2 Sample selection

2.1 Previous samples

Surveys of the molecular gas content in galaxies have in general been done on samples which are far-infrared selected, or galaxies selected exclusively for belonging to clusters or groups (often with a far-infrared selection criteria on top; e.g. Casoli et al. 1991; Combes et al. 1994; Leon et al. 1998). A few exceptions exist in the literature. For example, Sage (1993) presents the CO content of a distance limited sample of 65 non-strongly interacting spiral galaxies, and Horellou et al. (1995) present a CO and HI survey of spiral and lenticular galaxies in the Fornax cluster, both based on samples selected without a far-infrared criterion.

However, until now no survey of galaxies in different environments has included a rigorously selected control sample. For instance, the sample by Casoli et al. (1998) which contains a large sample of 582 objects is an important source of information concerning molecular gas in spiral galaxies. However, it was built by gathering data from various surveys and is very heterogeneous in terms of morphology and environment. It contains galaxies from several clusters as well as galaxies in the field.

2.2 Dense environment and Control Sample (HDS and CS)

In view of these biases plagueing existing samples we have selected our sample from the catalog by Maia et al. (1994) which contains objects in low and high density areas of the Southern sky. The selection of groups adopted by Maia et al. is similar to the methodology developed by Huchra & Geller (1982) with the adaptations described by Maia et al. (1989). The catalog was drawn from the ESO/Uppsala Survey of the ESO(B) Atlas (Lauberts 1982) and used velocity information from the Southern Sky Redshift Survey (e.g., da Costa et al. 1989). The groups are defined to be formed by the accumulation of galaxy pairs with a member in common.

• The high density sample (HDS, hereafter) is formed by galaxies that are in groups of three or more members. They have a density contrast $\delta\rho/\rho \geq 500$. This is equivalent to densities larger than 18 galaxies/Mpc³. All the selected objects have radial velocities (after correction for Virgo infall) smaller than 8000 km s^-1.

• The control sample (CS, hereafter) is made up of galaxies which are not members of any group and which are situated in regions with density contrast $\delta\rho/\rho \leq 0.01$, i.e. less than 0.0004 galaxies/Mpc³.

2.3 HDS versus compact groups and poor groups

Although a group-finding algorithm was used to generate the samples, the idea is not to identify groups (either loose or compact), but galaxies in high and low local density environments. The main difference between the HDS and compact groups of galaxies is the isolation criterion which is imposed by the groups selection (Hickson 1982; Coziol et al. 2000). The only 2 compact groups (HCG 21 and HCG 90) in the region searched by Maia et al. (1994) ( $b^{\rm II}\le{-}30^{\circ}$, $\delta <-17\hbox{$.\!\!^\circ$ }5$) have 3 galaxies of each group taking part of the HDS, but none of them take part in the present subsample analysis.

The HDS should also not be confused with poor groups which are defined as systems with less than five bright galaxies but which can have 20-50 faint members (e.g., Zabludoff & Mulchaey 1998; Willmer et al. 1999). Some galaxies in these poor groups are certainly part of the HDS, but since our selection includes only members with known redshift, the HDS will have only the brighter members which have measured redshift. The HDS and CS contain in total 151 and 179 galaxies, respectively.

2.4 Our subsample: Morphology selection

Maia et al. (1994) have analysed the morphology distribution of the HDS and CS and concluded that the HDS has an excess of early-type galaxies compared to the CS. This is interpreted as an effect of the morphology-density relationship (Dressler 1980); i.e. a correlation between morphological types and local density showing that the fraction of early-type galaxies increases as a function of local galaxy density while the fraction of later types decreases (see also Sanroma & Salvador-Solé 1990; Whitmore & Gilmore 1991). Since there are galaxies of all morphologies in the HDS and in the CS, the main goal of our work is to evaluate the effects of the environment in galaxies of the same morphological type when compared with isolated galaxies. The ideal survey would include all galaxies in the HDS and CS, however, due to large size of the samples we have imposed such a selection which is fundamental in order to avoid any bias due to the well-known correlation between morphology and physical properties of galaxies. Figures 2-4 of Roberts & Haynes (1994) summarize clearly how morphology is correlated with fundamental properties of galaxies such as, blue luminosity, far infrared lumninosity, total mass, and neutral hydrogen mass. One of their conclusions is that, although the scatter is large, Sa-Sc have near constant molecular gas normalized either by the blue luminosity or by the total mass. They also pointed out that later-type spirals have less molecular gas and suggest that this could also be due to the CO to H₂ conversion factor which would depend on morphology. Therefore, in order to have an homogeneous sample, we selected mostly intermediate spiral galaxies; i.e. Sb, Sbc, and Sc, avoiding Sa and Sd galaxies. In this work we present the analysis of the optical and millimetric data of a subsample of 47 spiral galaxies, 22 in the HDS and 25 in the CS, with velocities less than 5500 km s^-1.

Table 1 lists information taken from the NASA/IPAC Extragalactic Database (NED) on each galaxy as follows. Column 1: designation in the ESO-Uppsala catalog (LV89); Col. 2: designation in other catalogs; Col. 3: right ascension ( $^{\rm h\ m\ s}$) and declination ($^{\circ}$ ' '') for J2000; Col. 4: type of sample (control $\rm sample=CS$ and high density $\rm sample=HDS$) and morphological type (Lauberts & Valentijn 1989, hereafter LV89) $\rm 1=Sa$, $\rm 2=Sa$-b, $\rm 3=Sb$, $\rm 4=Sb$-c, $\rm 5=S$..., $\rm 6=Sc$, Sc-d, $\rm 7=S$../Irr, $\rm 8=Sd$; Col. 5: morphological type from The Third Reference Catalogue of Bright Galaxies (RC3; de Vaucouleurs et al. 1991); Col. 6: number of galaxies in the same group (Maia et al. 1989); Col. 7: mean pairwise separation in Mpc (Maia et al. 1989); Col. 8: $B_{\rm T}$ magnitude from RC3; Col. 9: IRAS 60 $\mu$m flux in Jy (Moshir et al. 1990), and Col. 10: IRAS 100 $\mu$m flux in Jy (Moshir et al. 1990).

 

 

Table 1: Observed sample.

ESO-LV	Other	Coord.	Sample &	Morph.	$N_{\rm g}$	$r_{\rm p}$	$B_{\rm T}$	$F_{\rm 60\,\mu m}$	$F_{\rm 100\,\mu m}$
Name	Name	J2000	Morph.	RC3		Mpc		Jy	Jy
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
5390050		00 17 10.1 -19 18 00	CS 5	SAB(rs)c?			13.53	0.977	2.972
3500140	N101	00 23 54.6 -32 32 09	CS 6	SAB(rs)c			13.37	0.549	1.754
3520530	N491	01 21 20.3 -34 03 48	HDS 3	SB(rs)b:	3	0.18	13.21	2.843	8.632
2960380		01 32 27.4 -38 40 40	CS 4	SAB(rs)c			13.99	0.516	1.779
4780060		02 09 19.1 -23 24 54	CS 4	Sbc			13.22	3.543	9.112
5450100	N907	02 23 01.7 -20 42 43	HDS 5	SBdm? sp	5	0.40	13.21	2.649	5.625
5450110	N908	02 23 04.8 -21 14 03	HDS 5	SA(s)c			10.83	14.770	43.670
3550260		02 32 17.5 -35 01 50	CS 4	SB(s)bc:			13.80	0.482	1.588
3550300		02 37 36.4 -32 55 28	CS 4	SB(rs:)bc:			13.59	0.881	3.137
0310050		02 58 06.0 -74 27 24	CS 3.5	SAB(rs)bc			14.07	1.043	3.887
3570190	N1310	03 21 03.7 -37 05 58	HDS 5	SB(rs)cd	55	0.82	12.55	0.881	3.345
5480070	N1325	03 24 25.6 -21 32 35	HDS 3.5	SA(s)bc	7	0.94	12.22	0.631	3.211
5480310	N1353	03 32 03.0 -20 49 04	HDS 3	SA(rs)bc	7	0.94	12.40	2.420	8.786
5480380	I1953	03 33 41.7 -21 28 45	HDS 6	SB(rs)d	7	0.94	12.24	8.470	11.128
4190030		03 42 11.2 -27 51 47	CS 4	(R')SAB(rs)c			13.60	1.334	3.361
4820430	N1459	03 46 58.0 -25 31 11	CS 4	SB(s)bc?			13.62	0.572	2.657
4200030		04 07 45.8 -29 51 30	CS 5	SA(rs)bc			13.52	0.704	2.172
2010220		04 08 59.3 -48 43 42	CS 5	Sbc			14.73	0.356	1.466
1570050	N1536	04 10 59.9 -56 28 48	HDS 5.5	SB(s)c pec:	46	1.30	13.15	0.475	1.649
4840250	N1591	04 29 30.7 -26 42 44	CS 2	SB(r)ab pec			13.77	1.929	5.001
1190060	N1688	04 48 23.8 -59 47 59	HDS 7.5	SB(rs)dm	14	0.85	12.57	2.683	6.677
1190190	N1703	04 52 51.9 -59 44 33	HDS 5	SA(s)c	14	0.85	11.90	2.122	7.723
3050140		05 12 34.1 -39 51 36	CS 5	SB(s)c			14.13	0.378	0.982
2030180	N1803	05 05 26.6 -49 34 05	CS 4	Sbc:			13.38	0.277	0.715
1420500	I4901	19 54 23.1 -58 42 50	CS 5	SAB(r)c			12.29	1.778	6.518
2340160		20 23 25.1 -50 32 43	HDS 5	SAB(s)bc pec	4	0.68	14.56	3.069	7.875
2850080	N6902	20 24 27.7 -43 39 09	HDS 4	SA(r)b	4	0.31	11.64	0.826	3.924
1060120	I5038	20 46 51.2 -65 01 00	CS 6	(R':)SB(s)bc			14.13	0.723	2.460
2350550		21 05 55.4 -48 12 23	HDS 5	(R')SAB(rs)bc	9	1.00	12.70	0.461	2.840
2350570		21 06 21.8 -48 10 14	HDS 4	Sbc: sp	9	1.00	14.45	0.461	3.368
2860820		21 15 45.4 -42 25 33	HDS 5	SAB(s)c	3	0.20	14.51	0.337	1.032
2370020	N7124	21 48 05.7 -50 33 51	CS 4.5	SB(rs)c			13.10	0.791	3.411
1890070	N7140	21 52 15.3 -55 34 10	CS 4	(R'2)SB(rs)b			12.20	2.183	5.886
2880260	N7162	21 59 39.0 -43 18 12	HDS 5	(R')SA(r)bc	4	0.20	13.29	0.484	1.656
5320090	N7167	22 00 30.9 -24 38 00	CS 5	SB(s)c:			13.22	1.314	3.588
6010040		22 01 30.4 -22 04 15	CS 4.6	SB(s)c:			14.58	0.227	0.877
1080130	N7191	22 06 51.3 -64 38 03	HDS 3.5	SAB(rs)c	5	0.48	13.80	0.570	2.061
1080200	I5176	22 11 55.0 -66 50 46	CS 3.9	SAB(s)bc?sp			13.54	3.031	11.21
1460090	N7205	22 08 34.4 -57 26 33	CS 5	SA(s)bc			11.55	8.861	25.960
4050180	N7267	22 24 21.6 -33 41 38	CS 1	(R'1)SB(rs)a			12.91	2.081	4.930
4060250	N7418	22 56 36.0 -37 01 47	HDS 5	SAB(rs)cd	32	1.31	11.66	4.344	15.010
4060330	I5270	22 57 54.7 -35 51 30	HDS 6	SB(rs)c	32	1.31	13.00	3.076	8.398
4070140		23 17 39.7 -34 47 24	CS 5	SB(s)c?			13.48	0.987	2.766
3470340	N7599	23 19 21.1 -42 15 20	HDS 3	SB(s)c	32	1.31	12.08	5.408	21.750
2400110		23 37 49.7 -47 43 42	HDS 4.8	Sb	3	0.18	13.20	0.956	5.612
2400130		23 39 26.9 -47 46 27	HDS 3	(R'1)SAB(rs)b	3	0.18	13.99	0.791	3.411
4710200	N7755	23 45 51.8 -30 31 19	CS 4.5	SB(r)bc			12.56	2.686	8.538

Column 4: $\rm CS=control$ sample, $\rm HDS=high$ density sample; morphological types are: $\rm 1=Sa$, $\rm 2=Sa$-b, $\rm 3=Sb$, $\rm 4=Sb$-c, $\rm 5=S$..., $\rm 6=Sc$, Sc-d, $\rm 7=S$../Irr, $\rm 8=Sd$. Column 6: $N_{\rm g}$ is the number of companions from Maia et al. (1989). Column 7: $r_{\rm p}$ is the mean pairwise separation from Maia et al. (1989).