72 research outputs found
An HSC view of the CMASS galaxy sample. Halo mass as a function of stellar mass, size and S\'ersic index
Aims. We wish to determine the distribution of dark matter halo masses as a
function of the stellar mass and the stellar mass profile, for massive galaxies
in the BOSS CMASS sample. Methods. We use grizy photometry from HSC to obtain
S\'ersic fits and stellar masses of CMASS galaxies for which HSC weak lensing
data is available, visually selected to have spheroidal morphology. We apply a
cut in stellar mass, ,selecting 10, 000
objects. Using a Bayesian hierarchical inference method, we first investigate
the distribution of S\'ersic index and size as a function of stellar mass.
Then, making use of shear measurements from HSC, we measure the distribution of
halo mass as a function of stellar mass, size and S\'ersic index. Results. Our
data reveals a steep stellar mass-size relation ,
with larger than unity, and a positive correlation between S\'ersic
index and stellar mass: . Halo mass scales approximately
with the 1.7 power of the stellar mass. We do not find evidence for an
additional dependence of halo mass on size or S\'ersic index at fixed stellar
mass. Conclusions. Our results disfavour galaxy evolution models that predict
significant differences in the size growth efficiency of galaxies living in low
and high mass halos.Comment: Accepted for publication on Astronomy & Astrophysics. 18 pages, 15
figure
Statistical strong lensing. III. Inferences with complete samples of lenses
Context. Existing samples of strong lenses have been assembled by giving
priority to sample size, at the cost of having a complex selection function.
With the advent of the next generation of wide-field photometric surveys,
however, it might become possible to identify subsets of the lens population
with well-defined selection criteria, trading sample size for completeness.
Aims. There are two main advantages of working with a complete sample of
lenses. First, it is possible to recover the properties of the general
population of galaxies, of which strong lenses are a biased subset. Second, the
relative number of lenses and non-detections can be used to further constrain
models of galaxy structure. This work illustrates how to carry out a
statistical strong lensing analysis that takes advantage of these features.
Methods. I introduced a general formalism for the statistical analysis of a
sample of strong lenses with known selection function, then tested it on
simulated data. The simulation consists of a population of galaxies with
an axisymmetric power-law density profile, a population of background point
sources, and a subset of strong lenses, complete above an
observational cut.
Results. The method allows to recover the distribution in Einstein radius and
mass density slope of the galaxy population in an unbiased way. The number of
non-lenses helps to constrain the model when magnification data are not
available.
Conclusions. Complete samples of lenses are a powerful asset to turn precise
strong lensing measurements into accurate statements on the properties of the
general galaxy population.Comment: Submitted to Astronomy & Astrophysics. A 2-minute summary video is
available at https://youtu.be/f6PziIr9US
A robust two-parameter description of the stellar profile of elliptical galaxies
The stellar density profile a galaxy is typically summarised with two
numbers: total stellar mass and half-light radius. The total mass of a galaxy,
however, is not a well-defined quantity, due to the finite depth of photometric
observations and the arbitrariness of the distinction between galaxy and
diffuse intra-group light. This limits our ability to make accurate comparisons
between models and observations. I wish to provide a more robust two-parameter
description of the stellar density distribution of elliptical galaxies, in
terms of quantities that can be measured unambiguously. I propose to use the
stellar mass enclosed within 10 kpc in projection, , and the
mass-weighted stellar density slope within the same aperture, ,
for this purpose. I measured the distribution in and
of a sample of elliptical galaxies from the Sloan Digital Sky Survey and the
Galaxy And Mass Assembly survey, using photometry from the Hyper Suprime-Cam
survey. The pair of values of can be used to predict
the stellar density profile in the inner 10 kpc of a galaxy with better than
20% accuracy. Similarly, and can be combined to
obtain a proxy for stellar velocity dispersion at least as good as the stellar
mass fundamental plane. As a first application, I then compared the observed
relation of elliptical galaxies with that of similarly
selected galaxies in the EAGLE Reference simulation. Observed and simulated
galaxies match at , but the EAGLE
relation is shallower and has a larger intrinsic
scatter compared to observations. This new parameterisation of the stellar
density profile of massive elliptical galaxies provides a more robust way of
comparing results from different photometric surveys and from hydrodynamical
simulations.Comment: 13 pages, 13 figures, submitted to Astronomy & Astrophysic
The effect of spiral arms on the S\'ersic photometry of galaxies
Context. The S\'ersic profile is a widely-used model to describe the surface
brightness distribution of galaxies. Spiral galaxies, however, are
qualitatively different from a S\'ersic model.
Aims. The goal of this study is to assess how accurately can the total flux
and half-light radius of a galaxy with spiral arms be recovered, when fitted
with a S\'ersic profile.
Methods. I selected a sample of bulge-dominated galaxies with spiral arms.
Using photometric data from the Hyper Suprime-Cam survey, I estimated the
contribution of the spiral arms to their total flux. Then I generated simulated
images of galaxies with similar characteristics, fitted them with a S\'ersic
model and quantified the error on the determination of the total flux and
half-light radius.
Results. Spiral arms can introduce biases on the photometry of galaxies, in a
way that depends on the underlying smooth surface brightness profile, the
location of the arms, and the depth of the photometric data. A set of spiral
arms accounting for 10% of the flux of a bulge-dominated galaxy typically
causes the total flux and the half-light radius to be overestimated by 15% and
30%, respectively. This bias, however, is much smaller if the galaxy is
disk-dominated.
Conclusions. Galaxies with a prominent bulge and a non-zero contribution from
spiral arms are the most susceptible to biases in the total flux and half-light
radius, when fitted with a S\'ersic profile. If photometric measurements with
high accuracy are required, then measurements over finite apertures are to be
preferred over global estimates of the flux.Comment: Submitted to Astronomy & Astrophysics. A 2-minute summary video is
available at https://youtu.be/y5ffIZP9XH
Statistical strong lensing. I. Constraints on the inner structure of galaxies from samples of a thousand lenses
Context: The number of known strong gravitational lenses is expected to grow
substantially in the next few years. The statistical combination of large
samples of lenses has the potential of providing strong constraints on the
inner structure of galaxies. Aims: We investigate to what extent we can
calibrate stellar mass measurements and constrain the average dark matter
density profile of galaxies by statistically combining strong lensing data from
thousands of lenses. Methods: We generate mock samples of axisymmetric lenses.
We assume that, for each lens, we have measurements of two image positions of a
strongly lensed background source, as well as magnification information from
full surface brightness modelling, and a stellar population synthesis-based
estimate of the lens stellar mass. We then fit models describing the
distribution of the stellar population synthesis mismatch parameter
(the ratio between the true stellar mass and the stellar
population synthesis-based estimate) and dark matter density profile of the
population of lenses to an ensemble of 1000 mock lenses. Results: The average
, projected dark matter mass and dark matter density slope can be
obtained with great precision and accuracy, compared with current constraints.
A flexible model and the knowledge of the lens detection efficiency as a
function of image configuration are required in order to avoid a biased
inference. Conclusions: Statistical strong lensing inferences from upcoming
surveys have the potential to calibrate stellar mass measurements and to
constrain the inner dark matter density profile of massive galaxies.Comment: Published on Astronomy & Astrophysics. A 2-minute summary video can
be found at this link: https://youtu.be/En0-uJobqE
RingFinder: automated detection of galaxy-scale gravitational lenses in ground-based multi-filter imaging data
We present RingFinder, a tool for finding galaxy-scale strong gravitational
lenses in multiband imaging data. By construction, the method is sensitive to
configurations involving a massive foreground early-type galaxy and a faint,
background, blue source. RingFinder detects the presence of blue residuals
embedded in an otherwise smooth red light distribution by difference imaging in
two bands. The method is automated for efficient application to current and
future surveys, having originally been designed for the 150-deg2 Canada France
Hawaii Telescope Legacy Survey (CFHTLS). We describe each of the steps of
RingFinder. We then carry out extensive simulations to assess completeness and
purity. For sources with magnification mu>4, RingFinder reaches 42% (resp. 25%)
completeness and 29% (resp. 86%) purity before (resp. after) visual inspection.
The completeness of RingFinder is substantially improved in the particular
range of Einstein radii 0.8 < REin < 2. and lensed images brighter than g =
22.5, where it can be as high as 70%. RingFinder does not introduce any
significant bias in the source or deflector population. We conclude by
presenting the final catalog of RingFinder CFHTLS galaxy-scale strong lens
candidates. Additional information obtained with Hubble Space Telescope and
Keck Adaptive Optics high resolution imaging, and with Keck and Very Large
Telescope spectroscopy, is used to assess the validity of our classification,
and measure the redshift of the foreground and the background objects. From an
initial sample of 640,000 early type galaxies, RingFinder returns 2500
candidates, which we further reduce by visual inspection to 330 candidates. We
confirm 33 new gravitational lenses from the main sample of candidates, plus an
additional 16 systems taken from earlier versions of RingFinder. First
applications are presented in the SL2S galaxy-scale Lens Sample paper series.Comment: 32 pages (aastex 2col format), 6 figs, ApJ Accepte
The SL2S Galaxy-scale Lens Sample. III. Lens Models, Surface Photometry and Stellar Masses for the final sample
We present Hubble Space Telescope (HST) imaging data and CFHT Near IR
ground-based images for the final sample of 56 candidate galaxy-scale lenses
uncovered in the CFHT Legacy Survey as part of the Strong Lensing in the Legacy
Survey (SL2S) project. The new images are used to perform lens modeling,
measure surface photometry, and estimate stellar masses of the deflector
early-type galaxies. Lens modeling is performed on the HST images (or CFHT when
HST is not available) by fitting the spatially extended light distribution of
the lensed features assuming a singular isothermal ellipsoid mass profile and
by reconstructing the intrinsic source light distribution on a pixelized grid.
Based on the analysis of systematic uncertainties and comparison with inference
based on different methods we estimate that our Einstein Radii are accurate to
\sim3%. HST imaging provides a much higher success rate in confirming
gravitational lenses and measuring their Einstein radii than CFHT imaging does.
Lens modeling with ground-based images however, when successful, yields
Einstein radius measurements that are competitive with spaced-based images.
Information from the lens models is used together with spectroscopic
information from the companion paper IV to classify the systems, resulting in a
final sample of 39 confirmed (grade-A) lenses and 17 promising candidates. The
redshifts of the main deflector span a range 0.3<zd< 0.8, providing an
excellent sample for the study of the cosmic evolution of the mass distribution
of early-type galaxies over the second half of the history of the Universe.Comment: Submitted to The Astrophysical Journa
Luminous and dark matter in early-type galaxies
Three open problems in our understanding of early-type galaxies are 1) identifying theprocess(es) responsible for their rapid size evolution, 2) accurately constraining the stellarIMF and its variations in the population, 3) measuring the density profile of their darkmatter halo. We use strong lensing as the main diagnostic tool to address these issues.We first dissected a massive elliptical galaxy in its stellar and dark matter components,measuring both its IMF and the inner slope of the dark matter halo. We then collecteda sample of 45 strong lenses in the redshift interval 0.2 < z < 0.8 and used them, incombination with lenses from other surveys, to measure the slope of the total densityprofile, the stellar IMF and the dark matter mass in the population of massive early-typegalaxies, and their time evolution. Finally, we used our measurements of the evolution ofthe density slope to test a galaxy growth scenario based on purely dissipationless mergers.Our main results are: the stellar IMF of massive early-type galaxies is significantly heavierthan that of the Milky Way and correlates with galaxy mass; the dark matter halo hasa steep slope in at least one system; more compact galaxies have less dark matter thantheir extended counterparts at fixed redshift and stellar mass; early-type galaxies evolvewhile keeping the slope of their total density profile approximately constant. This last result cannot be reproduced with purely dissipationless mergers, therefore a little amountof dissipation is required
The cosmic evolution of the stellar massvelocity dispersion relation of early-type galaxies
We study the evolution of the observed correlation between central stellar
velocity dispersion and stellar mass of massive
() early-type galaxies (ETGs) out
to redshift , exploiting a Bayesian hierarchical inference
formalism. Collecting ETGs from state-of-the-art literature samples, we build a
sample (), which is obtained with homogeneous
selection criteria, but also a less homogeneous sample (). Based on the fiducial sample, we find that the
- relation is well represented by
, with
independent of redshift and (at given ,
decreases for decreasing , for instance by a factor of
from to ). When the slope is allowed to evolve,
we find it increasing with redshift:
describes the data as well as constant . The intrinsic
scatter of the - relation is dex in
at given , independent of redshift. Our results
suggest that, on average, the velocity dispersion of massive
() ETGs decreases with time while they
evolve from to . The analysis of the extended sample
leads to results similar to that of the fiducial sample, with slightly stronger
redshift dependence of the normalisation () and weaker
redshift dependence of the slope () when varies with time. At ETGs with have, on average, higher
than ETGs of similar stellar mass at .Comment: 26 pages, 15 figures, accepted for publication in MNRA
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