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Compact groups of galaxies: small, dense, elusive

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. Compact groups: what are they?. SCG 0018-4854.

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Compact groups of galaxies: small, dense, elusive

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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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