arXiv:0906.0803 [ps, pdf, other]
    Title: Galaxy clusters identified from the SDSS DR6 and their
    properties
    Authors: Z. L. Wen, J. L. Han, F. S. Liu
    Comments: 18 pages, 27 figures, 2 tables; accepted for publication
    in ApJS
    Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

    Clusters of galaxies in most previous catalogs have redshifts
    z<0.3. Using the photometric redshifts of galaxies from the Sloan
    Digital Sky Survey Data Release 6 (SDSS DR6), we identify 39,716
    clusters in the redshift range 0.05< z <0.6 with more than 8
    luminous (M_r<-21) member galaxies. Cluster redshifts are
    estimated accurately with an uncertainty less than 0.022. The
    contamination rate of member galaxies is found to be roughly 20%,
    and the completeness of member galaxy detection reaches to
    ~90%. Monte Carlo simulations show that the cluster detection rate
    is more than 90% for massive (M_{200}>2\times10^{14} M_{\odot})
    clusters of z<0.42. The false detection rate is ~5%. We obtain the
    richness, the summed luminosity and the gross galaxy number within
    the determined radius for identified clusters. They are tightly
    related to the X-ray luminosity and temperature of
    clusters. Cluster mass is related to the richness and summed
    luminosity with M_{200}\propto R^{1.90\pm0.04} and M_{200}\propto
    L_r^{1.64\pm0.03}, respectively. In addition, 790 new candidates
    of X-ray clusters are found by cross-identification of our
    clusters with the source list of the ROSAT X-ray survey.
[

 arXiv:0905.3401 [ps, pdf, other]
    Title: The colour of galaxies in distant groups
    Authors: Michael L. Balogh (1), Sean L. McGee (1), Dave Wilman
    (2), Richard G. Bower (3), George Hau (4), Simon L. Morris (3),
    J.S. Mulchaey (5), A. Oemler Jr. (5), Laura Parker (6), Stephen
    Gwyn (7) ((1) U. Waterloo, (2) MPE, Garching, (3) Durham, (4)
    Swinburne, (5) OCIW, (6) McMaster, (7) CADC, HIA)
    Comments: Resubmitted to MNRAS following referee report
    Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

    (Abridged) We present new optical and near-infrared imaging for a
    sample of 98 spectroscopically-selected galaxy groups at
    0.25<z<0.55. We measure accurate colours for group members and the
    surrounding field population, statistically complete above a
    stellar mass limit of M=1E10 Msun. The overall colour distribution
    is bimodal in both the field and group samples; but at fixed
    luminosity the fraction of group galaxies populating the red peak
    is larger, by 20+/-7 per cent, than that of the field. In
    particular, group members with early-type morphologies, as
    identified in HST imaging, exhibit a tight red sequence, similar
    to that seen for more massive clusters. We show that approximately
    20-30 per cent of galaxies on the red sequence may be
    dust-reddened galaxies with non-negligible star formation and
    early-spiral morphologies. This is true of both the field and
    group sample, and shows little dependence on near infrared
    luminosity. Thus, the fraction of bright group members with no
    sign of star formation or AGN activity is 54+/-6 per cent. Our
    field sample, which includes galaxies in all environments,
    contains 35+/-3 per cent of such inactive galaxies, consistent
    with the amount expected if all such galaxies are located in
    groups and clusters. This reinforces our earlier conclusions, that
    dense environments at z<0.5 are associated with a premature
    cessation of star formation in some galaxies; in particular we
    find no evidence for significantly enhanced star formation in
    these environments. Simple galaxy formation models predict a
    quenching of star formation in groups that is too efficient,
    overpopulating the red sequence. Attempts to fix this by
    increasing the timescale of this quenching equally for all group
    members distorts the colour distribution in a way that is
    inconsistent with observations.