1,040 research outputs found
Enhanced lensing rate by clustering of massive galaxies: newly discovered systems in the SLACS fields
[Abridged] We exploit the clustering of massive galaxies to perform a high
efficiency imaging search for gravitational lenses. Our dataset comprises 44
fields imaged by the Hubble Space Telescope (HST) Advanced Camera for Surveys
(ACS), each of which is centered on a lens discovered by the Strong Lens ACS
Survey (SLACS). We compare four different search methods: 1) automated
detection with the HST Archive Galaxy-scale Gravitational Lens Survey (HAGGLeS)
robot, 2) examining cutout images of bright galaxies (BGs) after subtraction of
a smooth galaxy light distribution, 3) examining the unsubtracted BG cutouts,
and 4) performing a full-frame visual inspection of the ACS images. We compute
purity and completeness and consider investigator time for the four algorithms,
using the main SLACS lenses as a testbed. The first and second algorithms
perform the best. We present the four new lens systems discovered during this
comprehensive search, as well as one other likely candidate. For each new lens
we use the fundamental plane to estimate the lens velocity dispersion and
predict, from the resulting lens geometry, the redshifts of the lensed sources.
Two of these new systems are found in galaxy clusters, which include the SLACS
lenses in the two respective fields. Overall we find that the enhanced lens
abundance (30^{+24}_{-8} lenses/degree^2) is higher than expected for random
fields (12^{+4}_{-2} lenses/degree^2 for the COSMOS survey). Additionally, we
find that the gravitational lenses we detect are qualitatively different from
those in the parent SLACS sample: this imaging survey is largely probing
higher-redshift, and lower-mass, early-type galaxies.Comment: submitted to ApJ; 19 pages, 12 figure
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
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 mass profile of early-type galaxies in overdense environments: the case of the double source plane gravitational lens SL2SJ02176-0513
SL2SJ02176-0513 is a remarkable lens for the presence of two multiply-imaged
systems at different redshifts lensed by a foreground massive galaxy at : a bright cusp arc at and an additional
double-image system at an estimated redshift of based on
photometry and lensing geometry. The system is located about 400 kpc away from
the center of a massive group of galaxies. Mass estimates for the group are
available from X-ray observations and satellite kinematics. Multicolor
photometry provides an estimate of the stellar mass of the main lens galaxy.
The lensing galaxy is modeled with two components (stars and dark matter), and
we include the perturbing effect of the group environment, and all available
constraints. We find that classic lensing degeneracies, e.g. between external
convergence and mass density slope, are significantly reduced with respect to
standard systems and infer tight constraints on the mass density profile: (i)
the dark matter content of the main lens galaxy is in line with that of typical
galaxies ; (ii) the required mass
associated with the dark matter halo of the nearby group is consistent with
X-ray and weak-lensing estimates (); (iii)
accounting for the group contribution in the form of an external convergence,
the slope of the mass density profile of the main lens galaxy alone is found to
be , consistent with the isothermal ()
slope. We demonstrate that multiple source plane systems together with good
ancillary dataset can be used to disentangle local and environmental effects.Comment: 10 pages, 6 figures, submitted to A&
Constraints on the equation of state of dark energy and the Hubble constant from stellar ages and the CMB
We place tight constraints on the redshift-averaged, effective value of the
equation of state of dark energy, w, using only the absolute ages of Galactic
stars and the observed position of the first peak in the angular power spectrum
of the CMB. We find w<-0.8 at the 68% confidence level. If we further consider
that w > -1, this finding suggests that within our uncertainties, dark energy
is indistinguishable from a classical vacuum energy term.
We detect a correlation between the ages of the oldest galaxies and their
redshift. This opens up the possibility of measuring w(z) by computing the
relative ages of the oldest galaxies in the universe as a function of redshift,
dz/dt. We show that this is a realistic possibility by computing dz/dt at z~0
from SDSS galaxies and obtain an independent estimate for the Hubble constant,
H_0 = 69 \pm 12 km s-1 Mpc-1. The small number of galaxies considered at z>0.2
does not yield, currently, a precise determination of w(z), but shows that the
age--redshift relation is consistent with a Standard LCDM universe with .Comment: Submitted to Ap
The Mass Assembly History of Spheroidal Galaxies: Did Newly-Formed Systems Arise Via Major Mergers?
We examine the properties of a morphologically-selected sample of 0.4<z<1.0
spheroidal galaxies in the GOODS fields in order to ascertain whether their
increase in abundance with time arises primarily from mergers. To address this
question we determine scaling relations between the dynamical mass determined
from stellar velocity dispersions, and the stellar mass determined from optical
and infrared photometry. We exploit these relations across the larger sample
for which we have stellar masses in order to construct the first statistically
robust estimate of the evolving dynamical mass function over 0<z<1. The trends
observed match those seen in the stellar mass functions of Bundy et al. 2005
regarding the top-down growth in the abundance of spheroidal galaxies. By
referencing our dynamical masses to the halo virial mass we compare the growth
rate in the abundance of spheroidals to that predicted by the assembly of dark
matter halos. Our comparisons demonstrate that major mergers do not fully
account for the appearance of new spheroidals since z~1 and that additional
mechanisms, such as morphological transformations, are required to drive the
observed evolution.Comment: Accepted to ApJL; New version corrects the Millennium merger
predictions--further details at
http://www.astro.utoronto.ca/~bundy/millennium
HST followup observations of two bright z ~ 8 candidate galaxies from the BoRG pure-parallel survey
We present followup imaging of two bright (L > L*) galaxy candidates at z > 8
from the Brightest of Reionizing Galaxies (BoRG) survey with the F098M filter
on HST/WFC3. The F098M filter provides an additional constraint on the flux
blueward of the spectral break, and the observations are designed to
discriminate between low- and high-z photometric redshift solutions for these
galaxies. Our results confirm one galaxy, BoRG 0116+1425 747, as a highly
probable z ~ 8 source, but reveal that BoRG 0116+1425 630 - previously the
brightest known z > 8 candidate (mAB = 24.5) - is likely to be a z ~ 2
interloper. As this source was substantially brighter than any other z > 8
candidate, removing it from the sample has a significant impact on the derived
UV luminosity function in this epoch. We show that while previous BoRG results
favored a shallow power-law decline in the bright end of the luminosity
function prior to reionization, there is now no evidence for departure from a
Schechter function form and therefore no evidence for a difference in galaxy
formation processes before and after reionization.Comment: Accepted by ApJL, 7 pages, 4 figure
The Baryon Fractions and Mass-to-Light Ratios of Early-Type Galaxies
We jointly model 22 early-type gravitational lens galaxies with stellar
dynamical measurements using standard CDM halo models. The sample is
inhomogeneous in both its mass distributions and the evolution of its stellar
populations unless the true uncertainties are significantly larger than the
reported measurement errors. In general, the individual systems cannot
constrain halo models, in the sense that the data poorly constrains the stellar
mass fraction of the halo. The ensemble of systems, however, strongly
constrains the average stellar mass represented by the visible galaxies to
of the halo mass if we neglect adiabatic compression, rising to
of the halo mass if we include adiabatic compression. Both
estimates are significantly smaller than the global baryon fraction,
corresponding to a star formation efficiency for early-type galaxies of
. In the adiabatically compressed models, we find an average local
B-band stellar mass-to-light ratio of (M/L)_0 =
(7.2\pm0.5)(M_{\sun}/L_{\sun}) that evolves by
per unit redshift. Adjusting the isotropy of the stellar orbits has little
effect on the results. The adiabatically compressed models are strongly favored
if we impose either local estimates of the mass-to-light ratios of early-type
galaxies or the weak lensing measurements for the lens galaxies on 100 kpc
scales as model constraints.Comment: 9 figure
A twelve-image gravitational lens system in the z ~ 0.84 cluster Cl J0152.7-1357
Gravitational lens modeling is presented for the first discovered example of
a three-component source for which each component is quadruply imaged. The lens
is a massive galaxy member of the cluster Cl J0152.7-1357 at z ~ 0.84. Taking
advantage of this exceptional configuration and of the excellent angular
resolution of the HST/ACS, we measure the properties of the lens. Several
parametric macroscopic models were developed for the lens galaxy, starting from
pointlike to extended image models. For a lens model in terms of a singular
isothermal sphere with external shear, the Einstein radius is found to be R_{E}
= (9.54 +/- 0.15) kpc. The external shear points to the cluster's northern mass
peak. The unknown redshift of the source is determined to be higher than 1.9
and lower than 2.9. Our estimate of the lensing projected total mass inside the
Einstein radius, M_{len}(R < 9.54 kpc), depends on the source distance and lies
between 4.6 and 6.2 x 10^{11} M_{Sun}. This result turns out to be compatible
with the dynamical estimate based on an isothermal model. By considering the
constraint on the stellar mass-to-light ratio that comes from the evolution of
the Fundamental Plane, we can exclude the possibility that at more than 4 sigma
level the total mass enclosed inside the Einstein ring is only luminous matter.
Moreover, the photometric-stellar mass measurement within the Einstein radius
gives a minimum value of 50% (1 sigma) for the dark-to-total matter fraction.
The lensing analysis has allowed us to investigate the distribution of mass of
the deflector, also providing some interesting indications on scales that are
larger and smaller than the Einstein radius of the lens galaxy. The combination
of different diagnostics has proved to be essential in determining quantities
that otherwise would have not been directly measurable with the current data.Comment: 10 pages, 9 figures, accepted by Astronomy & Astrophysic
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