714 research outputs found
X-ray observations and mass determinations in the cluster of galaxies Cl0024+17
We present a detailed analysis of the mass distribution in the rich and
distant cluster of galaxies Cl0024+17. X-ray data come from both a deep
ROSAT/HRI image of the field (Bohringer et al. 1999) and ASCA spectral data.
Using a wide field CCD image of the cluster, we optically identify all the
faint X-ray sources, whose counts are compatible with deep X-ray number counts.
In addition we marginally detect the X-ray counter-part of the gravitational
shear perturbation detected by Bonnet et al. (1994) at a 2.5 level. A
careful spectral analysis of ASCA data is also presented. In particular, we
extract a low resolution spectrum of the cluster free from the contamination by
a nearby point source located 1.2 arcmin from the center. The X-ray temperature
deduced from this analysis is keV at the 90%
confidence level. The comparison between the mass derived from a standard X-ray
analysis and from other methods such as the Virial Theorem or the gravitational
lensing effect lead to a mass discrepancy of a factor 1.5 to 3. We discuss all
the possible sources of uncertainties in each method of mass determination and
give some indications on the way to reduce them. A complementary study of
optical data is in progress and may solve the X-ray/optical discrepancy through
a better understanding of the dynamics of the cluster.Comment: Revised version, accepted in Astronomy and Astrophysics (Main
Journal). Few changes in the discussio
The ESO-Sculptor Faint Galaxy Redshift Survey: The Photometric Sample
We present the photometric sample of a faint galaxy survey carried out in the
southern hemisphere, using CCDs on the 3.60m and NTT-3.5m telescopes at La
Silla (ESO). The survey area is a continuous strip of 0.2 deg x 1.53 deg
located at high galactic latitude (-83 deg) in the Sculptor constellation. The
photometric survey provides total magnitudes in the bands B, V (Johnson) and R
(Cousins) to limiting magnitudes of 24.5, 24.0, 23.5 respectively. To these
limits, the catalog contains about 9500, 12150, 13000 galaxies in B, V, R bands
respectively and is the first large digital multi-colour photometric catalog at
this depth. This photometric survey also provides the entry catalog for a
fully-sampled redshift survey of ~ 700 galaxies with R < 20.5 (Bellanger et al.
1995). In this paper, we describe the photometric observations and the steps
used in the data reduction. The analysis of objects and the star-galaxy
separation with a neural network are performed using SExtractor, a new
photometric software developed by E. Bertin (1996). The photometric accuracy of
the resulting catalog is ~ 0.05 mag for R < 22. The differential galaxy number
counts in B, V, R are in good agreement with previously published CCD studies
and confirm the evidence for significant evolution at faint magnitudes as
compared to a standard non evolving model (by factors 3.6, 2.6, 2.1). The
galaxy colour distributions B-R, B-V of our sample show a blueing trend of ~
0.5 mag between 21 < R < 23.5 in contrast to the V-R colour distribution where
no significant evolution is observed.Comment: LATEX, 18 Postscript figures, 20 pages. To appear July 1997. Modified
version of article. Abstract corrected for missing lin
Detection of weak lensing by a cluster of galaxies at z=0.83
We report the detection of weak gravitational lensing of faint, distant
background galaxies by the rich, X-ray luminous cluster of galaxies MS1054-03
at z=0.83. This is the first measurement of weak lensing by a bona fide cluster
at such a high redshift. We detect tangential shear at the 5% - 10% level over
a range of radii 50'' < r < 250'' centered on the optical position of the
cluster. Two-dimensional mass reconstruction using galaxies with 21.5 < I <
25.5 shows a strong peak which coincides with the peak of the smoothed cluster
light distribution. Splitting this sample by magnitude (at I = 23.5) and color
(at R-I = 0.7), we find that the brighter and redder subsamples are only very
weakly distorted, indicating that the faint blue galaxies (FBG's), which
dominate the shear signal, are relatively more distant. The derived cluster
mass is quite sensitive to the N(z) for the FBG's. At one extreme, if all the
FBG's are at z_s = 3, then the mass within a Mpc aperture is \h1 , and the mass-to-light ratio is in solar units. For the derived mass is 70\%
higher and . If follows the no evolution model (in
shape) then , and if all the FBG's lie at z_s\la 1 the
required exceeds . These data provide clear evidence that large,
dense mass concentrations existed at early epochs; that they can be weighed
efficiently by weak lensing observations; and that most of the FBG's are at
high redshift.Comment: Submitted to ApJ, 15 pages (incl 8 figs, 3 of which are plates).
Plate images not included, but are available from
ftp://hubble.ifa.hawaii.edu/pub/ger/ms1054/ms1054_fig[1,3,5].ps.
The descendents of Lyman Break Galaxies in galaxy clusters: spatial distribution and orbital properties
We combine semi-analytical methods with a ultra-high resolution simulation of
a galaxy cluster (of mass 2.3 10^14h-1Msolar, and 4 10^6 particles within its
virial radius) formed in a standard CDM universe to study the spatial
distribution and orbital properties of the present-day descendents of Lyman
Break Galaxies (LBGs). At the present time only five (out of 12) of halos
containing LBGs survive as separate entities inside the cluster virial radius.
Their circular velocities are in the range 200 - 550 km/sec. Seven halos merged
together to form the central object at the very center of the cluster. Using
semi-analytical modeling of galaxy evolution we show that descendents of halos
containing LBGs now host giant elliptical galaxies. Galaxy orbits are radial,
with a pericenter to apocenter ratio of about 1:5. The orbital eccentricities
of LBGs descendents are statistically indistinguishable from those of the
average galaxy population inside the cluster, suggesting that the orbits of
these galaxies are not significantly affected by dynamical friction decay after
the formation of the cluster's main body. In this cluster, possibly due to its
early formation time, the descendents of LBGs are contained within the central
60% of the cluster virial radius and have an orbital velocity dispersion lower
than the global galaxy population, originating a mild luminosity segregation
for the brightest cluster members. Mass estimates based only on LBGs
descendents (especially including the central cD) reflect this bias in space
and velocity and underestimate the total mass of this well virialized cluster
by up to a factor of two compared to estimates using at least 20 cluster
members.Comment: 6 Pages, 2 Postscript figures. Submitted to Ap
A Collision of Subclusters in Abell 754
We present direct evidence of a collision of subclusters in the galaxy
cluster Abell 754. Our comparison of new optical data and archival ROSAT PSPC
X-ray data reveal three collision signatures predicted by n-body/hydrodynamical
simulations of hierarchical cluster evolution. First, there is strong evidence
of a non-hydrostatic process; neither of the two major clumps in the galaxy
distribution lies on the off-center peak of the X-ray emission from the
intracluster gas. Second, the peak of the X-ray emission is elongated
perpendicular to the collision axis defined by the centroids of the two galaxy
clumps. Third, there is evidence of compression-heated gas; one of A754's two
X-ray temperature components (Henry & Briel 1995) is among the hottest observed
in any cluster and hotter than that inferred from the velocity dispersion of
the associated galaxy clump. These signatures are consistent with the
qualitative features of simulations (Evrard 1990a,b) in which two subclusters
have collided in the plane of the sky during roughly the last Gyr. The
detection of such collisions is crucial for understanding both the dynamics of
individual clusters and the underlying cosmology. First, for systems like A754,
estimating the cluster X-ray mass from assumptions of hydrostatic equilibrium
and isothermality is incorrect and may produce the discrepancies sometimes
found between X-ray masses and those derived from gravitational lens models
(Babul & Miralda-Escude 1994). Second, the fraction of nearby clusters in which
subclusters have collided in the last Gyr is especially sensitive to the mean
mass density parameter Omega_0 (cf. Richstone et al. 1992; Evrard et al. 1993;
Lacey & Cole 1993). With a large, well-defined cluster sample, it will be
possible to place a new and powerful constraint on cosmological models.Comment: 4 pages + 1 color figure (Postscript). Accepted for Publication in
ApJ Letter
Pure Luminosity Evolution Hypothesis for QSOs: From Luminosity Functions to Synthetic Catalogues
This paper describes the simulation of realistic Monte-Carlo extragalactic
catalogues, aimed at comparing the behaviour of cosmological tests versus input
parameters. QSO catalogues are built on a Luminosity Function derived from data
through suitable computation of individual maximum volumes in complete (but
magnitude- and redshift-limited) samples requiring neither of redshift nor of
apparent magnitude histogram. The values of the evolution parameter are derived
for various cosmologies, corresponding to =1/2 in the sample of 400
Ultra-Violet Excess (UVX) QSOs (Boyle et al 1990). The various luminosity
functions are compared, both for the whole sample and in redshift bins. An
evolution characteristic time is defined and computed, depending strongly on
the cosmology, but practically constant when expressed in terms of the age of
the Universe. Algorithms are given for producing unbiased or biased catalogues
based on the null hypothesis that the objects are uniformly distributed in
volume but suffer Pure Luminosity Evolution.Comment: uuencode compressed tar file of Latex and macros files. Tar
compressed poscript files of the papers and figures are also available by
anonymous ftp at ftp://summer.obs-mip.fr/pub/OUTGOING/paper2 or upon request
at [email protected]
The Nature of Star Formation in Lensed Galaxies at High Redshift
We present near-infrared photometry of all available gravitationally lensed âarcsâ with spectroscopic redshifts. By combining this photometry with optical data, we find that the bulk of the systems with z ~ 1 are intrinsically blue across the rest-frame spectral region 2000 Ă
to 1 ÎŒm. Using a combination of optical and optical-infrared colours, we demonstrate that these systems cannot be blue by virtue of a secondary burst of star formation superimposed on an evolved population, but we are unable to distinguish directly between major star formation events in a generic young galaxy and an extended era of constant star formation typical of late-type spirals. Using various arguments, we conclude that our arcs represent modest gravitational magnification of typical field galaxies. Consequently, if the star formation seen is representative of that in field galaxies at z â„ 1, the absence of high-redshift galaxies in current deep spectroscopic surveys to bJâ24bJâ24 supports the hypothesis that the bulk of the star formation in normal galaxies occurred over an extended era up to the epoch corresponding to z ~ 1
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