1,036 research outputs found
The evolution of galaxy intrinsic alignments in the MassiveBlackII universe
We investigate the redshift evolution of the intrinsic alignments (IAs) of galaxies in the MassiveBlackII (MBII) simulation. We select galaxy samples above fixed subhalo mass cuts (â Mh>1011,12,13Mâhâ1â ) at z = 0.6 and trace their progenitors to z = 3 along their merger trees. Dark matter components of z = 0.6 galaxies are more spherical than their progenitors while stellar matter components tend to be less spherical than their progenitors. The distribution of the galaxyâsubhalo misalignment angle peaks at âŒ10 deg with a mild increase with time. The evolution of the ellipticityâdirection (ED) correlation amplitude Ï(r) of galaxies (which quantifies the tendency of galaxies to preferentially point towards surrounding matter overdensities) is governed by the evolution in the alignment of underlying dark matter (DM) subhaloes to the matter density of field, as well as the alignment between galaxies and their DM subhaloes. At scales âŒ1 Mpchâ1â , the alignment between DM subhaloes and matter overdensity gets suppressed with time, whereas the alignment between galaxies and DM subhaloes is enhanced. These competing tendencies lead to a complex redshift evolution of Ï(r) for galaxies at âŒ1 Mpchâ1â . At scales >1 Mpchâ1â , alignment between DM subhaloes and matter overdensity does not evolve significantly; the evolution of the galaxyâsubhalo misalignment therefore leads to an increase in Ï(r) for galaxies by a factor of âŒ4 from z = 3 to 0.6 at scales >1 Mpchâ1â . The balance between competing physical effects is scale dependent, leading to different conclusions at much smaller scales (â âŒ0.1 Mpchâ1â )
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&
Exploring Dark Energy with Next-Generation Photometric Redshift Surveys
The coming decade will be an exciting period for dark energy research, during which astronomers will address the question of what drives the accelerated cosmic expansion as first revealed by type Ia supernova (SN) distances, and confirmed by later observations. The mystery of dark energy poses a challenge of such magnitude that, as stated by the Dark Energy Task Force (DETF), nothing short of a revolution in our understanding of fundamental physics will be required to achieve a full understanding of the cosmic acceleration. The lack of multiple complementary precision observations is a major obstacle in developing lines of attack for dark energy theory. This lack is precisely what next-generation surveys will address via the powerful techniques of weak lensing (WL) and baryon acoustic oscillations (BAO) -- galaxy correlations more generally -- in addition to SNe, cluster counts, and other probes of geometry and growth of structure. Because of their unprecedented statistical power, these surveys demand an accurate understanding of the observables and tight control of systematics. This white paper highlights the opportunities, approaches, prospects, and challenges relevant to dark energy studies with wide-deep multiwavelength photometric redshift surveys. Quantitative predictions are presented for a 20000 sq. deg. ground-based 6-band (ugrizy) survey with 5-sigma depth of r~27.5, i.e., a Stage 4 survey as defined by the DETF
Validation of the AKIN criteria definition using high-resolution ICU data from the MIMIC-II database
Testing a simple recipe for estimating galaxy masses from minimal observational data
The accuracy and robustness of a simple method to estimate the total mass
profile of a galaxy is tested using a sample of 65 cosmological
zoom-simulations of individual galaxies. The method only requires information
on the optical surface brightness and the projected velocity dispersion
profiles and therefore can be applied even in case of poor observational data.
In the simulated sample massive galaxies ( \kms) at
redshift have almost isothermal rotation curves for broad range of radii
(RMS for the circular speed deviations from a constant value over
). For such galaxies the method recovers the
unbiased value of the circular speed. The sample averaged deviation from the
true circular speed is less than with the scatter of
(RMS) up to . Circular speed estimates of massive
non-rotating simulated galaxies at higher redshifts ( and ) are also
almost unbiased and with the same scatter. For the least massive galaxies in
the sample ( \kms) at the RMS deviation is
and the mean deviation is biased low by about . We also derive the
circular velocity profile from the hydrostatic equilibrium (HE) equation for
hot gas in the simulated galaxies. The accuracy of this estimate is about RMS
for massive objects () and the HE
estimate is biased low by , which can be traced to the presence of
gas motions. This implies that the simple mass estimate can be used to
determine the mass of observed massive elliptical galaxies to an accuracy of
and can be very useful for galaxy surveys.Comment: 15 pages, 14 figures, 1 tabl
Scale-dependent bias induced by local non-Gaussianity: A comparison to N-body simulations
We investigate the effect of primordial non-Gaussianity of the local f_NL
type on the auto- and cross-power spectrum of dark matter haloes using
simulations of the LCDM cosmology. We perform a series of large N-body
simulations of both positive and negative f_NL, spanning the range between 10
and 100. Theoretical models predict a scale-dependent bias correction \Delta
b(k,f_NL) that depends on the linear halo bias b(M). We measure the power
spectra for a range of halo mass and redshifts covering the relevant range of
existing galaxy and quasar populations. We show that auto and cross-correlation
analyses of bias are consistent with each other. We find that for low
wavenumbers with k<0.03 h/Mpc the theory and the simulations agree well with
each other for biased haloes with b(M)>1.5. We show that a scale-independent
bias correction improves the comparison between theory and simulations on
smaller scales, where the scale-dependent effect rapidly becomes negligible.
The current limits on f_NL from Slosar et al. (2008) come mostly from very
large scales k<0.01 h/Mpc and, therefore, remain valid. For the halo samples
with b(M)<1.5-2 we find that the scale- dependent bias from non-Gaussianity
actually exceeds the theoretical predictions. Our results are consistent with
the bias correction scaling linearly with f_NL.Comment: 13 pages, 10 figures. (v2): substantial changes. added a physically
motivated scale-independent bias correction which improves significantly the
agreement with the simulations (v3): matches published versio
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The LSST DESC data challenge 1: Generation and analysis of synthetic images for next-generation surveys
Data Challenge 1 (DC1) is the first synthetic data set produced by the Rubin Observatory Legacy Survey of Space and Time (LSST) Dark Energy Science Collaboration (DESC). DC1 is designed to develop and validate data reduction and analysis and to study the impact of systematic effects that will affect the LSST data set. DC1 is comprised of r-band observations of 40 deg2 to 10 yr LSST depth. We present each stage of the simulation and analysis process: (a) generation, by synthesizing sources from cosmological N-body simulations in individual sensor-visit images with different observing conditions; (b) reduction using a development version of the LSST Science Pipelines; and (c) matching to the input cosmological catalogue for validation and testing. We verify that testable LSST requirements pass within the fidelity of DC1. We establish a selection procedure that produces a sufficiently clean extragalactic sample for clustering analyses and we discuss residual sample contamination, including contributions from inefficiency in star-galaxy separation and imperfect deblending. We compute the galaxy power spectrum on the simulated field and conclude that: (i) survey properties have an impact of 50 per cent of the statistical uncertainty for the scales and models used in DC1; (ii) a selection to eliminate artefacts in the catalogues is necessary to avoid biases in the measured clustering; and (iii) the presence of bright objects has a significant impact (2-6) in the estimated power spectra at small scales (> 1200), highlighting the impact of blending in studies at small angular scales in LSST
Size magnification as a complement to Cosmic Shear
We investigate the extent to which cosmic size magnification may be used to
com- plement cosmic shear in weak gravitational lensing surveys, with a view to
obtaining high-precision estimates of cosmological parameters. Using simulated
galaxy images, we find that size estimation can be an excellent complement,
finding that unbiased estimation of the convergence field is possible with
galaxies with angular sizes larger than the point-spread function (PSF) and
signal-to-noise ratio in excess of 10. The statistical power is similar to, but
not quite as good as, cosmic shear, and it is subject to different systematic
effects. Application to ground-based data will be challeng- ing, with
relatively large empirical corrections required to account for with biases for
galaxies which are smaller than the PSF, but for space-based data with 0.1
arcsecond resolution, the size distribution of galaxies brighter than i=24 is
ideal for accurate estimation of cosmic size magnification.Comment: 11 pages, 11 figures, accepted by MNRA
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