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Exploring Compact Groups of Galaxies: Origin, Evolution, and Surroundings

Discover the intriguing nature of compact groups of galaxies, their origin, evolution, and relation to surroundings. Dive into observations at low and high redshifts, including nearby compact groups and challenging high-z searches. Join the quest to understand these small, dense, and elusive galactic assemblies.

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Exploring Compact Groups of Galaxies: Origin, Evolution, and Surroundings

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

  2. Compact groups: what are they? SCG 0018-4854 “The criteria” (HCGs) Richness: n ≥ 4 Isolation:Risol > 3 x Rgroup no galaxy in isolation ring Compactness:mG < 26

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

  4. Compact groups at low z: the Southern Compact Groups survey Dm = mest - mint > 3.35 NEW! 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 bj ≤ 14.5

  5. Southern Compact Groups at low z: observations and results <tc> = 0.051 Ho-1 = 0.76 Gyr s3D = 330 km/s fs = 0.69 dr/r ~ 80 Rc ~ 50 Kpc <M> = 7.7 x 1012 M <M/LB> = 207 <z> <= 0.02 = 0.29 Gyr 3.6m+EFOSC2 ATCA, Pompei et al., 2007

  6. Compact groups at high z: what’s going on • Las Campanas:76 compact groups from redshift survey,<z> ~ 0.08 • 2dFcompact group survey: compact group catalog from redshift survey.<z> = 0.11 • SDSScompact group survey: compact group catalog from imaging and spectroscopic survey.r*lim=21, 14 ≤ r*brightest ≤ 18,<z> = 0.126 • DPOSS IIcompact group survey: 459 compact groups from a search on digitized DPOSS plates.r ~ 21, 16 ≤ r brightest≤ 17 ,<z> = 0.13 Spectroscopic follow-up completed. Phase 2

  7. Compact groups at high z: z~0.2 Z = 1 1’ = 340 Kpc Z = 0.1 1’ = 119 Kpc Z = 0.01 1’ = 13 Kpc <Rgroup> ÷ 50-70 Kpc IT’S DIFFICULT TO FIND COMPACT GROUPS!

  8. 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 Dmagcomp ≤ 2 Isolation:Risol ≥ 3 x Rgr Compactness:mgr < mlimit; mlimit, r = 24 • Found: 459 candidates, with: <z> = 0.13, extending out to z ~ 0.2 and an expected contamination: Nrandom/Nreal = 10% Phase 2

  9. Compact groups at high z: the DPOSS II Compact Groups survey NTT+EMMI spectroscopic follow-up (P.I. E. Pompei) 70% concordant groups s3D = 299 km/s <tc> = 0.018 Ho-1 = 0.21 Gyr dr/r ~ 80 Rc ~50Kpc <M> = 4.5 x 1012 M <M/LB> = 92 <z> = 0.12 ± 0.06 = 1.6 Gyr

  10. Compact groups of galaxies: the environment 56/96 groups are isolated CLASS A Z=0.132 40/96 groups are within larger scale structures CLASS B Pompei et al., in prep. Z=0.02

  11. Compact groups of galaxies: they like it small Class A 16 32 6 2 n=5 n=4 n=6 n=3 Class B 1 10 20 8 n=4 n=6 n=5 n=3

  12. 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 1012M and mass-to-light ratio (M/LB < 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.)

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

  14. 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 • Some of them are located at the edge of clusters of galaxies, in which they may be falling TTF+OSIRIS 3D-NTT • ~ 60% are isolated structures, sometimes with peculiar characteristics. We don’t know their final fate: isolated peculiar ellipticals?

  15. From isolated compact groups to… Density of compact groups: 2-4 x 10-5 h3 Mpc-3(Iovino et al. 2003) Density of fossil groups:1-4 x 10-6 h3 Mpc-3(Jones et al., 2003, Mendes de Oliveira et al., 2007) Density of clusters: 1-10 x 10-6 h3 Mpc-3(Bramel et al., 2000)

  16. ….. the progeny? ?

  17. ?

  18. Memola et al., 2007 Isolated ellipticals (maybe) • Merger remnant: MB ≥ -21.9+5log(h70) Pompei et al., 2008

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