24,175 research outputs found
Evaluating the effect of stellar multiplicity on the PSF of space-based weak lensing surveys
The next generation of space-based telescopes used for weak lensing surveys
will require exquisite point spread function (PSF) determination. Previously
negligible effects may become important in the reconstruction of the PSF, in
part because of the improved spatial resolution. In this paper, we show that
unresolved multiple star systems can affect the ellipticity and size of the PSF
and that this effect is not cancelled even when using many stars in the
reconstruction process. We estimate the error in the reconstruction of the PSF
due to the binaries in the star sample both analytically and with image
simulations for different PSFs and stellar populations. The simulations support
our analytical finding that the error on the size of the PSF is a function of
the multiple stars distribution and of the intrinsic value of the size of the
PSF, i.e. if all stars were single. Similarly, the modification of each of the
complex ellipticity components (e1,e2) depends on the distribution of multiple
stars and on the intrinsic complex ellipticity. Using image simulations, we
also show that the predicted error in the PSF shape is a theoretical limit that
can be reached only if large number of stars (up to thousands) are used
together to build the PSF at any desired spatial position. For a lower number
of stars, the PSF reconstruction is worse. Finally, we compute the effect of
binarity for different stellar magnitudes and show that bright stars alter the
PSF size and ellipticity more than faint stars. This may affect the design of
PSF calibration strategies and the choice of the related calibration fields.Comment: 10 pages, 6 figures, accepted in A&
The universal distribution of halo interlopers in projected phase space. Bias in galaxy cluster concentration and velocity anisotropy?
When clusters of galaxies are viewed in projection, one cannot avoid picking
up foreground/background interlopers (FBIs), that lie within the virial cone
(VC), but outside the virial sphere. Structural & kinematic deprojection
equations are not known for an expanding Universe, where the Hubble flow (HF)
stretches the line-of-sight (LOS) distribution of velocities. We analyze 93
mock relaxed clusters, built from a cosmological simulation. The stacked mock
cluster is well fit by an m=5 Einasto DM density profile (but only out to 1.5
virial radii [r_v]), with velocity anisotropy (VA) close to the Mamon-Lokas
model with VA radius equal to that of density slope -2. The surface density of
FBIs is nearly flat out to r_v, while their LOS velocity distribution shows a
dominant gaussian cluster-outskirts component and a flat field component. This
distribution of FBIs in projected phase space is nearly universal in mass. A
local k=2.7 sigma velocity cut returns the LOS velocity dispersion profile
(LOSVDP) expected from the NFW density and VA profiles measured in 3D. The HF
causes a shallower outer LOSVDP that cannot be well matched by the Einasto
model for any k. After this velocity cut, FBIs still account for 23% of DM
particles within the VC (close to the observed fraction of cluster galaxies
lying off the Red Sequence). The best-fit projected NFW/Einasto models
underestimate the 3D concentration by 6+/-6% (16+/-7%) after (before) the
velocity cut, unless a constant background is included in the fit. Assuming the
correct mass profile, the VA profile is well recovered from the measured
LOSVDP, with a slight bias towards more radial orbits in the outer regions.
These small biases are overshadowed by large cluster-cluster variations caused
by cosmic variance. An appendix provides an analytical approximation to the
surface density, projected mass and tangential shear profiles of the Einasto
model.Comment: Version published in A&A, 23 pages, 22 figure
A Hubble Space Telescope Imaging Study of Four FeLoBAL Quasar Host Galaxies
We study the host galaxies of four Iron Low-Ionization Broad Absorption-line
Quasars (FeLoBALs) using Hubble Space Telescope imaging data, investigating the
possibility that they represent a transition between an obscured AGN and an
ordinary optical quasar. In this scenario, the FeLoBALs represent the early
stage of merger-triggered accretion, in which case their host galaxies are
expected to show signs of an ongoing or recent merger. Using PSF subtraction
techniques, we decompose the images into host galaxy and AGN components at
rest-frame ultraviolet and optical wavelengths. The ultraviolet is sensitive to
young stars, while the optical probes stellar mass. In the ultraviolet we image
at the BAL absorption trough wavelengths so as to decrease the contrast between
the quasar and host galaxy emission. We securely detect an extended source for
two of the four FeLoBALs in the rest-frame optical; a third host galaxy is
marginally detected. In the rest-frame UV we detect no host emission; this
constrains the level of unobscured star formation. Thus, the host galaxies have
observed properties that are consistent with those of non-BAL quasars with the
same nuclear luminosity, i.e., quiescent or moderately starforming elliptical
galaxies. However, we cannot exclude starbursting hosts that have the stellar
UV emission obscured by modest amounts of dust reddening. Thus, our findings
also allow the merger-induced young quasar scenario. For three objects, we
identify possible close companion galaxies that may be gravitationally
interacting with the quasar hosts.Comment: 33 pages, 15 figures. Accepted for publication in MNRA
Cross-correlating the Thermal Sunyaev-Zel'dovich Effect and the Distribution of Galaxy Clusters
We present the analytical formulas, derived based on the halo model, to
compute the cross-correlation between the thermal Sunyaev-Zel'dovich (SZ)
effect and the distribution of galaxy clusters. By binning the clusters
according to their redshifts and masses, this cross-correlation, the so-called
stacked SZ signal, reveals the average SZ profile around the clusters. The
stacked SZ signal is obtainable from a joint analysis of an
arcminute-resolution cosmic microwave background (CMB) experiment and an
overlapping optical survey, which allows for detection of the SZ signals for
clusters whose masses are below the individual cluster detection threshold. We
derive the error covariance matrix for measuring the stacked SZ signal, and
then forecast for its detection from ongoing and forthcoming combined
CMB-optical surveys. We find that, over a wide range of mass and redshift, the
stacked SZ signal can be detected with a significant signal to noise ratio
(total S/N \gsim 10), whose value peaks for the clusters with intermediate
masses and redshifts. Our calculation also shows that the stacking method
allows for probing the clusters' SZ profiles over a wide range of scales, even
out to projected radii as large as the virial radius, thereby providing a
promising way to study gas physics at the outskirts of galaxy clusters.Comment: 11 pages, 6 figures, 3 tables, minor revisions reflect PRD published
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