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 mB=14 and vrec=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:

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 mB 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 LB >=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. 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 are in the process of acquiring X-ray data for the richer subset, 26 groups with (R = 4-8), and low-frequency radio observations of the full 53-group sample have been approved. The groups, along with a summary of the data available (or where observations have been approved), are listed below:

LGG BGE D sigma X-ray Radio   LGG BGE D sigma X-ray Radio
(Mpc) (km/s) (Mpc) (km/s)
High-Richness Subsample
072 NGC 1060 76 8 496 X 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 610,235 421 NGC 6658 63 4 241 X 610,235
Low-Richness Subsample
006 NGC 128 60 3 669 610,235 012 NGC 252 72 3 675 610,235
078 NGC 1106 64 3 776 610,235 097 NGC 1395 21 3 168 X,C 610,235
126 NGC 1779 45 3 443 610,235 138 NGC 2292 30 3 325 610,235
177 NGC 2911 45 3 288 610,235 205 NGC 3325 80 3 463 610,235
232 NGC 3613 32 3 442 610,235 314 NGC 4697 18 3 1022 X,C 610,235
341 NGC 5061 28 3 423 610,235 370 NGC 5444 60 3 316 610,235
398 NGC 5903 36 3 421 X 610,235 014 NGC 315 73 2 846 X,C 610,235
023 NGC 524 34 2 216 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 C 610,235 255 NGC 3923 20 2 207 X,C 610,235
329 NGC 4956 71 2 88 610,235 350 NGC 5127 72 2 794 610,235
360 NGC 5322 29 2 254 X,C 610,235 376 NGC 5490 71 2 303 610,235
383 NGC 5629 67 2 267 610,235 457 NGC 7252 66 2 397 C 610,235
463 NGC 7377 46 2 120 610,235

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.