540 research outputs found
Weak Lensing by High-Redshift Clusters of Galaxies II: Mean Redshift of the Faint Background Galaxy Population
We use weak lensing shear measurements of six z>0.5 clusters of galaxies to
derive the mean lensing redshift of the background galaxies used to measure the
shear. Five of these clusters are compared to X-ray mass models and verify a
mean lensing redshift for a 23<R<26.3, R-I<0.9 background galaxy population in
good agreement with photometric redshift surveys of the HDF-S. The lensing
strength of the six clusters is also analyzed as a function of the magnitude of
the background galaxies, and an increase in shear with increasing magnitude is
detected at moderate significance. The change in the strength of the shear is
presumed to be caused by an increase in the mean redshift of the background
galaxies with increasing magnitude, and the degree of change detected is also
in agreement with those in photometric redshift surveys of the HDF-S.Comment: 6 pages, 4 figures, accepted by A&
A search for gravitational lensing in 38 X-ray selected clusters of galaxies
We present the results of a CCD imaging survey for gravitational lensing in a
sample of 38 X-ray-selected clusters of galaxies. Our sample consists of the
most X-ray luminous (Lx>= 2x10^{44} erg s^{-1}) clusters selected from the
Einstein Observatory Extended Medium Sensitivity Survey (EMSS) that are
observable from Mauna Kea (dec > -40deg). The sample spans a redshift range of
0.15 0.5. CCD images of the
clusters were obtained in excellent seeing. There is evidence of strong
gravitational lensing in the form of giant arcs (length l > 8'', axis ratio l/w
> 10) in 8 of the 38 clusters. Two additional clusters contain shorter arclets,
and 6 more clusters contain candidate arcs that require follow-up observations
to confirm their lensing origin. Since the survey does not have a uniform
surface brightness limit we do not draw any conclusion based on the statistics
of the arcs found. We note, however, that 60% (3 of 5) of the clusters with Lx
> 10^{45} erg s^{-1}, and none of the 15 clusters with Lx < 4x10^{44} erg
s^{-1} contain giant arcs, thereby confirming that high X-ray luminosity does
identify the most massive systems, and thus X-ray selection is the preferred
method for finding true, rich clusters at intermediate and high redshifts. The
observed geometry of the arcs, most of which are thin, have large axis ratios
(l/w > 10), and are aligned orthogonal to the optical major axes of the
clusters, indicate the cluster core mass density profiles must be compact
(steeper than isothermal). In several cases, however, there is also some
evidence, in the form of possible radial arcs, for density profiles with finite
core radii.Comment: Latex file, 17 pages, 7 jpeg figures, to be published in Astronomy
and Astrophysics Supplement
Mass and Light in the Universe
We present a weak lensing and photometric study of six half by half degree
fields observed at the CFHT using the UH8K CCD mosaic camera. The fields were
observed for a total of 2 hours each in I and V, resulting in catalogs
containing ~ 20 000 galaxies per passband per field. We use V-I color and I
magnitude to select bright early type galaxies at redshifts 0.1 < z < 0.9. We
measure the gravitational shear from faint galaxies in the range 21 < m_I < 25
from a composite catalog and find a strong correlation with that predicted from
the early types if they trace the mass with mass-to-light ratio 300\pm75 h (in
solar units) for a flat (Omega_m0 = 0.3, Omega_l0 = 0.7) lambda cosmology and
400\pm100 h for Einstein-de Sitter. We make two-dimensional reconstructions of
the mass surface density. Cross-correlation of the measured mass surface
density with that predicted from the early type galaxy distribution shows a
strong peak at zero lag (significant at the 5.2-sigma level). We azimuthally
average the cross- and auto-correlation functions. We conclude that the
profiles are consistent with early type galaxies tracing mass on scales of > 45
arcsec (> 200 kpc at z = 0.5). We sub-divide our bright early type galaxies by
redshift and obtain similar conclusions. These mass-to-light ratios imply
\Omega_m0 = 0.10\pm0.02 (\Omega_m0 = 0.13\pm0.03 for Einstein-de Sitter) of
closure density.Comment: 27 pages, 19 figs (4 ps, 15 gif), 4 tables, accepted for publication
in Ap.J. (email Gillian for better resolution ps versions of gif greyscale
plots
Infall, the Butcher-Oemler Effect, and the Descendants of Blue Cluster Galaxies at z~0.6
Using wide-field HST/WFPC2 imaging and extensive Keck/LRIS spectroscopy, we
present a detailed study of the galaxy populations in MS2053--04, a massive,
X-ray luminous cluster at z=0.5866. Analysis of 149 confirmed cluster members
shows that MS2053 is composed of two structures that are gravitationally bound
to each other; their respective velocity dispersions are 865 km/s (113 members)
and 282 km/s (36 members). MS2053's total dynamical mass is 1.2x10^15 Msun.
MS2053 is a classic Butcher-Oemler cluster with a high fraction of blue members
(24%) and an even higher fraction of star-forming members (44%), as determined
from their [OII] emission. The number fraction of blue/star-forming galaxies is
much higher in the infalling structure than in the main cluster. This result is
the most direct evidence to date that the Butcher-Oemler effect is linked to
galaxy infall. In terms of their colors, luminosities, estimated internal
velocity dispersions, and [OII] equivalent widths, the infalling galaxies are
indistinguishable from the field population. MS2053's deficit of S0 galaxies
combined with its overabundance of blue spirals implies that many of these
late-types will evolve into S0 members. The properties of the blue cluster
members in both the main cluster and infalling structure indicate they will
evolve into low mass, L<L* galaxies with extended star formation histories like
that of low mass S0's in Coma. Our observations show that most of MS2053's blue
cluster members, and ultimately most of its low mass S0's, originate in the
field. Finally, we measure the redshift of the giant arc in MS2053 to be
z=3.1462; this object is one in only a small set of known strongly lensed
galaxies at z>3.Comment: Accepted by ApJ. Version with full resolution figures available at
http://www.exp-astro.phys.ethz.ch/tran/outgoing/ms2053.ps.g
- âŠ