10,306 research outputs found
Weak Lensing Analysis of the z~0.8 cluster CL 0152-1357 with the Advanced Camera for Surveys
We present a weak lensing analysis of the X-ray luminous cluster CL 0152-1357
at z~0.84 using HST/ACS observations. The unparalleled resolution and
sensitivity of ACS enable us to measure weakly distorted, faint background
galaxies to the extent that the number density reaches ~175 arcmin^-2. The PSF
of ACS has a complicated shape that also varies across the field. We construct
a PSF model for ACS from an extensive investigation of 47 Tuc stars in a
modestly crowded region. We show that this model PSF excellently describes the
PSF variation pattern in the cluster observation when a slight adjustment of
ellipticity is applied. The high number density of source galaxies and the
accurate removal of the PSF effect through moment-based deconvolution allow us
to restore the dark matter distribution of the cluster in great detail. The
direct comparison of the mass map with the X-ray morphology from Chandra
observations shows that the two peaks of intracluster medium traced by X-ray
emission are lagging behind the corresponding dark matter clumps, indicative of
an on-going merger. The overall mass profile of the cluster can be well
described by an NFW profile with a scale radius of r_s =309+-45 kpc and a
concentration parameter of c=3.7+-0.5. The mass estimates from the lensing
analysis are consistent with those from X-ray and Sunyaev-Zeldovich analyses.
The predicted velocity dispersion is also in good agreement with the
spectroscopic measurement from VLT observations. In the adopted WMAP cosmology,
the total projected mass and the mass-to-light ratio within 1 Mpc are estimated
to be 4.92+-0.44 10^14 solar mass and 95+-8 solar mass/solar luminosity,
respectively.Comment: Accepted for publication in Astrophysical Journal. 58 pages, 26
figures. Figures have been degraded to meet size limit; a higher resolution
version available at http://acs.pha.jhu.edu/~mkjee/ms_cl0152.pd
HST/ACS weak lensing analysis of the galaxy cluster RDCS 1252.9-2927 at z=1.24
We present a weak lensing analysis of one of the most distant massive galaxy
cluster known, RDCS 1252.9-2927 at z=1.24, using deep images from the Advanced
Camera for Survey (ACS) on board the Hubble Space Telescope (HST). By taking
advantage of the depth and of the angular resolution of the ACS images, we
detect for the first time at z>1 a clear weak lensing signal in both the i
(F775W) and z (F850LP) filters. We measure a 5-\sigma signal in the i band and
a 3-\sigma signal in the shallower z band image. The two radial mass profiles
are found to be in very good agreement with each other, and provide a
measurement of the total mass of the cluster inside a 1Mpc radius of M(<1Mpc) =
(8.0 +/- 1.3) x 10^14 M_\odot in the current cosmological concordance model h
=0.70, \Omega_m=0.3, \Omega_\Lambda=0.7, assuming a redshift distribution of
background galaxies as inferred from the Hubble Deep Fields surveys. A weak
lensing signal is detected out to the boundary of our field (3' radius,
corresponding to 1.5Mpc at the cluster redshift). We detect a small offset
between the centroid of the weak lensing mass map and the brightest cluster
galaxy, and we discuss the possible origin of this discrepancy. The cumulative
weak lensing radial mass profile is found to be in good agreement with the
X-ray mass estimate based on Chandr and XMM-Newton observations, at least out
to R_500=0.5Mpc.Comment: 38 pages, ApJ in press. Full resolution images available at
http://www.eso.org/~prosati/RDCS1252/Lombardi_etal_accepted.pd
The old and heavy bulge of M31 I. Kinematics and stellar populations
We present new optical long-slit data along 6 position angles of the bulge
region of M31. We derive accurate stellar and gas kinematics reaching 5 arcmin
from the center, where the disk light contribution is always less than 30%, and
out to 8 arcmin along the major axis, where the disk makes 55% of the total
light. We show that the velocity dispersions of McElroy (1983) are severely
underestimated (by up to 50 km/s) and previous dynamical models have
underestimated the stellar mass of M31's bulge by a factor 2. Moreover, the
light-weighted velocity dispersion of the galaxy grows to 166 km/s, thus
reducing the discrepancy between the predicted and measured mass of the black
hole at the center of M31. The kinematic position angle varies with distance,
pointing to triaxiality. We detect gas counterrotation near the bulge minor
axis. We measure eight emission-corrected Lick indices. They are approximately
constant on circles. We derive the age, metallicity and alpha-element
overabundance profiles. Except for the region in the inner arcsecs of the
galaxy, the bulge of M31 is uniformly old (>12 Gyr, with many best-fit ages at
the model grid limit of 15 Gyr), slightly alpha-elements overabundant
([alpha/Fe]~0.2) and at solar metallicity, in agreement with studies of the
resolved stellar components. The predicted u-g, g-r and r-i Sloan color
profiles match reasonably well the dust-corrected observations. The stellar
populations have approximately radially constant mass-to-light ratios (M/L_R ~
4-4.5 for a Kroupa IMF), in agreement with stellar dynamical estimates based on
our new velocity dispersions. In the inner arcsecs the luminosity-weighted age
drops to 4-8 Gyr, while the metallicity increases to above 3 times the solar
value.Comment: Accepted for publication in A&
Electronic doping of graphene by deposited transition metal atoms
We perform a phenomenological analysis of the problem of the electronic
doping of a graphene sheet by deposited transition metal atoms, which aggregate
in clusters. The sample is placed in a capacitor device such that the
electronic doping of graphene can be varied by the application of a gate
voltage and such that transport measurements can be performed via the
application of a (much smaller) voltage along the graphene sample, as reported
in the work of Pi et al. [Phys. Rev. B 80, 075406 (2009)]. The analysis allows
us to explain the thermodynamic properties of the device, such as the level of
doping of graphene and the ionisation potential of the metal clusters in terms
of the chemical interaction between graphene and the clusters. We are also
able, by modelling the metallic clusters as perfect conducting spheres, to
determine the scattering potential due to these clusters on the electronic
carriers of graphene and hence the contribution of these clusters to the
resistivity of the sample. The model presented is able to explain the
measurements performed by Pi et al. on Pt-covered graphene samples at the
lowest metallic coverages measured and we also present a theoretical argument
based on the above model that explains why significant deviations from such a
theory are observed at higher levels of coverage.Comment: 16 pages, 10 figure
The GALATEA Test-Facility for High Purity Germanium Detectors
GALATEA is a test facility designed to investigate bulk and surface effects
in high purity germanium detectors. A vacuum tank houses an infrared screened
volume with a cooled detector inside. A system of three stages allows an almost
complete scan of the detector. The main feature of GALATEA is that there is no
material between source and detector. This allows the usage of alpha and beta
sources as well as of a laser beam to study surface effects. A 19-fold
segmented true-coaxial germanium detector was used for commissioning
Probing the Universe with Weak Lensing
Gravitational lenses can provide crucial information on the geometry of the
Universe, on the cosmological scenario of formation of its structures as well
as on the history of its components with look-back time. In this review, I
focus on the most recent results obtained during the last five years from the
analysis of the weak lensing regime. The interest of weak lensing as a probe of
dark matter and the for study of the coupling between light and mass on scales
of clusters of galaxies, large scale structures and galaxies is discussed
first. Then I present the impact of weak lensing for the study of distant
galaxies and of the population of lensed sources as function of redshift.
Finally, I discuss the potential interest of weak lensing to constrain the
cosmological parameters, either from pure geometrical effects observed in
peculiar lenses, or from the coupling of weak lensing with the CMB.Comment: To appear Annual Review of Astronomy and Astrophysiscs Vol. 37. Latex
and psfig.sty. Version without figure, 54 pages, 73Kb. Complete version
including 13 figures (60 pages) available on ftp.iap.fr anonymous account in
/pub/from_users/mellier/AnnualReview ; file ARAAmellier.ps.gz 1.6 M
Hubble Space Telescope weak lensing study of the z=0.83 cluster MS 1054-03
We have measured the weak gravitational lensing signal of MS 1054-03, a rich
and X-ray luminous cluster of galaxies at a redshift of z=0.83, using a
two-colour mosaic of deep WFPC2 images. The small corrections for the size of
the PSF and the high number density of background galaxies obtained in these
observations result in an accurate and well calibrated measurement of the
lensing induced distortion. The strength of the lensing signal depends on the
redshift distribution of the background galaxies. We used photometric redshift
distributions from the Northern and Southern Hubble Deep Fields to relate the
lensing signal to the mass. The predicted variations of the signal as a
function of apparent source magnitude and colour agrees well with the observed
lensing signal. We determine a mass of (1.2+-0.2)x10^15 Msun within an aperture
of radius 1 Mpc. Under the assumption of an isothermal mass distribution, the
corresponding velocity dispersion is 1311^{+83}_{-89} km/s. For the
mass-to-light ratio we find 269+-37 Msun/Lsun. The errors in the mass and
mass-to-light ratio include the contribution from the random intrinsic
ellipticities of the source galaxies, but not the (systematic) error due to the
uncertainty in the redshift distribution. However, the estimates for the mass
and mass-to-light ratio of MS 1054-03 agree well with other estimators,
suggesting that the mass calibration works well. The reconstruction of the
projected mass surface density shows a complex mass distribution, consistent
with the light distribution. The results indicate that MS 1054-03 is a young
system. The timescale for relaxation is estimated to be at least 1 Gyr.
Averaging the tangential shear around the cluster galaxies, we find that the
velocity dispersion of an Lstar galaxy is 203+-33 km/s.Comment: 21 pages, Latex, with 27 figures (3 figures bitmapped), ApJ, in
press. Version (with non-bitmapped figures) available at
http://www.astro.rug.nl/~hoekstra/papers.htm
Large-Scale QSO-Galaxy Correlations and Weak Lensing
Several recent studies show that bright, intermediate and high redshift
optically and radio selected QSOs are positively correlated with nearby
galaxies on a range of angular scales up to a degree. Obscuration by unevenly
distributed Galactic dust can be ruled out as the cause, leaving weak
statistical lensing as the physical process responsible. However the amplitude
of correlations on < 1 degree scales is at least a factor of a few larger than
lensing model predictions. A possible way to reconcile the observations and
theory is to revise the weak lensing formalism. We extend the standard lensing
formulation to include the next higher order term (second order) in the
geodesic equation of motion for photons. We derive relevant equations
applicable in the weak lensing regime, and discuss qualitative properties of
the updated formulation. We then perform numerical integrations of the revised
equation and study the effect of the extra term using two different types of
cosmic mass density fluctuations. We find that nearby large-scale coherent
structures increase the amplitude of the predicted lensing-induced correlations
between QSOs and foreground galaxies by ~ 10% (not a factor of several required
by observations), while the redshift of the optimal, i.e. `most correlated'
structures is moved closer to the observer compared to what is predicted using
the standard lensing equation.Comment: extended Section 2; 20 pages, including 4 figures, accepted to Ap
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