56 research outputs found
The Maximum Optical Depth Towards Bulge Stars From Axisymmetric Models of the Milky Way
It has been known that recent microlensing results towards the bulge imply
mass densities that are surprisingly high given dynamical constraints on the
Milky Way mass distribution. We derive the maximum optical depth towards the
bulge that may be generated by axisymmetric structures in the Milky Way, and
show that observations are close to surpassing these limits. This result argues
in favor of a bar as a source of significantly enhanced microlensing. Several
of the bar models in the literature are discussed.Comment: Latex, 6 pages, 4 figures, uses aas2pp4 and epsf style files.
Accepted for publication in ApJ Letter
The mass distribution in an assembling super galaxy group at
We present a weak gravitational lensing analysis of supergroup SG11201202,
consisting of four distinct X-ray-luminous groups, that will merge to form a
cluster comparable in mass to Coma at . These groups lie within a
projected separation of 1 to 4 Mpc and within km s and
form a unique protocluster to study the matter distribution in a coalescing
system.
Using high-resolution {\em HST}/ACS imaging, combined with an extensive
spectroscopic and imaging data set, we study the weak gravitational distortion
of background galaxy images by the matter distribution in the supergroup. We
compare the reconstructed projected density field with the distribution of
galaxies and hot X-ray emitting gas in the system and derive halo parameters
for the individual density peaks.
We show that the projected mass distribution closely follows the locations of
the X-ray peaks and associated brightest group galaxies. One of the groups that
lies at slightly lower redshift () than the other three groups
() is X-ray luminous, but is barely detected in the
gravitational lensing signal. The other three groups show a significant
detection (up to in mass), with velocity dispersions between
and km s and masses between
and , consistent with independent measurements. These groups are
associated with peaks in the galaxy and gas density in a relatively
straightforward manner. Since the groups show no visible signs of interaction,
this supports the picture that we are catching the groups before they merge
into a cluster.Comment: 10 pages, 10 figures, accepted for publication by Astronomy &
Astrophysic
Microlensing by Cosmic Strings
We consider the signature and detectability of gravitational microlensing of
distant quasars by cosmic strings. Because of the simple image configuration
such events will have a characteristic light curve, in which a source would
appear to brighten by exactly a factor of two, before reverting to its original
apparent brightness. We calculate the optical depth and event rate, and
conclude that current predictions and limits on the total length of strings on
the sky imply optical depths of \la 10^{-8} and event rates of fewer than one
event per sources per year. Disregarding those predictions but replacing
them with limits on the density of cosmic strings from the CMB fluctuation
spectrum, leaves only a small region of parameter space (in which the sky
contains about strings with deficit angle of order 0.3
milli-arcseconds) for which a microlensing survey of exposure
source-years, spanning a 20--40-year period, might reveal the presence of
cosmic strings.Comment: 4 pages, accepted for publication in MNRA
Cinderella Strings
We investigate recent claims concerning a new class of cosmic string
solutions in the Weinberg-Salam model. They have the general form of previously
discussed semi-local and electroweak strings, but are modified by the presence
of a non-zero W-condensate in the core of the string. We explicitly construct
such solutions for arbitrary values of the winding number . We then prove
that they are gauge equivalent to bare electroweak strings with winding number
. We also develop new asymptotic expressions for large- strings.Comment: 11 pages, harvmac (b) and epsf (2 figures uuencoded
KiDS+VIKING-450:Improved cosmological parameter constraints from redshift calibration with self-organising maps
We present updated cosmological constraints for the KiDS+VIKING-450 cosmic
shear data set (KV450), estimated using redshift distributions and photometric
samples defined using self-organising maps (SOMs). Our fiducial analysis finds
marginal posterior constraints of ; smaller than, but otherwise consistent with,
previous work using this data set (). We analyse
additional samples and redshift distributions constructed in three ways:
excluding certain spectroscopic surveys during redshift calibration, excluding
lower-confidence spectroscopic redshifts in redshift calibration, and
considering only photometric sources which are jointly calibrated by at least
three spectroscopic surveys. In all cases, the method utilised here proves
robust: we find a maximal deviation from our fiducial analysis of for all samples defined and analysed using our SOM. To demonstrate
the reduction in systematic biases found within our analysis, we highlight our
results when performing redshift calibration without the DEEP2 spectroscopic
data set. In this case we find marginal posterior constraints of
; a difference with respect to the fiducial that
is both significantly smaller than, and in the opposite direction to, the
equivalent shift from previous work. These results suggest that our improved
cosmological parameter estimates are insensitive to pathological
misrepresentation of photometric sources by the spectroscopy used for direct
redshift calibration, and therefore that this systematic effect cannot be
responsible for the observed difference between estimates made with KV450
and Planck CMB probes.Comment: 10 pages, 3 figures, 4 appendices, accepted for publication in A&A
Letter
Pure-mode correlation functions for cosmic shear and application to KiDS-1000
One probe for systematic effects in gravitational lensing surveys is the presence of so-called B modes in the cosmic shear two-point correlation functions, ξ ± (ϑ), since lensing is expected to produce only E-mode shear. Furthermore, there exist ambiguous modes that cannot uniquely be assigned to either E-or B-mode shear. In this paper we derive explicit equations for the pure-mode shear correlation functions, ξ E/B ± (ϑ), and their ambiguous components, ξ amb ± (ϑ), that can be derived from the measured ξ ± (ϑ) on a finite angular interval, ϑ min ≤ ϑ ≤ ϑ max , such that ξ ± (ϑ) can be decomposed uniquely into pure-mode functions as ξ + = ξ E + + ξ B + + ξ amb + and ξ − = ξ E − − ξ B − + ξ amb −. The derivation is obtained by defining a new set of Complete Orthogonal Sets of E and B mode-separating Integrals (COSEBIs), for which explicit relations are obtained and which yields a smaller covariance between COSEBI modes. We derive the relation between ξ E/B/amb ± and the underlying E-and B-mode power spectra. The pure-mode correlation functions can provide a diagnostic of systematics in configuration space. We then apply our results to Scinet LIght Cone Simulations (SLICS) and the Kilo-Degree Survey (KiDS-1000) cosmic shear data, calculate the new COSEBIs and the pure-mode correlation functions, as well as the corresponding covariances, and show that the new statistics fit equally well to the best fitting cosmological model as the previous KiDS-1000 analysis and recover the same level of (insignificant) B modes. We also consider in some detail the ambiguous modes at the first-and second-order level, finding some surprising results. For example, the shear field of a point mass, when cut along a line through the center, cannot be ascribed uniquely to an E-mode shear and is thus ambiguous; additionally, the shear correlation functions resulting from a random ensemble of point masses, when measured over a finite angular range, correspond to an ambiguous mode
The dark matter halo masses of elliptical galaxies as a function of observationally robust quantities
Context. The assembly history of the stellar component of a massive elliptical galaxy is closely related to that of its dark matter halo. Measuring how the properties of galaxies correlate with their halo mass can therefore help to understand their evolution. Aims. We investigate how the dark matter halo mass of elliptical galaxies varies as a function of their properties, using weak gravi-tational lensing observations. To minimise the chances of biases, we focus on the following galaxy properties that can be determined robustly: the surface brightness profile and the colour. Methods. We selected 2409 central massive elliptical galaxies (log M * /M 11.4) from the Sloan Digital Sky Survey spectroscopic sample. We first measured their surface brightness profile and colours by fitting Sérsic models to photometric data from the Kilo-Degree Survey (KiDS). We fitted their halo mass distribution as a function of redshift, rest-frame r−band luminosity, half-light radius, and rest-frame u − g colour, using KiDS weak lensing measurements and a Bayesian hierarchical approach. For the sake of robustness with respect to assumptions on the large-radii behaviour of the surface brightness, we repeated the analysis replacing the total luminosity and half-light radius with the luminosity within a 10 kpc aperture, L r,10 , and the light-weighted surface brightness slope, Γ 10. Results. We did not detect any correlation between the halo mass and either the half-light radius or colour at fixed redshift and luminosity. Using the robust surface brightness parameterisation, we found that the halo mass correlates weakly with L r,10 and anti-correlates with Γ 10. At fixed redshift, L r,10 and Γ 10 , the difference in the average halo mass between galaxies at the 84th percentile and 16th percentile of the colour distribution is 0.00 ± 0.11 dex. Conclusions. Our results indicate that the average star formation efficiency of massive elliptical galaxies has little dependence on their final size or colour. This suggests that the origin of the diversity in the size and colour distribution of these objects lies with properties other than the halo mass
Towards emulating cosmic shear data:Revisiting the calibration of the shear measurements for the Kilo-Degree Survey
Exploiting the full statistical power of future cosmic shear surveys will
necessitate improvements to the accuracy with which the gravitational lensing
signal is measured. We present a framework for calibrating shear with image
simulations that demonstrates the importance of including realistic
correlations between galaxy morphology, size and more importantly, photometric
redshifts. This realism is essential so that selection and shape measurement
biases can be calibrated accurately for a tomographic cosmic shear analysis. We
emulate Kilo-Degree Survey (KiDS) observations of the COSMOS field using
morphological information from {\it Hubble} Space Telescope imaging, faithfully
reproducing the measured galaxy properties from KiDS observations of the same
field. We calibrate our shear measurements from lensfit, and find through a
range of sensitivity tests that lensfit is robust and unbiased within the
allowed 2 per cent tolerance of our study. Our results show that the
calibration has to be performed by selecting the tomographic samples in the
simulations, consistent with the actual cosmic shear analysis, because the
joint distributions of galaxy properties are found to vary with redshift.
Ignoring this redshift variation could result in misestimating the shear bias
by an amount that exceeds the allowed tolerance. To improve the calibration for
future cosmic shear analyses, it will be essential to also correctly account
for the measurement of photometric redshifts, which requires simulating
multi-band observations.Comment: 31 pages, 17 figures and 2 tables. Accepted for publication in A&A.
Matches the published versio
KiDS-Legacy calibration: unifying shear and redshift calibration with the SKiLLS multi-band image simulations
We present SKiLLS, a suite of multi-band image simulations for the weak
lensing analysis of the complete Kilo-Degree Survey (KiDS), dubbed KiDS-Legacy
analysis. The resulting catalogues enable joint shear and redshift calibration,
enhancing the realism and hence accuracy over previous efforts. To create a
large volume of simulated galaxies with faithful properties and to a sufficient
depth, we integrated cosmological simulations with high-quality imaging
observations. We also improved the realism of simulated images by allowing the
point spread function (PSF) to differ between CCD images, including stellar
density variations and varying noise levels between pointings. Using realistic
variable shear fields, we accounted for the impact of blended systems at
different redshifts. Although the overall correction is minor, we found a clear
redshift-bias correlation in the blending-only variable shear simulations,
indicating the non-trivial impact of this higher-order blending effect. We also
explored the impact of the PSF modelling errors and found a small yet
noticeable effect on the shear bias. Finally, we conducted a series of
sensitivity tests, including changing the input galaxy properties. We conclude
that our fiducial shape measurement algorithm, lensfit, is robust within the
requirements of lensing analyses with KiDS. As for future weak lensing surveys
with tighter requirements, we suggest further investments in understanding the
impact of blends at different redshifts, improving the PSF modelling algorithm
and developing the shape measurement method to be less sensitive to the galaxy
properties.Comment: 28 pages, 31 figures, 2 tables, minor revisions to match the final
accepted versio
KiDS-1000: Combined halo-model cosmology constraints from galaxy abundance, galaxy clustering, and galaxy-galaxy lensing
We present constraints on the flat Λ cold dark matter cosmological model through a joint analysis of galaxy abundance, galaxy clustering, and galaxy-galaxy lensing observables with the Kilo-Degree Survey. Our theoretical model combines a flexible conditional stellar mass function, which describes the galaxy-halo connection, with a cosmological N-body simulation-calibrated halo model, which describes the non-linear matter field. Our magnitude-limited bright galaxy sample combines nine-band optical-to-near-infrared photometry with an extensive and complete spectroscopic training sample to provide accurate redshift and stellar mass estimates. Our faint galaxy sample provides a background of accurately calibrated lensing measurements. We constrain the structure growth parameter to S8 = σ8√Ωm/0.3 =√0.773−0.030+0.028 and the matter density parameter to Ωm = 0.290−0.017+0.021. The galaxy-halo connection model adopted in the work is shown to be in agreement with previous studies. Our constraints on cosmological parameters are comparable to, and consistent with, joint ‘3 × 2pt’ clustering-lensing analyses that additionally include a cosmic shear observable. This analysis therefore brings attention to the significant constraining power in the often excluded non-linear scales for galaxy clustering and galaxy-galaxy lensing observables. By adopting a theoretical model that accounts for non-linear halo bias, halo exclusion, scale-dependent galaxy bias, and the impact of baryon feedback, this work demonstrates the potential for, and a way towards, including non-linear scales in cosmological analyses. Varying the width of the satellite galaxy distribution with an additional parameter yields a strong preference for sub-Poissonian variance, improving the goodness of fit by 0.18 in terms of the reduced χ2 value (and increasing the p-value by 0.25) compared to a fixed Poisson distribution
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