1 / 29

Galaxy groups

Galaxy groups. Michael Balogh Department of Physics and Astronomy University of Waterloo. Outline. Where do groups fit in the hierarchy? Group selection methods Properties of galaxies in groups Theoretical challenges. What is a group?. ~few L* galaxies

tala
Download Presentation

Galaxy groups

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Galaxy groups Michael Balogh Department of Physics and Astronomy University of Waterloo

  2. Outline • Where do groups fit in the hierarchy? • Group selection methods • Properties of galaxies in groups • Theoretical challenges

  3. What is a group? • ~few L* galaxies • Mhalo~1012-5x1013 (s<500 km/s) • Physically associated – but not necessarily virialized • At higher masses, galaxy population seems to be weakly dependent on halo mass

  4. Buildup of structure • Group abundance evolves strongly • Fraction of galaxies in groups (N>6) increases by about a factor 3 since z=1 Knobel et al. (2009)

  5. Cluster growth via groups • Clusters grow via: • Major mergers between clusters • Accretion of groups • Accretion of isolated galaxies • Low-mass clusters may accrete much of their mass directly from the field Berrier et al. (2008)

  6. Cluster growth via groups • M=1014.2 clustersaccrete 35% of galaxies via groups • For Coma-like clusters, fraction is 50%. McGee et al. (2009), using Font et al. (2008) model

  7. Pre-processing • Importance of groups also depends on how long these galaxies reside in group environment. And main progenitor was itself a group at some point. • Use “processed galaxies” as tracer of accretion histories. • Assume galaxies “transform” T Gyr after first accretion into a halo >M.

  8. Fraction of processed galaxies Halo mass Slow truncation • Without preprocessing: not only would groups be field-like, but clusters would show much more scatter McGee et al. (2009)

  9. Fraction of processed galaxies Halo mass Slow truncation • And z evolution would be rapid • Ellingson et al. (2001) used this argument to support long (T~3Gyr) timescales from CNOC clusters McGee et al. (2009)

  10. Group preprocessing • Slow timescale, low mass threshold predicts: • Tight red sequence at z=0 • Weak dependence on halo mass • Moderate evolution: negligible red fraction by z=1.5 McGee et al. (2009) Halo mass

  11. Group Selection Methods • Redshift surveys • Xray • Photometric surveys

  12. Redshift surveys • 2dFGRS/SDSS • >4500 sq degrees • >5000 groups with z<0.1 • CNOC2 • 1.5 sq degrees • 200 groups 0.2<z<0.55 • Extensive follow-up of ~30 groups • zCOSMOS • 1.7 sq degree • 800 groups 0.1<z<1 • DEEP II • 1 sq degree • 899 groups with 2 or more members • 0.7<z<1.4

  13. X-ray selection: low-z • ROSAT able to detect nearby systems with s~100 km/s or greater • Zabludoff & Mulchaey (1998) • Osmond & Ponman (2004) • Rasmussen et al. (2008) Mulchaey & Zabludoff (1998)

  14. X-ray selection: higher z • XMM-LSS (~10 ks) • Willis et al. (2005) • Mulchaey et al. (2007); Jeltema et al. (2007, 2008) • Nine X-ray groups at 0.2<z<0.6, from ROSAT DCS • These probe low-mass cluster regime, but not true groups Mulchaey et al. (2006)

  15. X-ray selection: higher z • CNOC2 fields: Chandra and XMM data – combined depth equivalent to 469 ksec (Chandra) • c.f. ~160 ks in COSMOS z=0.4 See also Knobel et al. (2009) Finoguenov et al. (in prep)

  16. Photometric selection • McConnachie et al. (2008) use SDSS to detect 7400 compact groups, photometrically. • Attempt to correct for contamination using simulations

  17. Photometric selection • RCS: not effective in the group regime • Completeness trusted down to s~300 km/s. Gilbank et al. (2007)

  18. Group properties

  19. SDSS groups • Weak correlation with halo mass for clusters • Evidence for larger blue fractions in groups Bamford et al. (2009)

  20. Groups and clusters • Low-mass satellite galaxies show dependence on halo mass on group scales Also Weinmann et al. 2006, Pasquali et al. 2009 Kimm et al. 2009

  21. Properties of X-ray groups • Spiral fraction in X-ray groups correlates with s, Tx • X-ray bright groups tend to be spiral-poor (e.g. Brough et al. 2006) • Significant scatter in early fraction (Mulchaey & Zabludoff 1998) • HI deficiency independent of X-ray properties in compact groups (Rasmussen et al. 2008) Osmond & Ponman (2004)

  22. Groups at z=0.5 • At fixed stellar mass, groups have fewer blue galaxies than the field Balogh et al. (2009)

  23. Groups at z=0.5 Balogh et al. (2009)

  24. Groups and clusters at z=0.5 • Galaxies show a halo-mass dependence: • Red fractions of groups intermediate between cluster and field environments Balogh et al. (2009)

  25. Low-sfr galaxies • Mounting evidence that there may be a transition population of dust-reddened, low-sfr galaxies found in intermediate environments • STAGES supercluster: Wolf et al. (2008); Gallazzi et al. (2008) • SDSS: Skibba et al. (2008); Bamford et al. (2008) • Virgo: Crowl & Kenney (2008); Hughes et al. (2009) • HCGs: Johnson et al. (2007); Gallagher et al. (2008)

  26. Theoretical challenges

  27. Rapid strangulation • Compare z=0.5 group galaxy colour distribution with models • Narrow range of NIR luminosity • Simple models overpredict the red fraction (but actually do a pretty good job) • The blue galaxies are near the group halo – but not actually subhaloes Balogh et al. (2009)

  28. Slow strangulation • Models which slow the rate of transformation • Destroys distinct bimodality • Maybe only a fraction of group galaxies should be affected; orbit-dependent? • Puzzle: strangulation should be slow for low-mass galaxies (e.g. Haines, Rasmussen)… why so quick in GALFORM? Balogh et al. (2009)

  29. Conclusions • Robust samples of groups at 0<z<1 now routinely available • All require good mock catalogues to account for contamination, selection effects • Need more precise measures of SFH • Dust-obscured star formation • SF on long vs short timescales • Need to find source of scatter in group properties • Lx-M residuals? Concentration? Dynamics? Associated large-scale structure?

More Related