. 1995; 21Casoli, F., Dupraz C.,Combes F. 1992, A&A 264, 55; 22Gusten R., Serabyn E., Kasemann C., et al. 1993, ApJ 402, 537 (M 82); 23Garay G., Mardones D., MirabelI.F. 1993, A&A 277, 405; 24Sage L. & Isbell D.W. 1991 A&A 247, 320 (NGC 253); 25Rigopoulou D., et al. 1996, A&A, 305, 747; 26Neiningeret al. (1996).

low-mass spiral, irregular and blue compact galaxies (log LH< 10.0 LH) which generally have lower abundances and dust-to-gas ratios than large spirals (Bothun et al. 1984; Zaritsky etal. 1994) while the star formation rate per unit stellar mass ormass of gas is generally larger (Gavazzi 1993). In these objectssuch as the SMC, both the lower metallicity and the higher SFRcontribute to increase the mean far-UV radiation field and yieldchanges in X . These changes can be large (in particular whenthe CO emission is dominated by clumpy regions); they are notnecessarily monotonous nor there is a one-to-one relation be-tween X , Z and the UV radiation flux. This may induce a largedispersion in the values ofX , accounting for the lack of relationbetween Mi(H2) and star formation rate for these objects (seeFig. 10b).

Table A1 summarizes the situations described in this Ap-pendix. An unfortunate fact is that there are no determination ofX in intermediate cases between say our Galaxy and the SMC,so that any prediction for low-mass galaxies is hazardous.

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the molecular gas content of spiral galaxies in the coma...

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