272 research outputs found
Coma cluster object populations down to M_R~-9.5
This study follows a recent analysis of the galaxy luminosity functions and
colour-magnitude red sequences in the Coma cluster (Adami et al. 2007). We
analyze here the distribution of very faint galaxies and globular clusters in
an east-west strip of arcmin crossing the Coma cluster
center (hereafter the CS strip) down to the unprecedented faint absolute
magnitude of M. This work is based on deep images obtained at the
CFHT with the CFH12K camera in the B, R, and I bands. The analysis shows that
the observed properties strongly depend on the environment, and thus on the
cluster history. When the CS is divided into four regions, the westernmost
region appears poorly populated, while the regions around the brightest
galaxies NGC 4874 and NGC 4889 (NGC 4874 and NGC 4889 being masked) are
dominated by faint blue galaxies. They show a faint luminosity function slope
of -2, very significantly different from the field estimates. Results are
discussed in the framework of galaxy destruction (which can explain part of the
very faint galaxy population) and of structures infalling on to Coma.Comment: To be published in A&
Effect of Gravitational Lensing on Measurements of the Sunyaev-Zel'dovich Effect
The Sunyaev-Zel'dovich (SZ) effect of a cluster of galaxies is usually
measured after background radio sources are removed from the cluster field.
Gravitational lensing by the cluster potential leads to a systematic deficit in
the residual intensity of unresolved sources behind the cluster core relative
to a control field far from the cluster center. As a result, the measured
decrement in the Rayleigh-Jeans temperature of the cosmic microwave background
is overestimated. We calculate the associated systematic bias which is
inevitably introduced into measurements of the Hubble constant using the SZ
effect. For the cluster A2218, we find that observations at 15 GHz with a beam
radius of 0'.4 and a source removal threshold of 100 microJy underestimate the
Hubble constant by 6-10%. If the profile of the gas pressure declines more
steeply with radius than that of the dark matter density, then the ratio of
lensing to SZ decrements increases towards the outer part of the cluster.Comment: 11 pages, 3 figures, submitted to ApJ
Properties of Faint Distant Galaxies as seen through Gravitational Telescopes
This paper reviews the most recent developments related to the use of lensing
clusters of galaxies as Gravitational Telescopes in deep Universe studies. We
summarize the state of the art and the most recent results aiming at studying
the physical properties of distant galaxies beyond the limits of conventional
spectroscopy. The application of photometric redshift techniques in the context
of gravitational lensing is emphasized for the study of both lensing structures
and the background population of lensed galaxies. A presently ongoing search
for the first building blocks of galaxies behind lensing clusters is presented
and discussed.Comment: Review lecture given at "Gravitational Lensing: a unique tool for
cosmology",Aussois, France, January 2003. To appear in ASP Conf. S., eds. D.
Valls-Gabaud & J.-P. Kneib, 26 pages, 8 figure
Cluster Mass Estimate and a Cusp of the Mass Density Distribution in Clusters of Galaxies
We study density cusps in the center of clusters of galaxies to reconcile
X-ray mass estimates with gravitational lensing masses. For various mass
density models with cusps we compute X-ray surface brightness distribution, and
fit them to observations to measure the range of parameters in the density
models. The Einstein radii estimated from these density models are compared
with Einstein radii derived from the observed arcs for Abell 2163, Abell 2218,
and RX J1347.5-1145. The X-ray masses and lensing masses corresponding to these
Einstein radii are also compared. While steeper cusps give smaller ratios of
lensing mass to X-ray mass, the X-ray surface brightnesses estimated from
flatter cusps are better fits to the observations. For Abell 2163 and Abell
2218, although the isothermal sphere with a finite core cannot produce giant
arc images, a density model with a central cusp can produce a finite Einstein
radius, which is smaller than the observed radii. We find that a total mass
density profile which declines as produces the largest radius
in models which are consistent with the X-ray surface brightness profile. As
the result, the extremely large ratio of the lensing mass to the X-ray mass is
improved from 2.2 to 1.4 for Abell 2163, and from 3 to 2.4 for Abell 2218. For
RX J1347.5-1145, which is a cooling flow cluster, we cannot reduce the mass
discrepancy.Comment: 23 pages, 10 figures, Latex, uses aasms4.sty, accepted for
publication in ApJ, Part
Chandra Observations of Galaxy Cluster Abell 2218
We present results from two observations (combined exposure of ~17 ks) of
galaxy cluster A2218 using the Advanced CCD Imaging Spectrometer on board the
Chandra X-ray Observatory that were taken on October 19, 1999. Using a
Raymond-Smith single temperature plasma model corrected for galactic absorption
we find a mean cluster temperature of kT = 6.9+/-0.5 keV, metallicity of
0.20+/-0.13 (errors are 90 % CL) and rest-frame luminosity in the 2-10 keV
energy band of 6.2x10^{44} erg/s in a LambdaCDM cosmology with H_0=65 km/s/Mpc.
The brightness distribution within 4'.2 of the cluster center is well fit by a
simple spherical beta model with core radius 66".4 and beta = 0.705 . High
resolution Chandra data of the inner 2' of the cluster show the x-ray
brightness centroid displaced ~22" from the dominant cD galaxy and the presence
of azimuthally asymmetric temperature variations along the direction of the
cluster mass elongation. X-ray and weak lensing mass estimates are in good
agreement for the outer parts (r > 200h^{-1}) of the cluster; however, in the
core the observed temperature distribution cannot reconcile the x-ray and
strong lensing mass estimates in any model in which the intracluster gas is in
thermal hydrostatic equilibrium. Our x-ray data are consistent with a scenario
in which recent merger activity in A2218 has produced both significant
non-thermal pressure in the core and substructure along the line of sight; each
of these phenomena probably contributes to the difference between lensing and
x-ray core mass estimates.Comment: 33 pages, 6 figures, uses AASTeX 5.02, ApJ submitte
Caustics of Compensated Spherical Lens Models
We consider compensated spherical lens models and the caustic surfaces they
create in the past light cone. Examination of cusp and crossover angles
associated with particular source and lens redshifts gives explicit lensing
models that confirm previous claims that area distances can differ by
substantial factors from angular diameter distances even when averaged over
large angular scales. `Shrinking' in apparent sizes occurs, typically by a
factor of 3 for a single spherical lens, on the scale of the cusp caused by the
lens; summing over many lenses will still leave a residual effect.Comment: 21 pages, 5 ps figures, eps
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