61 research outputs found
Exploiting the full potential of photometric quasar surveys: Optimal power spectra through blind mitigation of systematics
We present optimal measurements of the angular power spectrum of the XDQSOz
catalogue of photometric quasars from the Sloan Digital Sky Survey. These
measurements rely on a quadratic maximum likelihood estimator that
simultaneously measures the auto- and cross-power spectra of four redshift
samples, and provides minimum-variance, unbiased estimates even at the largest
angular scales. Since photometric quasars are known to be strongly affected by
systematics such as spatially-varying depth and stellar contamination, we
introduce a new framework of extended mode projection to robustly mitigate the
impact of systematics on the power spectrum measurements. This technique
involves constructing template maps of potential systematics, decorrelating
them on the sky, and projecting out modes which are significantly correlated
with the data. Our method is able to simultaneously process several thousands
of nonlinearly-correlated systematics, and mode projection is performed in a
blind fashion. Using our final power spectrum measurements, we find a good
agreement with theoretical predictions, and no evidence for further
contamination by systematics. Extended mode projection not only obviates the
need for aggressive sky and quality cuts, but also provides control over the
level of systematics in the measurements, enabling the search for small signals
of new physics while avoiding confirmation bias.Comment: 13 pages, 8 figures. v2: version accepted by MNRAS. v3: systematics
templates publicly available on www.earlyuniverse.org/code, no change to
pape
No new cosmological concordance with massive sterile neutrinos
It has been claimed recently that massive sterile neutrinos could bring about
a new concordance between observations of the cosmic microwave background
(CMB), the large-scale structure (LSS) of the Universe, and local measurements
of the Hubble constant, . We demonstrate that this apparent concordance
results from combining datasets which are in significant tension, even within
this extended model, possibly indicating remaining systematic biases in the
measurements. We further show that this tension remains when the cosmological
model is further extended to include significant tensor modes, as suggested by
the recent BICEP2 results. Using the Bayesian evidence, we show that the
minimal CDM model is strongly favoured over its neutrino extensions by
various combinations of datasets. Robust data combinations yield stringent
limits of eV and eV at CL for the sum of active and sterile neutrinos,
respectively.Comment: 6 pages, version accepted by PR
Hierarchical Bayesian inference of galaxy redshift distributions from photometric surveys
Accurately characterizing the redshift distributions of galaxies is essential
for analysing deep photometric surveys and testing cosmological models. We
present a technique to simultaneously infer redshift distributions and
individual redshifts from photometric galaxy catalogues. Our model constructs a
piecewise constant representation (effectively a histogram) of the distribution
of galaxy types and redshifts, the parameters of which are efficiently inferred
from noisy photometric flux measurements. This approach can be seen as a
generalization of template-fitting photometric redshift methods and relies on a
library of spectral templates to relate the photometric fluxes of individual
galaxies to their redshifts. We illustrate this technique on simulated galaxy
survey data, and demonstrate that it delivers correct posterior distributions
on the underlying type and redshift distributions, as well as on the individual
types and redshifts of galaxies. We show that even with uninformative priors,
large photometric errors and parameter degeneracies, the redshift and type
distributions can be recovered robustly thanks to the hierarchical nature of
the model, which is not possible with common photometric redshift estimation
techniques. As a result, redshift uncertainties can be fully propagated in
cosmological analyses for the first time, fulfilling an essential requirement
for the current and future generations of surveys.Comment: 10 pages, matches version accepted in MNRAS, including new appendix
describing the effect of Bayesian shrinkage in a simplified settin
Spurious correlations between galaxies and multi-epoch image stacks in the DESI Legacy Surveys
A non-negligible source of systematic bias in cosmological analyses of galaxy
surveys is the on-sky modulation caused by foregrounds and variable image
characteristics such as observing conditions. Standard mitigation techniques
perform a regression between the observed galaxy density field and sky maps of
the potential contaminants. Such maps are ad-hoc, lossy summaries of the
heterogeneous sets of co-added exposures that contribute to the survey. We
present a methodology to address this limitation, and extract the spurious
correlations between the observed distribution of galaxies and arbitrary stacks
of single-epoch exposures. We study four types of galaxies (LRGs, ELGs, QSOs,
LBGs) in the three regions of the DESI Legacy Surveys (North, South, DES),
which results in twelve samples with varying levels and type of contamination.
We find that the new technique outperforms the traditional ones in all cases,
and is able to remove higher levels of contamination. This paves the way for
new methods that extract more information from multi-epoch galaxy survey data
and mitigate large-scale biases more effectively.Comment: 18 pages, 8 figures. To be submitted to APJ
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