162,687 research outputs found
A Comparison of Simple Mass Estimators for Galaxy Clusters
High-resolution N-body simulations are used to investigate systematic trends
in the mass profiles and total masses of clusters as derived from 3 simple
estimators: (1) the weak gravitational lensing shear field under the assumption
of an isothermal cluster potential, (2) the dynamical mass obtained from the
measured velocity dispersion under the assumption of an isothermal cluster
potential, and (3) the classical virial estimator. The clusters consist of
order 2.5e+05 particles of mass m_p \simeq 10^{10} \Msun, have triaxial mass
distributions, and significant substructure exists within their virial radii.
Not surprisingly, the level of agreement between the mass profiles obtained
from the various estimators and the actual mass profiles is found to be
scale-dependent.
The virial estimator yields a good measurement of the total cluster mass,
though it is systematically underestimated by of order 10%. This result
suggests that, at least in the limit of ideal data, the virial estimator is
quite robust to deviations from pure spherical symmetry and the presence of
substructure. The dynamical mass estimate based upon a measurement of the
cluster velocity dispersion and an assumption of an isothermal potential yields
a poor measurement of the total mass. The weak lensing estimate yields a very
good measurement of the total mass, provided the mean shear used to determine
the equivalent cluster velocity dispersion is computed from an average of the
lensing signal over the entire cluster (i.e. the mean shear is computed
interior to the virial radius). [abridged]Comment: Accepted for publication in The Astrophysical Journal. Complete
paper, including 3 large colour figures can also be obtained from
http://bu-ast.bu.edu/~brainerd/preprints
Intrinsic galaxy shapes and alignments II: Modelling the intrinsic alignment contamination of weak lensing surveys
Intrinsic galaxy alignments constitute the major astrophysical systematic of
forthcoming weak gravitational lensing surveys but also yield unique insights
into galaxy formation and evolution. We build analytic models for the
distribution of galaxy shapes based on halo properties extracted from the
Millennium Simulation, differentiating between early- and late-type galaxies as
well as central galaxies and satellites. The resulting ellipticity correlations
are investigated for their physical properties and compared to a suite of
current observations. The best-faring model is then used to predict the
intrinsic alignment contamination of planned weak lensing surveys. We find that
late-type galaxy models generally have weak intrinsic ellipticity correlations,
marginally increasing towards smaller galaxy separation and higher redshift.
The signal for early-type models at fixed halo mass strongly increases by three
orders of magnitude over two decades in galaxy separation, and by one order of
magnitude from z=0 to z=2. The intrinsic alignment strength also depends
strongly on halo mass, but not on galaxy luminosity at fixed mass, or galaxy
number density in the environment. We identify models that are in good
agreement with all observational data, except that all models over-predict
alignments of faint early-type galaxies. The best model yields an intrinsic
alignment contamination of a Euclid-like survey between 0.5-10% at z>0.6 and on
angular scales larger than a few arcminutes. Cutting 20% of red foreground
galaxies using observer-frame colours can suppress this contamination by up to
a factor of two.Comment: 23 pages, 14 figures; minor changes to match version published in
MNRA
Soliton attenuation and emergent hydrodynamics in fragile matter
Disordered packings of soft grains are fragile mechanical systems that loose
rigidity upon lowering the external pressure towards zero. At zero pressure, we
find that any infinitesimal strain-impulse propagates initially as a non-linear
solitary wave progressively attenuated by disorder. We demonstrate that the
particle fluctuations generated by the solitary-wave decay, can be viewed as a
granular analogue of temperature. Their presence is manifested by two emergent
macroscopic properties absent in the unperturbed granular packing: a finite
pressure that scales with the injected energy (akin to a granular temperature)
and an anomalous viscosity that arises even when the microscopic mechanisms of
energy dissipation are negligible. Consistent with the interpretation of this
state as a fluid-like thermalized state, the shear modulus remains zero.
Further, we follow in detail the attenuation of the initial solitary wave
identifying two distinct regimes : an initial exponential decay, followed by a
longer power law decay and suggest simple models to explain these two regimes.Comment: 8 pages, 3 Figure
Galaxy-galaxy and galaxy-cluster lensing with the SDSS and FIRST surveys
We perform a galaxy-galaxy lensing study by correlating the shapes of
2.7 10 galaxies selected from the VLA FIRST radio survey
with the positions of 38.5 million SDSS galaxies, 132000 BCGs and
78000 SDSS galaxies that are also detected in the VLA FIRST survey. The
measurements are conducted on angular scales 1200 arcsec.
On scales 200 arcsec we find that the measurements are
corrupted by residual systematic effects associated with the instrumental beam
of the VLA data. Using simulations we show that we can successfully apply a
correction for these effects. Using the three lens samples (the SDSS DR10
sample, the BCG sample and the SDSS-FIRST matched object sample) we measure a
tangential shear signal that is inconsistent with zero at the 10,
3.8 and 9 level respectively. Fitting an NFW model to the
detected signals we find that the ensemble mass profile of the BCG sample
agrees with the values in the literature. However, the mass profiles of the
SDSS DR10 and the SDSS-FIRST matched object samples are found to be shallower
and steeper than results in the literature respectively. The best-fitting
Virial masses for the SDSS DR10, BCG and SDSS-FIRST matched samples, derived
using an NFW model and allowing for a varying concentration factor, are
M = (1.2 0.4) 10M,
M = (1.4 1.3) 10M and
M = 8.0 4.2 10M
respectively. These results are in good agreement (within 2)
with values in the literature. Our findings suggest that for galaxies to be
both bright in the radio and in the optical they must be embedded in very dense
environment on scales R 1Mpc.Comment: 15 pages, 9 figures and 2 table
Multiple Weak Deflections in Galaxy-Galaxy Lensing
The frequency and effects of multiple weak deflections in galaxy-galaxy
lensing are investigated via Monte Carlo simulations. The lenses in the
simulations are galaxies with known redshifts and known rest-frame blue
luminosities. The frequency of multiple deflections above a given threshold
shear value is quantified for discrete source redshifts, as well as for a set
of sources that are broadly distributed in redshift space. In general, the
closest lens in projection on the sky is not the only lens for a given source.
In addition, ~50% of the time the closest lens is not the most important lens
for a given source. Compared to a naive single-deflection calculation in which
only the lensing due to the closest weak lens is considered, a full
multiple-deflection calculation yields a higher net shear for individual
sources, as well as a higher mean tangential shear around the lens centers. The
full multiple-deflection calculation also shows that galaxy-galaxy lensing may
contribute a substantial amount to cosmic shear on small angular scales. The
degree to which galaxy-galaxy lensing contributes to the small-scale cosmic
shear is, however, quite sensitive to the mass adopted for the halos of L_B*
galaxies. Changing the halo mass by a factor of ~2.5 changes the contribution
of galaxy-galaxy lensing to the cosmic shear by a factor of ~3 on scales of
order 1 arcmin. The contribution of galaxy-galaxy lensing to cosmic shear
decreases rapidly with angular scale and extrapolates to zero at scales of
order 5 arcmin. This last result is roughly independent of the halo mass and
suggests that for scales greater than about 5 arcmin, cosmic shear is
insensitive to the details of the gravitational potentials of large galaxies.Comment: accepted for publication in ApJ; 35 pages, 15 figures; full text with
high-resolution Figure 1 available at
http://firedrake.bu.edu/preprints/preprints.htm
Revisiting the bulge-halo conspiracy I: Dependence on galaxy properties and halo mass
We carry out a systematic investigation of the total mass density profile of
massive (Mstar>2e11 Msun) early-type galaxies and its dependence on galactic
properties and host halo mass with the aid of a variety of lensing/dynamical
data and large mock galaxy catalogs. The latter are produced via semi-empirical
models that, by design, are based on just a few basic input assumptions.
Galaxies, with measured stellar masses, effective radii and S\'{e}rsic indices,
are assigned, via abundance matching relations, host dark matter halos
characterized by a typical LCDM profile. Our main results are as follows: (i)
In line with observational evidence, our semi-empirical models naturally
predict that the total, mass-weighted density slope at the effective radius
gamma' is not universal, steepening for more compact and/or massive galaxies,
but flattening with increasing host halo mass. (ii) Models characterized by a
Salpeter or variable initial mass function and uncontracted dark matter
profiles are in good agreement with the data, while a Chabrier initial mass
function and/or adiabatic contractions/expansions of the dark matter halos are
highly disfavored. (iii) Currently available data on the mass density profiles
of very massive galaxies (Mstar>1e12 Msun), with Mhalo>3e14 Msun, favor instead
models with a stellar profile flatter than a S\'{e}rsic one in the very inner
regions (r<3-5 kpc), and a cored NFW or Einasto dark matter profile with median
halo concentration a factor of ~2 or <1.3, respectively, higher than those
typically predicted by N-body numerical simulations.Comment: 23 pages, 10 figures, 3 Appendices (with an extra 7 figures). ApJ,
accepted. Main results in Figures 3, 5, 6,
Modeling the galaxy/light-mass connection with cosmological simulations
I review some results on the galaxy/light-mass connection obtained by
dissipationless simulations in combination with a simple, non-parametric model
to connect halo circular velocity to the luminosity of the galaxy they would
host. I focus on the galaxy-mass correlation and mass-to-light ratios obtained
from galaxy up to cluster scales. The predictions of this simple scheme are
shown to be in very good agreement with SDSS observations.Comment: proceedings of the 6th International Workshop on The Identification
of Dark Matter, 11 - 16 September 2006, Rhodes Island, Greece; to be
published by World Scientifi
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