405 research outputs found
Redshift-space distortions with split densities
Accurate modelling of redshift-space distortions (RSD) is challenging in the
non-linear regime for two-point statistics e.g. the two-point correlation
function (2PCF). We take a different perspective to split the galaxy density
field according to the local density, and cross-correlate those densities with
the entire galaxy field. Using mock galaxies, we demonstrate that combining a
series of cross-correlation functions (CCFs) offers improvements over the 2PCF
as follows: 1. The distribution of peculiar velocities in each split density is
nearly Gaussian. This allows the Gaussian streaming model for RSD to perform
accurately within the statistical errors of a (Gpc) volume for
almost all scales and all split densities. 2. The PDF of the density field at
small scales is non-Gaussian, but the CCFs of split densities capture the
non-Gaussianity, leading to improved cosmological constraints over the 2PCF. We
can obtain unbiased constraints on the growth parameter at the
per-cent level, and Alcock-Paczynski (AP) parameters at the sub-per-cent level
with the minimal scale of . This is a 30 per cent
and 6 times improvement over the 2PCF, respectively. The diverse and
steep slopes of the CCFs at small scales are likely to be responsible for the
improved constraints of AP parameters. 3. Baryon acoustic oscillations (BAO)
are contained in all CCFs of split densities. Including BAO scales helps to
break the degeneracy between the line-of-sight and transverse AP parameters,
allowing independent constraints on them. We discuss and compare models for RSD
around spherical densities.Comment: 23 pages, 16 figures, MNRAS accepted version after peer review, minor
comment
Void Dynamics
Cosmic voids are becoming key players in testing the physics of our Universe.
Here we concentrate on the abundances and the dynamics of voids as these are
among the best candidates to provide information on cosmological parameters.
Cai, Padilla \& Li (2014) use the abundance of voids to tell apart Hu \&
Sawicki models from General Relativity. An interesting result is that
even though, as expected, voids in the dark matter field are emptier in
gravity due to the fifth force expelling away from the void centres, this
result is reversed when haloes are used to find voids. The abundance of voids
in this case becomes even lower in compared to GR for large voids.
Still, the differences are significant and this provides a way to tell apart
these models. The velocity field differences between and GR, on the
other hand, are the same for halo voids and for dark matter voids. Paz et al.
(2013), concentrate on the velocity profiles around voids. First they show the
necessity of four parameters to describe the density profiles around voids
given two distinct void populations, voids-in-voids and voids-in-clouds. This
profile is used to predict peculiar velocities around voids, and the
combination of the latter with void density profiles allows the construction of
model void-galaxy cross-correlation functions with redshift space distortions.
When these models are tuned to fit the measured correlation functions for voids
and galaxies in the Sloan Digital Sky Survey, small voids are found to be of
the void-in-cloud type, whereas larger ones are consistent with being
void-in-void. This is a novel result that is obtained directly from redshift
space data around voids. These profiles can be used to remove systematics on
void-galaxy Alcock-Pacinsky tests coming from redshift-space distortions.Comment: 8 pages, 4 figures, to appear in the proceedings of IAU308 Symposium
"The Zeldovich Universe
Modeling the environmental dependence of the growth rate of cosmic structure
International audienceThe growth rate of cosmic structure is a powerful cosmological probe for extracting information on the gravitational interactions and dark energy. In the late-time Universe, the growth rate becomes nonlinear and is usually probed by measuring the two-point statistics of galaxy clustering in redshift space up to a limited scale, retaining the constraint on the linear growth rate f. In this paper, we present an alternative method to analyze the growth of structure in terms of local densities, i.e., f(Δ). Using N-body simulations, we measure the function of f(Δ) and show that structure grows faster in high-density regions and slower in low-density regions. We demonstrate that f(Δ) can be modeled using a log-normal Monte Carlo random walk approach, which provides a means to extract cosmological information from f(Δ). We discuss prospects for applying this approach to galaxy surveys
Cosmology without cosmic variance
We examine the improvements in constraints on the linear growth factor G and
its derivative f=d ln G / dln a that are available from the combination of a
large-scale galaxy redshift survey with a weak gravitational lensing survey of
background sources. In the linear perturbation theory limit, the
bias-modulation method of McDonald & Seljak allows one to distinguish the
real-space galaxy clustering from the peculiar velocity signal in each Fourier
mode. The ratio of lensing signal to galaxy clustering in transverse modes
yields the bias factor b of each galaxy subset (as per Pen 2004), hence
calibrating the conversion from galaxy real-space density to matter density in
every mode. In combination these techniques permit measure of the growth rate f
in each Fourier mode. This yields a measure of the growth rate free of sample
variance, i.e. the uncertainty in f can be reduced without bound by increasing
the number of redshifts within a finite volume. In practice, the gain from the
absence of sample variance is bounded by the limited range of bias modulation
among dark-matter halos. Nonetheless, the addition of background weak lensing
data to a redshift survey increases information on G and f by an amount
equivalent to a 10-fold increase in the volume of a standard redshift-space
distortion measurement---if the lensing signal can be measured to sub-percent
accuracy. This argues that a combined lensing and redshift survey over a common
low-redshift volume is a more powerful test of general relativity than an
isolated redshift survey over larger volume at high redshift. An example case
is that a survey of ~10^6 redshifts over half the sky in the redshift range
can determine the growth exponent \gamma for the model
to an accuracy of , using only modes with
k<0.1h/Mpc, but only if a weak lensing survey is conducted in concert.
[Abridged]Comment: 9 pages, 3 figures, accepted by MNRAS, minor changes to match the
accepted versio
Galaxy clustering in the DESI Legacy Survey and its imprint on the CMB
We use data from the DESI Legacy Survey imaging to probe the galaxy density
field in tomographic slices covering the redshift range . After
careful consideration of completeness corrections and galactic cuts, we obtain
a sample of galaxies covering 17 739 deg. We derive
photometric redshifts with precision , and
compare with alternative estimates. Cross-correlation of the tomographic galaxy
maps with Planck maps of CMB temperature and lensing convergence probe the
growth of structure since . The signals are compared with a fiducial
Planck CDM model, and require an overall scaling in amplitude of
for the lensing cross-correlation and for the temperature cross-correlation, interpreted as the
Integrated Sachs-Wolfe effect. The ISW amplitude is consistent with the
fiducial CDM prediction, but lies significantly below the prediction
of the AvERA model of R\'acz et al. (2017), which has been proposed as an
alternative explanation for cosmic acceleration. Within CDM, our low
amplitude for the lensing cross-correlation requires a reduction either in
fluctuation normalization or in matter density compared to the Planck results,
so that . In combination with the total
amplitude of CMB lensing, this favours a shift mainly in density:
. We discuss the consistency of this figure with
alternative evidence. A conservative compromise between lensing and primary CMB
constraints would require , where the 95% confidence
regions of both probes overlap.Comment: 18 pages, 18 figures, revised to match the accepted version on MNRA
Probing the missing baryons with the Sunyaev-Zel'dovich effect from filaments
Observations of galaxies and galaxy clusters in the local universe can
account for only of the total baryon content. Cosmological
simulations predict that the `missing baryons' are spread throughout
filamentary structures in the cosmic web, forming a low-density gas with
temperatures of K. We search for this warm-hot intergalactic
medium (WHIM) by stacking the Planck Compton -parameter map of the thermal
Sunyaev-Zel'dovich (tSZ) effect for 1,002,334 pairs of CMASS galaxies from the
Sloan Digital Sky Survey. We model the contribution from the galaxy halo pairs
assuming spherical symmetry, finding a residual tSZ signal at the
2.9\mbox{\sigma} level from a stacked filament of length with a Compton parameter magnitude . We
consider possible sources of contamination and conclude that bound gas in
haloes may contribute only up to of the measured filamentary signal. To
estimate the filament gas properties we measure the gravitational lensing
signal for the same sample of galaxy pairs; in combination with the tSZ signal,
this yields an inferred gas density of with a temperature K. This result is consistent with the predicted WHIM properties, and
overall the filamentary gas can account for of the total baryon
content of the Universe. We also see evidence that the gas filament extends
beyond the galaxy pair. Averaging over this longer baseline boosts the
significance of the tSZ signal and increases the associated baryon content to
of the global value.Comment: 13 pages, 8 figures; accepted for publication in A&
Intergalactic filaments spin
Matter in the Universe is arranged in a cosmic web, with a filament of matter
typically connecting each neighbouring galaxy pair, separated by tens of
millions of light-years. A quadrupolar pattern of the spin field around
filaments is known to influence the spins of galaxies and haloes near them, but
it remains unknown whether filaments themselves spin. Here, we measure
dark-matter velocities around filaments in cosmological simulations, finding
that matter generally rotates around them, much faster than around a randomly
located axis. It also exhibits some coherence along the filament. The net
rotational component is comparable to, and often dominant over, the known
quadrupolar flow. The evidence of net rotations revises previous emphasis on a
quadrupolar spin field around filaments. The full picture of rotation in the
cosmic web is more complicated and multiscale than a network of spinning
filamentary rods, but we argue that filament rotation is substantial enough to
be an essential part of the picture. It is likely that the longest coherently
rotating objects in the Universe are filaments. Also, we speculate that this
rotation could provide a mechanism to generate or amplify intergalactic
magnetic fields in filaments.Comment: MNRAS, in press. Illustrative animation at
https://www.youtube.com/watch?v=h1-a-htHAx
Measuring cosmic filament spin with the kinetic Sunyaev-Zel'dovich effect
The spin of intergalactic filaments has been predicted from simulations, and
supported by tentative evidence from redshift-space filament shapes in a galaxy
redshift survey: generally, a filament is redshifted on one side of its axis,
and blueshifted on the other. Here, we investigate whether filament spins could
have a measurable kinetic Sunyaev-Zel'dovich (kSZ) signal, from CMB photons
being scattered by moving ionised gas; this pure velocity information is
complementary to filament redshift-space shapes. We propose to measure the kSZ
dipole by combining galaxy redshift surveys with CMB experiments. We base our
S/N analyses first on an existing filament catalogue, and its combination with
Planck data. We then investigate the detectability of the kSZ dipole using the
combination of DESI or SKA-2 with next-stage CMB experiments. We find that the
gas halos of filament galaxies co-rotating with filaments induce a stronger kSZ
dipole signal than that from the diffuse filamentary gas, but both signals seem
too small to be detected in near-term surveys such as DESI+future CMB
experiments. But the combination of SKA-2 with future CMB experiments could
give a more than 10 detection. The gain comes mainly from an increased
area overlap and an increased number of filaments, but also the low noise and
high resolution in future CMB experiments are important to capture signals from
filaments small on the sky. Successful detection of the signals may help to
find the gravitomagnetic effect in large-scale structure and advance our
understanding of baryons in the cosmic web.Comment: Minor revisions, MNRAS accepte
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