60 research outputs found
The impact of cosmic variance on simulating weak lensing surveys
Upcoming weak lensing surveys will survey large cosmological volumes to
measure the growth of cosmological structure with time and thereby constrain
dark energy. One major systematic uncertainty in this process is the
calibration of the weak lensing shape distortions, or shears. Most upcoming
surveys plan to test several aspects of their shear estimation algorithms using
sophisticated image simulations that include realistic galaxy populations based
on high-resolution data from the Hubble Space Telescope (HST). However,
existing datasets from the (HST) cover very small cosmological volumes, so
cosmic variance could cause the galaxy populations in them to be atypical. A
narrow redshift slice from such surveys could be dominated by a single large
overdensity or underdensity. In that case, the morphology-density relation
could alter the local galaxy populations and yield an incorrect calibration of
shear estimates as a function of redshift. We directly test this scenario using
the COSMOS survey, the largest-area (HST) survey to date, and show how the
statistical distributions of galaxy shapes and morphological parameters (e.g.,
S\'{e}rsic ) are influenced by redshift-dependent cosmic variance. The
typical variation in RMS ellipticity due to environmental effects is 5 per cent
(absolute, not relative) for redshift bins of width , which
could result in uncertain shear calibration at the 1 per cent level. We
conclude that the cosmic variance effects are large enough to exceed the
systematic error budget of future surveys, but can be mitigated with careful
choice of training dataset and sufficiently large redshift binning.Comment: 18 pages, 16 figures, 3 tables. v2 matches the accepted version for
MNRA
Testing the presence of multiple photometric components in nearby early-type galaxies using SDSS
We investigate two-dimensional image decomposition of nearby, morphologically
selected early-type galaxies (ETGs). We are motivated by recent observational
evidence of significant size growth of quiescent galaxies and theoretical
development advocating a two-phase formation scenario for ETGs. We find that a
significant fraction of nearby ETGs show changes in isophotal shape that
require multi-component models. The characteristic sizes of the inner and outer
component are and kpc. The inner component lies on the
mass-size relation of ETGs at , while the outer component
tends to be more elliptical and hints at a stochastic buildup process. We find
real physical differences between the single- and double-component ETGs, with
the double-component galaxies being younger and more metal-rich. The fraction
of double component ETGs increases with increasing and decreases in
denser environments. We hypothesize that double-component systems were able to
accrete gas and small galaxies until later times, boosting their central
densities, building up their outer parts, and lowering their typical central
ages. In contrast, the oldest galaxies, perhaps due to residing in richer
environments, have no remaining hints of their last accretion episode.Comment: resubmitted to ApJ after referee's repor
Soft Finger Model with Adaptive Contact Geometry for Grasping and Manipulation Tasks
This paper presents a method for building analytical contact models for soft fingers. Friction constraints are derived based on general expressions for non-planar contacts of elastic bodies, taking into account the local geometry and structure of the objects in contact. These constraints are then formulated as a linear complementarity problem, the solution of which provides the normal and frictional forces applied at each contact, as well as the relative velocity of the bodies involved. This approach captures frictional effects such as coupling between tangential force and frictional torque. We illustrate this method by analyzing manipulation tasks performed by an anthropomorphic robotic hand equipped with soft fingerpads
Understanding the Unique Assembly History of Central Group Galaxies
Central Galaxies (CGs) in massive halos live in unique environments with
formation histories closely linked to that of the host halo. In local clusters
they have larger sizes () and lower velocity dispersions (sigma) at fixed
stellar mass M_star, and much larger R_e at a fixed than field and
satellite galaxies (non-CGs). Using spectroscopic observations of group
galaxies selected from the COSMOS survey, we compare the dynamical scaling
relations of early-type CGs and non-CGs at z~0.6, to distinguish possible
mechanisms that produce the required evolution. CGs are systematically offset
towards larger R_e at fixed compared to non-CGs with similar M_star.
The CG R_e-M_star relation also shows differences, primarily driven by a
sub-population (~15%) of galaxies with large , while the M_star-sigma
relations are indistinguishable. These results are accentuated when double
Sersic profiles, which better fit light in the outer regions of galaxies, are
adopted. They suggest that even group-scale CGs can develop extended components
by these redshifts that can increase total and M_star estimates by
factors of ~2. To probe the evolutionary link between our sample and cluster
CGs, we also analyze two cluster samples at z~0.6 and z~0. We find similar
results for the more massive halos at comparable z, but much more distinct CG
scaling relations at low-z. Thus, the rapid, late-time accretion of outer
components, perhaps via the stripping and accretion of satellites, would appear
to be a key feature that distinguishes the evolutionary history of CGs.Comment: 18 pages, 14 Figures, ApJ in pres
Luminous Red Galaxies in Clusters: Central Occupation, Spatial Distributions, and Mis-centering
Luminous Red Galaxies (LRG) from the Sloan Digital Sky Survey are considered
among the best understood samples of galaxies, and they are employed in a broad
range of cosmological studies. Because they form a relatively homogeneous
population, with high stellar masses and red colors, they are expected to
occupy halos in a relatively simple way. In this paper, we study how LRGs
occupy massive halos via direct counts in clusters and we reveal several
unexpected trends suggesting that the connection between LRGs and dark matter
halos may not be straightforward. Using the redMaPPer cluster catalog, we
derive the central occupation of LRGs as a function richness, Ncen({\lambda}).
Assuming no correlation between cluster mass and central galaxy luminosity at
fixed richness, we show that clusters contain a significantly lower fraction of
central LRGs than predicted from the two-point correlation function. At halo
masses of 10^14.5 Msun, we find Ncen=0.73, compared to Ncen of 0.89 from
correlation studies. Our central occupation function for LRGs converges to 0.95
at large halo masses. A strong anti-correlation between central luminosity and
cluster mass at fixed richness is required to reconcile our results with those
based on clustering studies. We also derive P_BNC, the probability that the
brightest cluster member is not the central galaxy. We find P_BNC ~ 20-30%
which is a factor of ~2 lower than the value found by Skibba et al. 2011.
Finally, we study the radial offsets of bright non-central LRGs from cluster
centers and show that bright non-central LRGs follow a different radial
distribution compared to red cluster members, which follow a
Navarro-Frank-White profile. This work demonstrates that even the most massive
clusters do not always have an LRG at the center, and that the brightest galaxy
in a cluster is not always the central galaxy.Comment: 18 pages, 9 figures, 4 tables, submitted to MNRAS, included the
referee comment
The Third Gravitational Lensing Accuracy Testing (GREAT3) Challenge Handbook
The GRavitational lEnsing Accuracy Testing 3 (GREAT3) challenge is the third
in a series of image analysis challenges, with a goal of testing and
facilitating the development of methods for analyzing astronomical images that
will be used to measure weak gravitational lensing. This measurement requires
extremely precise estimation of very small galaxy shape distortions, in the
presence of far larger intrinsic galaxy shapes and distortions due to the
blurring kernel caused by the atmosphere, telescope optics, and instrumental
effects. The GREAT3 challenge is posed to the astronomy, machine learning, and
statistics communities, and includes tests of three specific effects that are
of immediate relevance to upcoming weak lensing surveys, two of which have
never been tested in a community challenge before. These effects include
realistically complex galaxy models based on high-resolution imaging from
space; spatially varying, physically-motivated blurring kernel; and combination
of multiple different exposures. To facilitate entry by people new to the
field, and for use as a diagnostic tool, the simulation software for the
challenge is publicly available, though the exact parameters used for the
challenge are blinded. Sample scripts to analyze the challenge data using
existing methods will also be provided. See http://great3challenge.info and
http://great3.projects.phys.ucl.ac.uk/leaderboard/ for more information.Comment: 30 pages, 13 figures, submitted for publication, with minor edits
(v2) to address comments from the anonymous referee. Simulated data are
available for download and participants can find more information at
http://great3.projects.phys.ucl.ac.uk/leaderboard
Semen CD4+ T cells and macrophages are productively infected at all stages of SIV infection in macaques.
International audienceThe mucosal events of HIV transmission have been extensively studied, but the role of infected cells present in the genital and rectal secretions, and in the semen, in particular, remains a matter of debate. As a prerequisite to a thorough in vivo investigation of the early transmission events through infected cells, we characterized in detail by multi-parameter flow cytometry the changes in macaque seminal leukocytes during SIVmac251 infection, focusing on T cells, macrophages and dendritic cells. Using immunocytofluorescence targeting SIV proteins and real-time quantitative PCR targeting SIV DNA, we investigated the nature of the infected cells on sorted semen leukocytes from macaques at different stages of infection. Finally, we cocultured semen CD4(+) T cells and macrophages with a cell line permissive to SIV infection to assess their infectivity in vitro. We found that primary infection induced strong local inflammation, which was associated with an increase in the number of leukocytes in semen, both factors having the potential to favor cell-associated virus transmission. Semen CD4(+) T cells and macrophages were productively infected at all stages of infection and were infectious in vitro. Lymphocytes had a mucosal phenotype and expressed activation (CD69 & HLA-DR) and migration (CCR5, CXCR4, LFA-1) markers. CD69 expression was increased in semen T cells by SIV infection, at all stages of infection. Macrophages predominated at all stages and expressed CD4, CCR5, MAC-1 and LFA-1. Altogether, we demonstrated that semen contains the two major SIV-target cells (CD4+ T cells and macrophages). Both cell types can be productively infected at all stages of SIV infection and are endowed with markers that may facilitate transmission of infection during sexual exposure
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