Compact groups of galaxies: small, dense, elusive by: E. Pompei - European Southern Observatory A. Iovino - Osservatorio astronomico di Brera-Milano V. Presotto - Osservatorio astronomico di Brera-Milano M. Dahlem - ATCA

Compact groups: what are they? “The criteria” (HCGs) Richness: n ≥ 4 Isolation: R isol > 3 x R group no galaxy in isolation ring Compactness:  G < 26 SCG

Compact groups: open questions What is their origin? What is their evolution? What is their relation with their surroundings? ? ?

Compact groups at low z: the Southern Compact Groups survey  Southern Compact Groups (SCGs): nearby (z < 0.03) compact groups. Imaging and spectroscopic follow-up, groups with high percentage of spirals 120 groups over 5200 sq. deg. 60 groups with b j ≤ 14.5  m = m est - m int > 3.35 NEW!

Southern Compact Groups at low z: observations and results = H o -1 = 0.76 Gyr  3D = 330 km/s f s = 0.69  /  ~ 80 R c ~ 50 Kpc = 7.7 x M  = 207 <= 0.02 = 0.29 Gyr 3.6m+EFOSC2 ATCA, Pompei et al., 2007

Phase 2 Compact groups at high z: what’s going on  Las Campanas: 76 compact groups from redshift survey, ~ 0.08  2dF compact group survey: compact group catalog from redshift survey. = 0.11  SDSS compact group survey: compact group catalog from imaging and spectroscopic survey. r* lim =21, 14 ≤ r* brightest ≤ 18, =  DPOSS II compact group survey: 459 compact groups from a search on digitized DPOSS plates. r ~ 21, 16 ≤ r brightest ≤ 17, = 0.13 Spectroscopic follow-up completed.

Z = ’ = 13 Kpc Z = 0.1 1’ = 119 Kpc Z = 1 1’ = 340 Kpc ÷ Kpc IT’S DIFFICULT TO FIND COMPACT GROUPS! Compact groups at high z: z~0.2

Phase 2 Compact groups at high z: DPOSS II survey  Automated search of ~ 6200 sq. degrees of DPOSS II plates for small, high density groups  Search criteria: Richness: n ≥ 4, with  mag comp ≤ 2 Isolation: R isol ≥ 3 x R gr Compactness:  gr <  limit ;  limit, r = 24  Found: 459 candidates, with: = 0.13, extending out to z ~ 0.2 and an expected contamination: N random /N real = 10%

Compact groups at high z: the DPOSS II Compact Groups survey 70% concordant groups  3D = 299 km/s = H o -1 = 0.21 Gyr  /  ~ 80 R c ~50Kpc = 4.5 x M  = 92 = 0.12 ± 0.06 = 1.6 Gyr NTT+EMMI spectroscopic follow-up (P.I. E. Pompei)

Compact groups of galaxies: the environment Z=0.02 Z= /96 groups are isolated CLASS A 40/96 groups are within larger scale structures CLASS B Pompei et al., in prep.

Compact groups of galaxies: they like it small Class A Class B n=3 n=4 n=5 n=

Isolated compact groups of galaxies: characteristics  EW(H  ) > 6 Å for 2 or more confirmed group galaxies the CG is a late type group: 40% of the total  Groups with smaller velocity dispersion (  < 200 km/s), smaller mass (M < 2 x M   and mass-to-light ratio (M/L B < 15) are much more likely to be late type groups  No Sy1 has been found in isolated confirmed compact groups  HCG 16 siblings are rare both in SCGs and in DPOSS II groups: ~5% of the confirmed isolated CGs (Pompei et al., in prep.)

Compact groups of galaxies: groups indeed! Pompei et al., 2007 G. Trinchieri, A. Iovino, E. Pompei et al., 2008 T ~ 0.2 keV

Compact groups of galaxies: conclusions  Compact groups are real and physically bound structures  We observe them over a time span longer than their expected lifetime S Some of them are located at the edge of clusters of galaxies, in which they may be falling TTF+OSIRIS 3D-NTT S ~ 60% are isolated structures, sometimes with peculiar characteristics. We don’t know their final fate: isolated peculiar ellipticals?

From isolated compact groups to… Density of compact groups: 2-4 x h 3 Mpc -3 (Iovino et al. 2003) Density of fossil groups: 1-4 x h 3 Mpc -3 (Jones et al., 2003, Mendes de Oliveira et al., 2007) Density of clusters: 1-10 x h 3 Mpc -3 (Bramel et al., 2000)

….. the progeny?

Isolated ellipticals (maybe) Merger remnant: M B ≥ log(h 70 ) Memola et al., 2007 Pompei et al., 2008