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Nearby groups of galaxies. J. Vennik, T õravere 21.06.2005. Introduction. Groups of galaxies (GG) provide sites in which: to study e nvironmental dependences of galaxy properties (field-groups-clusters); to study interactions btw. galaxies and hot/cold intra-group matter (IGM),

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nearby groups of galaxies

Nearby groups of galaxies

J. Vennik,Tõravere 21.06.2005

introduction
Introduction

Groups of galaxies (GG) provide sites in which:

  • to study environmental dependences of galaxy properties (field-groups-clusters);
  • to study interactions btw. galaxies and hot/cold intra-group matter (IGM),
  • to trace large-scale structure (LSS) of the Universe.

Outstanding questions regarding the groups:

  • are groups bound physical systems?
  • if bound, why have more of them merged, given their high density and short crossing time?
  • typical mass of the group and their contibution to mass density of Universe,
  • how is the evolution of galaxies different in groups nd clusters?
  • evolution of groups in the time.

To this purpose: extensive, homogeneous catalogs are needed, representing

hierarchy of bound (?) systems in local and distant Universe.

group catalogs
Group catalogs
    • Local(many catalogs, selected either by FoF or hierarchical (dendrogram) algorithms)
  • UZC-SSRS2 (USGC)– Ramella et al. (2002)
  •  mlim=15.5, cz ≤ 12000 km/s, 4.69 str
  • FoF, δρ/ρ = 80
  • 1168 groups (ngal ≥ 3),
  • 411 groups (ngal ≥ 5),
  • 61 X-ray (RASSCALS) groups
    • Deep surveys (LCRS, 2dFGRS,SDSS, DPOSS, CNOC2, DEEP2, VIMOS)
  • Suitable for statistical studies
  • 2dFGRS: (1300 deg2, bj ≤ 19.45)
  • Merchan & Zandivares (2002)
  • Eke et al. (2004): 2PIGG: 7020 groups (ngal ≥ 4) / z ~ 0.11, σv ~ 260 km/s, 55% in groups/
  • Yang et al.(2005)
  • SDSS: (4000 deg2, r ≤ 17.77)
2dfgrs gr oups luminosity function
2dFGRS groups: Luminosity function
  • Schechter function:L*increases and faint-end slope (α) decreases with increasing group total mass
2dfgrs gr oups star formation properties
2dFGRS groups: Star-formation properties
  • 2 populations with very different on-going SFE exist;
  • relative ratios of 2 pop-s changes with environment (density);
  • pop. with high SFE [W0(Hα) > 4] appears weakly sensitive to environment except centers of rich clusters, where W0(Hα) is strongly depleted;
  • possible explanation - galaxy pre-processing in pairs/groups, at higher z could be responsible for observed trends at z ~ 0.
2dfgrs gr oups mass to light ratio
2dFGRS groups: Mass-to-light ratio
  • M/L ratio increases (i.e. SF efficiencydecreases) at larger group(halo) masses in the range L = 1011 - 1012 LO
  • Expected upturn in SFE at LG masses (1012 MO) is not yet reached;
  • (M/L)med[3 1011 LO] = 466 yields Ώm = 0.26 ± 0.3 in agreement with LCDM theory and CMB data.
slide7

Possible evolutionary bottom-up scenario

    • at high z small groups could start forming in high-density regions; interactions (mergers, tidal interactions) take place and have sufficient time to modify galaxy properties - that is past pre-processing, responsible for today’s galaxy properties;
    • ~ 30% quiescent galaxies in the field could be fossile groups – end products of some compact groups (CGs), while having higher LX/Lvisratio, suggesting old age;
    • in local low σv (σv <150) groups higher SFR is observed; their brightest galaxies are bluer – i.e. low σv and high density enhance interactions;
    • CGs are proper to probe how galaxy-galaxy low velocity encounters and merging can shape galaxy properties (CGs should be less sensitive to ram-pressure stripping and galaxy harassments, which work in clusters).
    • 75% of CGs show X-ray emission & signs of interactions in bright member-galaxies,
  • but CGs are different (true CGs, core + halo, in loose groups).
environment of poor groups its effect on galaxy population
Environment of poor groups – its effect on galaxy population
  • / z-survey of 12 groups (vlim = 19), incl. 9 X-ray groups – Zabludoff & Mulchaey (2000) /
  • X-ray groups:
    • typically 30-60 members, show substructure – evidently still accreting subgroups;
    • σv (R) not decreasing out to Rvir – i.e. significant mass located in group halo, preventing merging to single central galaxy;
    • fraction of early-type galaxies is prop. to σv - similar to clusters
    • σv > 400 km/s is cluster domain – merging ceases;
    • 6 bright X-ray groups (log Lx > 41.2) show morphology – radius relation, i.e. B/D-ratio decreases with increasing projected radius;
    • disk-dominated gal-s are less asymmetric and with lower W0(Hα) than those in the field, i.e. X-ray groups are old, dynamically evolved, and signs of past interactions have been washed out
  • Non-X-ray groups:
    • have fewer ( < 10) members/faint satellites - are they bound ?
    • lower σv and lower fraction early-type galaxies;
    • their LF show dip btw. –19 < MB < -17, probably a result of recent merging of intermediate luminosity galaxies;
    • Local Group as prototype is bound and probably collapsing for the first time.
slide9

Conclusions:

  • spiral-rich, low-mass groups formed more recently, should observe higher SF activity than in the field, more morphological disturbances, spiral-fraction lower or equal than in the field.
  • observations corroborate possible evolutionary sequence from low σv , spiral-dominated, actively SF groups to higher σv , early-type dominated, X-ray groups.
  • Where are young groups ?
  • HCG 16 / cz =3400, σv(8) = 85, LX = 2.4 1040 h-2 erg/s /
  • In extremely active phase: 1 Sy + 2 LINERs + 3 STBGs - prototype of on-going galaxy pre-processing (?)
  • SCG 0018-4854 / cz = 3400, σv(5) = 67, LX = 1.2 1040 h-2 /
  • Spiral-only group but detected in X-rays by XMM Newton
slide10

Identified as: KPG 548 (2) - Karachentsev ( ) WBL 666 (7) - White et al. (1999) RSCG 78 (4) - Barton et al. (1996) UZC-CG 272 (6) - Focardi et al. (2002) USGC (9) - Ramella et al. (2002)

NGC 6962 group

RA = 20h47m19.1s

DEC = 00d19m15s

z = 0.014

Dist = 55 Mpc

m-M = 33.84

slide17

IC 65 groupRA = 01h00m55.4sDEC = 47d40m50sb = - 15.2 degcz = 2890 km/sDist = 38.5 Mpcm-M = 32.93Early identification:LGG 016 (3) – Garcia (1993)WSRT - HI, v. Moorsel (1983)

summary
concentrated, core + halo

clear morphological segregation

but luminous E’s are missing

non X-ray group (consistent w. frac(E), low σv and M/L)

substructure (probably collapsing, but short crossing time - ?)

pure S + Irr group

non X-ray group

HI-rich, active SF (both in bright and dwarf members)

Summary

NGC 6962 group IC 65 group

n: 20 8/6

σv (km/s): 238 77

Rh (kpc): 186 136

LBT (1010 LO): 1.4 5.8

M (1012 MO): 23.1 2.3

M/L (1010 M/LO): 170 38

tcrosH0: 0.46 0.09

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