340 research outputs found
Can Effects of Dark Matter be Explained by the Turbulent Flow of Spacetime?
For the past forty years the search for dark matter has been one of the
primary foci of astrophysics, although there has yet to be any direct evidence
for its existence (Porter et al. 2011). Indirect evidence for the existence of
dark matter is largely rooted in the rotational speeds of stars within their
host galaxies, where, instead of having a ~ r^1/2 radial dependence, stars
appear to have orbital speeds independent of their distance from the galactic
center, which led to proposed existence of dark matter (Porter et al. 2011;
Peebles 1993). We propose an alternate explanation for the observed stellar
motions within galaxies, combining the standard treatment of a fluid-like
spacetime with the possibility of a "bulk flow" of mass through the Universe.
The differential "flow" of spacetime could generate vorticies capable of
providing the "perceived" rotational speeds in excess of those predicted by
Newtonian mechanics. Although a more detailed analysis of our theory is
forthcoming, we find a crude "order of magnitude" calculation can explain this
phenomena. We also find that this can be used to explain the graviational
lensing observed around globular clusters like "Bullet Cluster".Comment: 5 pages, Accepted for publication in Journal of Modern Physics:
Gravitation and Cosmology (Sept. 2012
Strong grain neighbour effects in polycrystals
Anisotropy in single crystal properties of polycrystals controls both the overall response of the aggregates and patterning of local stress/strain distributions, the extremes of which govern failure processes. Improving the understanding of grain-grain interactions has important consequences for in-service performance limits. Three-dimensional synchrotron X-ray diffraction was used to study the evolution of grain resolved stresses over many contiguous grains in Zr and Ti polycrystals deformed in situ. In a significant fraction of grains the stress along the loading axis was found to decrease during tensile plastic flow just beyond the macroscopic yield point; this is in the absence of deformation twinning and is a surprising behaviour. It is shown that this phenomenon is controlled by the crystallographic orientation of the grain and its immediate neighbours, particularly those adjacent along the loading axis
Photometric Redshift Calibration with Self Organising Maps
Accurate photometric redshift calibration is central to the robustness of all
cosmology constraints from cosmic shear surveys. Analyses of the KiDS
re-weighted training samples from all overlapping spectroscopic surveys to
provide a direct redshift calibration. Using self-organising maps (SOMs) we
demonstrate that this spectroscopic compilation is sufficiently complete for
KiDS, representing of the effective 2D cosmic shear sample. We use the
SOM to define a represented `gold' cosmic shear sample, per tomographic
bin. Using mock simulations of KiDS and the spectroscopic training set, we
estimate the uncertainty on the SOM redshift calibration, and find that
photometric noise, sample variance, and spectroscopic selection effects
(including redshift and magnitude incompleteness) induce a combined maximal
scatter on the bias of the redshift distribution reconstruction () of
in all tomographic bins. We show
that the SOM calibration is unbiased in the cases of noiseless photometry and
perfectly representative spectroscopic datasets, as expected from theory. The
inclusion of both photometric noise and spectroscopic selection effects in our
mock data introduces a maximal bias of , or at
confidence, once quality flags have been applied to the SOM. The method
presented here represents a significant improvement over the previously adopted
direct redshift calibration implementation for KiDS, owing to its diagnostic
and quality assurance capabilities. The implementation of this method in future
cosmic shear studies will allow better diagnosis, examination, and mitigation
of systematic biases in photometric redshift calibration.Comment: 22 pages, 10 figures, 4 appendices, accepted for publication in A&
KiDS+VIKING+GAMA: Halo occupation distributions and correlations of satellite numbers with a new halo model of the galaxy-matter bispectrum for galaxy-galaxy-galaxy lensing
Halo models and halo occupation distributions (HODs) are important tools to
model the galaxy and matter distribution. We present and assess a new method
for constraining the parameters of HODs using the gravitational lensing shear
around galaxy pairs, galaxy-galaxy-galaxy-lensing (G3L). In contrast to
galaxy-galaxy-lensing, G3L is sensitive to correlations between the per-halo
numbers of galaxies from different populations. We use G3L to probe these
correlations and test the default hypothesis that they are negligible. We
derive a halo model for G3L and validate it with realistic mock data from the
Millennium Simulation and a semi-analytic galaxy model. Then, we analyse public
data from the Kilo-Degree Survey (KiDS), the VISTA Infrared Kilo-Degree Galaxy
Survey (VIKING) and data from the Galaxy And Mass Assembly Survey (GAMA) to
infer the HODs of galaxies at in five different stellar mass bins
between and and two colours
(red and blue), as well as correlations between satellite numbers. The analysis
recovers the true HODs in the simulated data within the credibility
range. The inferred HODs vary significantly with colour and stellar mass. There
is also strong evidence () for correlations, increasing with halo
mass, between the numbers of red and blue satellites and galaxies with stellar
masses below $10^{10} \Msun. Possible causes of these correlations are the
selection of similar galaxies in different samples, the survey flux limit, or
physical mechanisms like a fixed ratio between the satellite numbers of
distinct populations. The decorrelation for halos with smaller masses is
probably an effect of shot noise by low-occupancy halos. The inferred HODs can
be used to complement galaxy-galaxy-lensing or galaxy clustering HOD studies or
as input to cosmological analyses and improved mock galaxy catalogues.Comment: 20 pages + Appendix, 14 Figures. Submitted to Astronomy &
Astrophysics. Abstract is abridge
Luminous red galaxies in the Kilo Degree Survey: selection with broad-band photometry and weak lensing measurements
We use the overlap between multiband photometry of the Kilo-Degree Survey
(KiDS) and spectroscopic data based on the Sloan Digital Sky Survey (SDSS) and
Galaxy And Mass Assembly (GAMA) to infer the colour-magnitude relation of
red-sequence galaxies. We then use this inferred relation to select luminous
red galaxies (LRGs) in the redshift range of over the entire KiDS
Data Release 3 footprint. We construct two samples of galaxies with different
constant comoving densities and different luminosity thresholds. The selected
red galaxies have photometric redshifts with typical photo-z errors of
that are nearly uniform with respect to
observational systematics. This makes them an ideal set of galaxies for lensing
and clustering studies. As an example, we use the KiDS-450 cosmic shear
catalogue to measure the mean tangential shear signal around the selected LRGs.
We detect a significant weak lensing signal for lenses out to
Improved Weak Lensing Photometric Redshift Calibration via StratLearn and Hierarchical Modeling
Discrepancies between cosmological parameter estimates from cosmic shear
surveys and from recent Planck cosmic microwave background measurements
challenge the ability of the highly successful CDM model to describe
the nature of the Universe. To rule out systematic biases in cosmic shear
survey analyses, accurate redshift calibration within tomographic bins is key.
In this paper, we improve photo- calibration via Bayesian hierarchical
modeling of full galaxy photo- conditional densities, by employing
, a recently developed statistical methodology, which
accounts for systematic differences in the distribution of the spectroscopic
training/source set and the photometric target set. Using realistic simulations
that were designed to resemble the KiDS+VIKING-450 dataset, we show that
-estimated conditional densities improve the galaxy
tomographic bin assignment, and that our -Bayesian
framework leads to nearly unbiased estimates of the target population means.
This leads to a factor of improvement upon the previously best
photo- calibration method. Our approach delivers a maximum bias per
tomographic bin of , with an
average absolute bias of across the five tomographic bins.Comment: 24 pages, 20 figures, 3 appendice
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
Cosmic star formation history with tomographic CIB-galaxy cross-correlation
In this work, we probe the star formation history of the Universe using
tomographic cross-correlation between the cosmic infrared background (CIB) and
galaxy samples. The galaxy samples are from the Kilo-Degree Survey (KiDS),
while the CIB maps are made from \planck\, sky maps. We measure the
cross-correlation in harmonic space with a significance of 43. We model
the cross-correlation with a halo model, which links CIB anisotropies to star
formation rates (SFR) and galaxy abundance. We assume that SFR has a lognormal
dependence on halo mass, while galaxy abundance follows the halo occupation
distribution (HOD) model. The cross-correlations give a best-fit maximum star
formation efficiency of at a halo
mass . The derived
star formation rate density (SFRD) is well constrained up to . The
constraining power at high redshift is mainly limited by the KiDS survey depth.
A combination with external SFRD measurements from previous studies gives
. This tightens
the SFRD constraint up to , yielding a peak SFRD of
at
, corresponding to a lookback time of
Gyr. Both constraints are consistent, and the derived
SFRD agrees with previous studies and simulations. Additionally, we estimate
the galaxy bias of KiDS galaxies from the constrained HOD parameters and
yield an increasing bias from at to
at . Finally, we provide a forecast for future
galaxy surveys and conclude that, due to their considerable depth, future
surveys will yield a much tighter constraint on the evolution of the SFRD.Comment: 22 pages, 14 figures, 3 tables, the abstract is abridge
G10/COSMOS : 38 band (far-UV to far-IR) panchromatic photometry using LAMBDAR
We present a consistent total flux catalogue for a ∼1 deg2 subset of the Cosmic Evolution Survey (COSMOS) region (RA ∈ [149∘.55, 150∘.65], Dec. ∈ [1∘.80, 2∘.73]) with near-complete coverage in 38 bands from the far-ultraviolet to the far-infrared. We produce aperture matched photometry for 128 304 objects with i < 24.5 in a manner that is equivalent to the Wright et al. catalogue from the low-redshift (z < 0.4) Galaxy and Mass Assembly (GAMA) survey. This catalogue is based on publicly available imaging from GALEX, Canada–France–Hawaii Telescope, Subaru, Visible and Infrared Survey Telescope for Astronomy, Spitzer and Herschel, contains a robust total flux measurement or upper limit for every object in every waveband and complements our re-reduction of publicly available spectra in the same region. We perform a number of consistency checks, demonstrating that our catalogue is comparable to existing data sets, including the recent COSMOS2015 catalogue. We also release an updated Davies et al. spectroscopic catalogue that folds in new spectroscopic and photometric redshift data sets. The catalogues are available for download at http://cutout.icrar.org/G10/dataRelease.php. Our analysis is optimised for both panchromatic analysis over the full wavelength range and for direct comparison to GAMA, thus permitting measurements of galaxy evolution for 0 < z < 1 while minimizing the systematic error resulting from disparate data reduction methods.Publisher PDFPeer reviewe
- …