989 research outputs found
Towards an understanding of third-order galaxy-galaxy lensing
Third-order galaxy-galaxy lensing (G3L) is a next generation galaxy-galaxy
lensing technique that either measures the excess shear about lens pairs or the
excess shear-shear correlations about lenses. It is clear that these statistics
assess the three-point correlations between galaxy positions and projected
matter density. For future applications of these novel statistics, we aim at a
more intuitive understanding of G3L to isolate the main features that possibly
can be measured. We construct a toy model ("isolated lens model"; ILM) for the
distribution of galaxies and associated matter to determine the measured
quantities of the two G3L correlation functions and traditional galaxy-galaxy
lensing (GGL) in a simplified context. The ILM presumes single lens galaxies to
be embedded inside arbitrary matter haloes that, however, are statistically
independent ("isolated") from any other halo or lens position. In the ILM, the
average mass-to-galaxy number ratio of clusters of any size cannot change. GGL
and galaxy clustering alone cannot distinguish an ILM from any more complex
scenario. The lens-lens-shear correlator in combination with second-order
statistics enables us to detect deviations from a ILM, though. This can be
quantified by a difference signal defined in the paper. We demonstrate with the
ILM that this correlator picks up the excess matter distribution about galaxy
pairs inside clusters. The lens-shear-shear correlator is sensitive to
variations among matter haloes. In principle, it could be devised to constrain
the ellipticities of haloes, without the need for luminous tracers, or maybe
even random halo substructure. [Abridged]Comment: 14 pages, 3 figures, 1 table, accepted by A&A; some
"lens-shear-shear" were falsely "lens-lens-shear
Galaxy-galaxy(-galaxy) lensing as a sensitive probe of galaxy evolution
The gravitational lensing effect provides various ways to study the mass
environment of galaxies. We investigate how galaxy-galaxy(-galaxy) lensing can
be used to test models of galaxy formation and evolution. We consider two
semi-analytic galaxy formation models based on the Millennium Run N-body
simulation: the Durham model by Bower et al. (2006) and the Garching model by
Guo et al. (2011). We generate mock lensing observations for the two models,
and then employ Fast Fourier Transform methods to compute second- and
third-order aperture statistics in the simulated fields for various galaxy
samples. We find that both models predict qualitatively similar aperture
signals, but there are large quantitative differences. The Durham model
predicts larger amplitudes in general. In both models, red galaxies exhibit
stronger aperture signals than blue galaxies. Using these aperture measurements
and assuming a linear deterministic bias model, we measure relative bias ratios
of red and blue galaxy samples. We find that a linear deterministic bias is
insufficient to describe the relative clustering of model galaxies below ten
arcmin angular scales. Dividing galaxies into luminosity bins, the aperture
signals decrease with decreasing luminosity for brighter galaxies, but increase
again for fainter galaxies. This increase is likely an artifact due to too many
faint satellite galaxies in massive group and cluster halos predicted by the
models. Our study shows that galaxy-galaxy(-galaxy) lensing is a sensitive
probe of galaxy evolution.Comment: 11 pages, 8 figures, accepted in A&
Constraints on the shapes of galaxy dark matter haloes from weak gravitational lensing
We study the shapes of galaxy dark matter haloes by measuring the anisotropy
of the weak gravitational lensing signal around galaxies in the second
Red-sequence Cluster Survey (RCS2). We determine the average shear anisotropy
within the virial radius for three lens samples: all galaxies with
19<m_r'<21.5, and the `red' and `blue' samples, whose lensing signals are
dominated by massive low-redshift early-type and late-type galaxies,
respectively. To study the environmental dependence of the lensing signal, we
separate each lens sample into an isolated and clustered part and analyse them
separately. We also measure the azimuthal dependence of the distribution of
physically associated galaxies around the lens samples. We find that these
satellites preferentially reside near the major axis of the lenses, and
constrain the angle between the major axis of the lens and the average location
of the satellites to =43.7 deg +/- 0.3 deg for the `all' lenses,
=41.7 deg +/- 0.5 deg for the `red' lenses and =42.0 deg +/- 1.4
deg for the `blue' lenses. For the `all' sample, we find that the anisotropy of
the galaxy-mass cross-correlation function =0.23 +/- 0.12, providing
weak support for the view that the average galaxy is embedded in, and
preferentially aligned with, a triaxial dark matter halo. Assuming an
elliptical Navarro-Frenk-White (NFW) profile, we find that the ratio of the
dark matter halo ellipticity and the galaxy ellipticity
f_h=e_h/e_g=1.50+1.03-1.01, which for a mean lens ellipticity of 0.25
corresponds to a projected halo ellipticity of e_h=0.38+0.26-0.25 if the halo
and the lens are perfectly aligned. For isolated galaxies of the `all' sample,
the average shear anisotropy increases to =0.51+0.26-0.25 and
f_h=4.73+2.17-2.05, whilst for clustered galaxies the signal is consistent with
zero. (abridged)Comment: 28 pages, 23 figues, accepted for publication in A&
Direct measurement of dark matter halo ellipticity from two-dimensional lensing shear maps of 25 massive clusters
We present new measurements of dark matter distributions in 25 X-ray luminous
clusters by making a full use of the two-dimensional (2D) weak lensing signals
obtained from high-quality Subaru/Suprime-Cam imaging data. Our approach to
directly compare the measured lensing shear pattern with elliptical model
predictions allows us to extract new information on the mass distributions of
individual clusters, such as the halo ellipticity and mass centroid. We find
that these parameters on the cluster shape are little degenerate with cluster
mass and concentration parameters. By combining the 2D fitting results for a
subsample of 18 clusters, the elliptical shape of dark matter haloes is
detected at 7\sigma significance level. The mean ellipticity is found to be e =
0.46 \pm 0.04 (1\sigma), which is in excellent agreement with the standard
collisionless CDM model prediction. The mass centroid can be constrained with a
typical accuracy of ~20" (~50 kpc/h) in radius for each cluster with some
significant outliers, enabling to assess one of the most important systematic
errors inherent in the stacked cluster weak lensing technique, the mass
centroid uncertainty. In addition, the shape of the dark mass distribution is
found to be only weakly correlated with that of the member galaxy distribution.
We carefully examine possible sources of systematic errors in our measurements,
finding none of them to be significant. Our results demonstrate the power of
high-quality imaging data for exploring the detailed spatial distribution of
dark matter (Abridged).Comment: 17 pages, 10 figures, MNRAS in pres
Galaxy density profiles and shapes -- I. simulation pipeline for lensing by realistic galaxy models
Studies of strong gravitational lensing in current and upcoming wide and deep
photometric surveys, and of stellar kinematics from (integral-field)
spectroscopy at increasing redshifts, promise to provide valuable constraints
on galaxy density profiles and shapes. However, both methods are affected by
various selection and modelling biases, whch we aim to investigate in a
consistent way. In this first paper in a series we develop a flexible but
efficient pipeline to simulate lensing by realistic galaxy models. These galaxy
models have separate stellar and dark matter components, each with a range of
density profiles and shapes representative of early-type, central galaxies
without significant contributions from other nearby galaxies. We use Fourier
methods to calculate the lensing properties of galaxies with arbitrary surface
density distributions, and Monte Carlo methods to compute lensing statistics
such as point-source lensing cross-sections. Incorporating a variety of
magnification bias modes lets us examine different survey limitations in image
resolution and flux. We rigorously test the numerical methods for systematic
errors and sensitivity to basic assumptions. We also determine the minimum
number of viewing angles that must be sampled in order to recover accurate
orientation-averaged lensing quantities. We find that for a range of
non-isothermal stellar and dark matter density profiles typical of elliptical
galaxies, the combined density profile and corresponding lensing properties are
surprisingly close to isothermal around the Einstein radius. The converse
implication is that constraints from strong lensing and/or stellar kinematics,
which are indeed consistent with isothermal models near the Einstein radius,
cannot trivially be extrapolated to smaller and larger radii.Comment: 31 pages, 15 figures; paper II at arXiv:0808.2497; accepted for
publication in MNRAS; PDF file with full resolution figures at
http://www.sns.ias.edu/~glenn/paper1.pd
Galaxy density profiles and shapes -- II. selection biases in strong lensing surveys
[Abridged] Many current and future astronomical surveys will rely on samples
of strong gravitational lens systems to draw conclusions about galaxy mass
distributions. We use a new strong lensing pipeline (presented in Paper I of
this series) to explore selection biases that may cause the population of
strong lensing systems to differ from the general galaxy population. Our focus
is on point-source lensing by early-type galaxies with two mass components
(stellar and dark matter) that have a variety of density profiles and shapes
motivated by observational and theoretical studies of galaxy properties. We
seek not only to quantify but also to understand the physics behind selection
biases related to: galaxy mass, orientation and shape; dark matter profile
parameters such as inner slope and concentration; and adiabatic contraction. We
study how all of these properties affect the lensing Einstein radius, total
cross-section, quad/double ratio, and image separation distribution. We find
significant (factors of several) selection biases with mass; orientation, for a
given galaxy shape at fixed mass; cusped dark matter profile inner slope and
concentration; concentration of the stellar and dark matter deprojected Sersic
models. Interestingly, the intrinsic shape of a galaxy does not strongly
influence its lensing cross-section when we average over viewing angles. Our
results are an important first step towards understanding how strong lens
systems relate to the general galaxy population.Comment: 26 pages, 15 figures; paper I at arXiv:0808.2493; accepted for
publication in MNRAS (minor revisions); PDF file with full resolution figures
at http://www.sns.ias.edu/~rmandelb/paper2.pd
The Origin of the Hubble Sequence in Lambda-CDM Cosmology
The Galform semi-analytic model of galaxy formation is used to explore the
mechanisms primarily responsible for the three types of galaxies seen in the
local universe: bulge, bulge+disk and disk, identified with the visual
morphological types E, S0/a-Sbc, and Sc-Scd, respectively. With a suitable
choice of parameters the Galform model can accurately reproduce the observed
local K_s-band luminosity function (LF) for galaxies split by visual
morphological type. The successful set of model parameters is used to populate
the Millennium Simulation with 9.4 million galaxies and their dark matter
halos. The resulting catalogue is then used to explore the evolution of
galaxies through cosmic history. The model predictions concur with recent
observational results including the galaxy merger rate, the star formation rate
and the seemingly anti-hierarchical evolution of ellipticals. However, the
model also predicts significant evolution of the elliptical galaxy LF that is
not observed. The discrepancy raises the possibility that samples of z~1
galaxies which have been selected using colour and morphological criteria may
be contaminated with galaxies that are not actually ellipticals.Comment: Accepted for publication in MNRAS. Missing reference adde
Physical properties underlying observed kinematics of satellite galaxies
We study the kinematics of satellites around isolated galaxies selected from
the Sloan Digital Sky Survey (SDSS) spectroscopic catalog. Using a model of the
phase-space density previously measured for the halos of LCDM dark matter
cosmological simulations, we determine the properties of the halo mass
distribution and the orbital anisotropy of the satellites as a function of the
colour-based morphological type and the stellar mass of the central host
galaxy. We place constraints on the halo mass and the concentration parameter
of dark matter and the satellite number density profiles. We obtain a
concentration-mass relation for galactic dark matter haloes that is consistent
with predictions of a standard LCDM cosmological model. At given halo or
stellar mass, red galaxies have more concentrated halos than their blue
counterparts. The fraction of dark matter within a few effective radii is
minimal for 11.25<log M_star<11.5. The number density profile of the satellites
appears to be shallower than of dark matter, with the scale radius typically 60
per cent larger than of dark matter. The orbital anisotropy around red hosts
exhibits a mild excess of radial motions, in agreement with the typical
anisotropy profiles found in cosmological simulations, whereas blue galaxies
are found to be consistent with an isotropic velocity distribution. Our new
constraints on the halo masses of galaxies are used to provide analytic
approximations of the halo-to-stellar mass relation for red and blue galaxies.Comment: 12 pages, 11 figures; accepted for publication in MNRAS (updated
references
Fitting functions for a disk-galaxy model with different LCDM-halo profiles
We present an adaptation of the standard scenario of disk-galaxy formation to
the concordant LCDM cosmology aimed to derive analytical expressions for the
scale length and rotation speed of present-day disks that form within four
different, cosmologically motivated protogalactic dark matter halo-density
profiles. We invoke a standard galaxy-formation model that includes virial
equilibrium of spherical dark halos, specific angular momentum conservation
during gas cooling, and adiabatic halo response to the gas inflow. The mean
mass-fraction and mass-to-light ratio of the central stellar disk are treated
as free parameters whose values are tuned to match the zero points of the
observed size-luminosity and circular speed-luminosity relations of galaxies.
We supply analytical formulas for the characteristic size and rotation speed of
disks built inside Einasto r^{1/6}, Hernquist, Burkert, and Navarro-Frenk-White
dark matter halos. These expressions match simultaneously the observed zero
points and slopes of the different correlations that can be built in the RVL
space of disk galaxies from plausible values of the galaxy- and star-formation
efficiencies
The degeneracy between star-formation parameters in dwarf galaxy simulations and the Mstar-Mhalo relation
We present results based on a set of N-Body/SPH simulations of isolated dwarf
galaxies. The simulations take into account star formation, stellar feedback,
radiative cooling and metal enrichment. The dark matter halo initially has a
cusped profile, but, at least in these simulations, starting from idealised,
spherically symmetric initial conditions, a natural conversion to a core is
observed due to gas dynamics and stellar feedback.
A degeneracy between the efficiency with which the interstellar medium
absorbs energy feedback from supernovae and stellar winds on the one hand, and
the density threshold for star formation on the other, is found. We performed a
parameter survey to determine, with the aid of the observed kinematic and
photometric scaling relations, which combinations of these two parameters
produce simulated galaxies that are in agreement with the observations.
With the implemented physics we are unable to reproduce the relation between
the stellar mass and the halo mass as determined by Guo et al. (2010), however
we do reproduce the slope of this relation.Comment: Accepted for publication in MNRAS | 12 pages, 8 figure
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