CLoGS - The Complete Local-Volume Groups Sample

Sample Selection

Since group catalogues based on SDSS/2dFGRS cover limited areas of the sky and rarely include large nearby galaxies, we start from the shallow, all-sky LGG catalogue of Garcia (1993) and test the reality of the groups using the best available optical data. The LGG sample is based on an early version of the LEDA galaxy catalogue containing 23490 galaxies, complete to m_B=14 and v_rec=5500 km/s (equivalent to D<80 Mpc, correcting for Virgocentric flow). Groups were identied by friends-of-friends and hierarchical clustering algorithms, the final sample consisting of systems identified by both method. We select groups within this sample which have:

4 or more member galaxies, excluding small associations which may lack a group-scale dark halo,
1 or more early-type member galaxy,
optical luminosity L_B of at least 3x10¹⁰L_sol for the brightest early-type galaxy, and
Declination >-30°, to ensure visibility from the GMRT and VLA.

Groups with early-type galaxies are typically more X-ray luminous (Mulchaey et al. 2003) and are more likely to be collapsed systems able to drive galaxy evolution through mergers.

We next refine and expand the group membership using the HyperLEDA catalogue which contains ~10⁵ galaxies with known m_B within 10,000 km/s. We select galaxies within 1 Mpc and 2000 km/s of the brightest group-member early-type (BGE) galaxy and iteratively determine the mean group velocity and velocity dispersion using a robust estimator (the gapper algorithm, Beers et al. 1990). Galaxies within 3 sigma of the mean velocity are considered group members. The spatial distribution of galaxies is then examined using isopleth maps (see the Figure below). We exclude cluster subgroups and groups with no clear central galaxy density peak, and cases where the BGE has been mis-identified are re-evaluated. The groups thus selected typically have >20 member galaxies.

Maps of galaxy position, velocity and
morphology for three CLoGS groups, with contours of local galaxy density
overlaid.

We define a richness parameter R as the number of galaxies with log L_B >=10.2 (equivalent to the 90% completeness limit of LEDA at our distance limit) within 1 Mpc and 3 sigma of the BGE. R > 10 systems are known clusters and are excluded. We discard six groups with R = 1 which, while probably bound systems (e.g., HCG 42), are too poor to allow reliable determination of physical parameters.

Of the remaining 53 groups, only 24 (45%) had been observed by Chandra or XMM-Newton prior to this project. The observed systems have typical temperatures of 0.5-1.7 keV.

The images below show the locations of the groups among the filamentary structures of the local volume, projected on the sky or in velocity space. Blue circles mark member galaxies, red points are dominant ellipticals, all other galaxies in the local volume are marked in black.

Structure in the local volume. HyperLEDA galaxies with recession velocities of 250-5500 km/s, with CLoGS group members marked in blue, and dominant ellipticals in red.

Structure in local velocity space. HyperLEDA galaxies with recession velocities of 250-5500 km/s, with CLoGS group members marked in blue, and dominant ellipticals in red.

High- and Low-Richness Subsamples

Since the high oversubscription rates of Chandra and XMM-Newton make acquisition of new data for large samples difficult, we divided the CLoGS groups into two statistically complete sub-samples. We now have complete X-ray and low-frequency radio observations of the full 53-group sample. The groups, along with a summary of the data available (or where observations have been approved), are listed below:

LGG	BGE	D	R	sigma	X-ray	Radio	LGG	BGE	D	R	sigma	X-ray	Radio
		(Mpc)		(km/s)					(Mpc)		(km/s)
High-Richness Subsample
072	NGC 1060	76	8	496	X,C	610,235	473	NGC 7619	54	8	466	X,C	610,235
009	NGC 193	74	7	493	C	610,235	031	NGC 677	78	7	249	X	610,235
066	NGC 978	69	7	209	X	610,235	278	NGC 4261	32	7	743	X,C	610,235
351	NGC 5153	60	7	251	C	610,235	363	NGC 5353	35	7	240	X,C	610,235
018	NGC 410	77	6	393	X,C	610,235	158	NGC 2563	65	6	379	X,C	610,235
185	NGC 3078	34	6	1013	C	610,235	042	NGC 777	73	5	291	X,C	610,235
338	NGC 5044	38	5	336	X,C	610,235	393	NGC 5846	26	5	717	X,C	610,235
027	NGC 584	25	4	160	C	610,235	058	NGC 940	74	4	229	X	610,235
061	NGC 924	64	4	126	X	610,235	080	NGC 1167	72	4	94	X	610,235
103	NGC 1453	63	4	537	X	610,235	117	NGC 1587	51	4	126	C	610,235
262	NGC 4008	54	4	230	X	610,235	276	NGC 4169	45	4	93	X	610,235
310	ESO 507-25	45	4	557	C	610,235	345	NGC 5084	23	4	680	C	610,235
402	NGC 5982	44	4	230	X,C	610,235	421	NGC 6658	63	4	241	X	610,235
Low-Richness Subsample
006	NGC 128	60	3	669	X,C	610,235	012	NGC 252	72	3	675	X	610,235
078	NGC 1106	64	3	776	X	610,235	097	NGC 1395	21	3	168	X,C	610,235
126	NGC 1779	45	3	443	X	610,235	138	NGC 2292	30	3	325	X	610,235
177	NGC 2911	45	3	288	X	610,235	205	NGC 3325	80	3	463	X	610,235
232	NGC 3613	32	3	442	X,C	610,235	314	NGC 4697	18	3	1022	X,C	610,235
341	NGC 5061	28	3	423	X	610,235	370	NGC 5444	60	3	316	X	610,235
398	NGC 5903	36	3	421	X,C	610,235	014	NGC 315	73	2	846	X,C	610,235
023	NGC 524	34	2	216	X,C	610,235	100	NGC 1407	23	2	368	X,C	610,235
113	NGC 1550	53	2	576	X,C	610,235	167	NGC 2768	23	2	706	C	610,235
236	NGC 3665	32	2	91	X,C	610,235	255	NGC 3923	20	2	207	X,C	610,235
329	NGC 4956	71	2	88	X	610,235	350	NGC 5127	72	2	794	X	610,235
360	NGC 5322	29	2	254	X,C	610,235	376	NGC 5490	71	2	303	X,C	610,235
383	NGC 5629	67	2	267	X	610,235	457	NGC 7252	66	2	397	X,C	610,235
463	NGC 7377	46	2	120	X	610,235

C and X indicate Chandra and XMM-Newton data respectively. Values in the Radio column indicate whether GMRT 610 or 235 MHz data are available; in some cases 327 and 150 MHz data are also in the archive. Bold entries indicate data awarded to our proposals for this project.