2,938 research outputs found
Inference of the Cold Dark Matter substructure mass function at z=0.2 using strong gravitational lenses
We present the results of a search for galaxy substructures in a sample of 11
gravitational lens galaxies from the Sloan Lens ACS Survey. We find no
significant detection of mass clumps, except for a luminous satellite in the
system SDSS J0956+5110. We use these non-detections, in combination with a
previous detection in the system SDSS J0946+1006, to derive constraints on the
substructure mass function in massive early-type host galaxies with an average
redshift z ~ 0.2 and an average velocity dispersion of 270 km/s. We perform a
Bayesian inference on the substructure mass function, within a median region of
about 32 kpc squared around the Einstein radius (~4.2 kpc). We infer a mean
projected substructure mass fraction at the 68
percent confidence level and a substructure mass function slope < 2.93
at the 95 percent confidence level for a uniform prior probability density on
alpha. For a Gaussian prior based on Cold Dark Matter (CDM) simulations, we
infer and a slope of =
1.90 at the 68 percent confidence level. Since only one
substructure was detected in the full sample, we have little information on the
mass function slope, which is therefore poorly constrained (i.e. the Bayes
factor shows no positive preference for any of the two models).The inferred
fraction is consistent with the expectations from CDM simulations and with
inference from flux ratio anomalies at the 68 percent confidence level.Comment: Accepted for publication on MNRAS, some typos corrected and some
important references adde
The Sloan Lens ACS Survey. VIII. The relation between environment and internal structure of early-type galaxies
We study the relation between the internal structure of early-type galaxies
and their environment using 70 strong gravitational lenses from the Sloan ACS
Lens Survey. The Sloan database is used to determine two measures of
overdensity of galaxies around each lens: the projected number density of
galaxies inside the tenth nearest neighbor (\Sigma_{10}) and within a cone of
radius one h^{-1} Mpc (D_1). Our main results are: 1) The average overdensity
is somewhat larger than unity, consistent with lenses preferring overdense
environments as expected for massive early-type galaxies (12/70 lenses are in
known groups/clusters). 2) The distribution of overdensities is
indistinguishable from that of "twin" non-lens galaxies selected from SDSS to
have the same redshift and stellar velocity dispersion \sigma_*. Thus, within
our errors, lens galaxies are an unbiased population, and the SLACS results can
be generalized to the overall population of early-type galaxies. 3) Typical
contributions from external mass distribution are no more than a few per cent,
reaching 10-20% (~0.05-0.10 external convergence) only in the most extreme
overdensities. 4) No significant correlation between overdensity and slope of
the mass density profile of the lens is found. 5) Satellite galaxies (those
with a more luminous companion) have marginally steeper mass density profiles
than central galaxies (those without). This result suggests that tidal
stripping may affect the mass structure of early-type galaxies down to kpc
scales probed by strong lensing, when they fall into larger structures
[ABRIDGED].Comment: ApJ, in press; minor changes with respect to v
A Determination of H_0 with the CLASS Gravitational Lens B1608+656: I. Time Delay Measurements with the VLA
We present the results of a program to monitor the four-image gravitational
lens B1608+656 with the VLA. The system was observed over a seven month period
from 1996 October to 1997 May. The 64 epochs of observation have an average
spacing of 3.6~d. The light curves of the four images of the background source
show that the flux density of the background source has varied at the ~5%
level. We measure time delays in the system based on common features that are
seen in all four light curves. The three independent time delays in the system
are found to be Delta t_{BA} = 31 +/- 7~d, Delta t_{BC} = 36 +/- 7~d, and Delta
t_{BD} = 76^{+9}_{-10}~d at 95% confidence. This is the first gravitational
lens system for which three independent time delays have been measured. A
companion paper presents a mass model for the lensing galaxy which correctly
reproduces the observed image positions, flux density ratios, and time delay
ratios. The last condition is crucial for determining H_0 with a four-image
lens. We combine the time delays with the model to obtain a value for the
Hubble constant of H_0 = 59^{+8}_{-7} km/s/Mpc at 95% confidence (statistical)
for (Omega_M, Omega_{Lambda}) = (1,0). In addition, there is an estimated
systematic uncertainty of +/- 15 km/s/Mpc from uncertainties in modeling the
radial mass profiles of the lensing galaxies. The value of H_0 presented in
this paper is comparable to recent measurements of H_0 from the gravitational
lenses 0957+561, PG1115+080, B0218+357, and PKS1830-211.Comment: Accepted for publication in ApJ. 20 pages, 13 figure
The Sloan Lens ACS Survey. IX. Colors, Lensing and Stellar Masses of Early-type Galaxies
We present the current photometric dataset for the Sloan Lens ACS (SLACS)
Survey, including HST photometry from ACS, WFPC2, and NICMOS. These data have
enabled the confirmation of an additional 15 grade `A' (certain) lens systems,
bringing the number of SLACS grade `A' lenses to 85; including 13 grade `B'
(likely) systems, SLACS has identified nearly 100 lenses and lens candidates.
Approximately 80% of the grade `A' systems have elliptical morphologies while
~10% show spiral structure; the remaining lenses have lenticular morphologies.
Spectroscopic redshifts for the lens and source are available for every system,
making SLACS the largest homogeneous dataset of galaxy-scale lenses to date. We
have developed a novel Bayesian stellar population analysis code to determine
robust stellar masses with accurate error estimates. We apply this code to
deep, high-resolution HST imaging and determine stellar masses with typical
statistical errors of 0.1 dex; we find that these stellar masses are unbiased
compared to estimates obtained using SDSS photometry, provided that informative
priors are used. The stellar masses range from 10^10.5 to 10^11.8 M and
the typical stellar mass fraction within the Einstein radius is 0.4, assuming a
Chabrier IMF. The ensemble properties of the SLACS lens galaxies, e.g. stellar
masses and projected ellipticities, appear to be indistinguishable from other
SDSS galaxies with similar stellar velocity dispersions. This further supports
that SLACS lenses are representative of the overall population of massive
early-type galaxies with M* >~ 10^11 M, and are therefore an ideal
dataset to investigate the kpc-scale distribution of luminous and dark matter
in galaxies out to z ~ 0.5.Comment: 20 pages, 18 figures, 5 tables, published in Ap
The X-shooter Lens Survey - II. Sample presentation and spatially resolved kinematics
We present the X-shooter Lens Survey (XLENS) data. The main goal of XLENS is
to disentangle the stellar and dark matter content of massive early-type
galaxies (ETGs), through combined strong gravitational lensing, dynamics and
spectroscopic stellar population studies. The sample consists of 11 lens
galaxies covering the redshift range from to and having stellar
velocity dispersions between and . All
galaxies have multi-band, high-quality HST imaging. We have obtained long-slit
spectra of the lens galaxies with X-shooter on the VLT. We are able to
disentangle the dark and luminous mass components by combining lensing and
extended kinematics data-sets, and we are also able to precisely constrain
stellar mass-to-light ratios and infer the value of the low-mass cut-off of the
IMF, by adding spectroscopic stellar population information. Our goal is to
correlate these IMF parameters with ETG masses and investigate the relation
between baryonic and non-baryonic matter during the mass assembly and structure
formation processes. In this paper we provide an overview of the survey,
highlighting its scientific motivations, main goals and techniques. We present
the current sample, briefly describing the data reduction and analysis process,
and we present the first results on spatially resolved kinematics.Comment: Accepted for publication in MNRA
The Structure & Dynamics of Massive Early-type Galaxies: On Homology, Isothermality and Isotropy inside one Effective Radius
Based on 58 SLACS strong-lens early-type galaxies with direct total-mass and
stellar-velocity dispersion measurements, we find that inside one effective
radius massive elliptical galaxies with M_eff >= 3x10^10 M_sun are
well-approximated by a power-law ellipsoid with an average logaritmic density
slope of = -dlog(rho_tot)/dlog(r)=2.085^{+0.025}_{-0.018} (random
error on mean) for isotropic orbits with beta_r=0, +-0.1 (syst.) and
sigma_gamma' <= 0.20^{+0.04}_{-0.02} intrinsic scatter (all errors indicate the
68 percent CL). We find no correlation of gamma'_LD with galaxy mass (M_eff),
rescaled radius (i.e. R_einst/R_eff) or redshift, despite intrinsic differences
in density-slope between galaxies. Based on scaling relations, the average
logarithmic density slope can be derived in an alternative manner, fully
independent from dynamics, yielding =1.959 +- 0.077. Agreement
between the two values is reached for =0.45 +- 0.25, consistent with
mild radial anisotropy. This agreement supports the robustness of our results,
despite the increase in mass-to-light ratio with total galaxy mass: M_eff ~
L_{V,eff}^(1.363+-0.056). We conclude that massive early-type galaxies are
structurally close-to homologous with close-to isothermal total density
profiles (<=10 percent intrinsic scatter) and have at most some mild radial
anisotropy. Our results provide new observational limits on galaxy formation
and evolution scenarios, covering four Gyr look-back time.Comment: Accepted for publication by ApJL; 4 pages, 2 figure
Political mobilisation by minorities in Britain: negative feedback of ‘race relations'?
This article uses a political opportunity approach to study the relationship of minority groups to the political community in Britain. The main argument is that the
British race relations approach established in the 1960s had an important effect that still shapes the patterns of political contention by different minority groups today. Original data on political claims-making by minorities demonstrate that British 'racialised' cultural pluralism has structured an inequality of opportunities for the two main groups, African-Caribbeans and Indian subcontinent minorities. African-Caribbeans mobilise along racial lines, use a strongly assimilative 'black' identity, conventional action forms, and target state institutions with demands for justice that are framed within the recognised framework of race relations. Conversely, a high proportion of the Indian subcontinent minority mobilisation is by Muslim groups, a non-assimilative religious identity. These are autonomously organised, but largely make public demands for extending the principle of racial equality to their non-racial group. Within the Indian subcontinent minorities, the relative absence of mobilisation by Indian, Sikh and Hindu minorities, who have achieved much better levels of socio-economic success than Pakistani and Bangladeshi Muslims, suggests that there is also a strong socioeconomic basis for shared experiences and grievances as Muslims in Britain. This relativises the notion that Muslim mobilisation is Britain is purely an expression of the right for cultural difference per se, and sees it as a product of the paradoxes of British race relations
Galaxy Masses
Galaxy masses play a fundamental role in our understanding of structure
formation models. This review addresses the variety and reliability of mass
estimators that pertain to stars, gas, and dark matter. The different sections
on masses from stellar populations, dynamical masses of gas-rich and gas-poor
galaxies, with some attention paid to our Milky Way, and masses from weak and
strong lensing methods, all provide review material on galaxy masses in a
self-consistent manner.Comment: 145 pages, 28 figures, to appear in Reviews of Modern Physics. Figure
22 is missing here, and Figs. 15, 26-28 are at low resolution. This version
has a slightly different title and some typos fixed in Chapter 5. For the
full review with figures, please consult:
http://www.astro.queensu.ca/~courteau/GalaxyMasses_28apr2014.pd
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