68 research outputs found

### Galaxy 2-Point Covariance Matrix Estimation for Next Generation Surveys

We perform a detailed analysis of the covariance matrix of the spherically
averaged galaxy power spectrum and present a new, practical method for
estimating this within an arbitrary survey without the need for running mock
galaxy simulations that cover the full survey volume. The method uses
theoretical arguments to modify the covariance matrix measured from a set of
small-volume cubic galaxy simulations, which are computationally cheap to
produce compared to larger simulations and match the measured small-scale
galaxy clustering more accurately than is possible using theoretical modelling.
We include prescriptions to analytically account for the window function of the
survey, which convolves the measured covariance matrix in a non-trivial way. We
also present a new method to include the effects of supersample covariance and
modes outside the small simulation volume which requires no additional
simulations and still allows us to scale the covariance matrix. As validation,
we compare the covariance matrix estimated using our new method to that from a
brute force calculation using 500 simulations originally created for analysis
of the Sloan Digital Sky Survey Main Galaxy Sample (SDSS-MGS). We find
excellent agreement on all scales of interest for large scale structure
analysis, including those dominated by the effects of the survey window, and on
scales where theoretical models of the clustering normally break-down, but the
new method produces a covariance matrix with significantly better
signal-to-noise. Although only formally correct in real-space, we also discuss
how our method can be extended to incorporate the effects of Redshift Space
Distortions.Comment: 18 pages, 9 figures. Accepted for publication in MNRAS. Added new
references to introduction and slightly updated text accordingl

### Cosmology with Peculiar Velocities: Observational Effects

In this paper we investigate how observational effects could possibly bias
cosmological inferences from peculiar velocity measurements. Specifically, we
look at how bulk flow measurements are compared with theoretical predictions.
Usually bulk flow calculations try to approximate the flow that would occur in
a sphere around the observer. Using the Horizon Run 2 simulation we show that
the traditional methods for bulk flow estimation can overestimate the magnitude
of the bulk flow for two reasons: when the survey geometry is not spherical
(the data do not cover the whole sky), and when the observations undersample
the velocity distributions. Our results may explain why several bulk flow
measurements found bulk flow velocities that seem larger than those expected in
standard {\Lambda}CDM cosmologies. We recommend a different approach when
comparing bulk flows to cosmological models, in which the theoretical
prediction for each bulk flow measurement is calculated specifically for the
geometry and sampling rate of that survey. This means that bulk flow values
will not be comparable between surveys, but instead they are comparable with
cosmological models, which is the more important measure.Comment: 11 pages, 5 figures. Accepted for publication in MNRA

### L-PICOLA: A parallel code for fast dark matter simulation

Robust measurements based on current large-scale structure surveys require
precise knowledge of statistical and systematic errors. This can be obtained
from large numbers of realistic mock galaxy catalogues that mimic the observed
distribution of galaxies within the survey volume. To this end we present a
fast, distributed-memory, planar-parallel code, L-PICOLA, which can be used to
generate and evolve a set of initial conditions into a dark matter field much
faster than a full non-linear N-Body simulation. Additionally, L-PICOLA has the
ability to include primordial non-Gaussianity in the simulation and simulate
the past lightcone at run-time, with optional replication of the simulation
volume. Through comparisons to fully non-linear N-Body simulations we find that
our code can reproduce the $z=0$ power spectrum and reduced bispectrum of dark
matter to within 2% and 5% respectively on all scales of interest to
measurements of Baryon Acoustic Oscillations and Redshift Space Distortions,
but 3 orders of magnitude faster. The accuracy, speed and scalability of this
code, alongside the additional features we have implemented, make it extremely
useful for both current and next generation large-scale structure surveys.
L-PICOLA is publicly available at https://cullanhowlett.github.io/l-picolaComment: 22 Pages, 20 Figures. Accepted for publication in Astronomy and
Computin

### Cosmological Forecasts for Combined and Next Generation Peculiar Velocity Surveys

Peculiar velocity surveys present a very promising route to measuring the
growth rate of large-scale structure and its scale dependence. However,
individual peculiar velocity surveys suffer from large statistical errors due
to the intrinsic scatter in the relations used to infer a galaxy's true
distance. In this context we use a Fisher Matrix formalism to investigate the
statistical benefits of combining multiple peculiar velocity surveys. We find
that for all cases we consider there is a marked improvement on constraints on
the linear growth rate $f\sigma_{8}$. For example, the constraining power of
only a few peculiar velocity measurements is such that the addition of the
2MASS Tully-Fisher survey (containing only $\sim2,000$ galaxies) to the full
redshift and peculiar velocity samples of the 6-degree Field Galaxy Survey
(containing $\sim 110,000$ redshifts and $\sim 9,000$ velocities) can improve
growth rate constraints by $\sim20\%$. Furthermore, the combination of the
future TAIPAN and WALLABY+WNSHS surveys has the potential to reach a $\sim3\%$
error on $f\sigma_{8}$, which will place tight limits on possible extensions to
General Relativity. We then turn to look at potential systematics in growth
rate measurements that can arise due to incorrect calibration of the peculiar
velocity zero-point and from scale-dependent spatial and velocity bias. For
next generation surveys, we find that neglecting velocity bias in particular
has the potential to bias constraints on the growth rate by over $5\sigma$, but
that an offset in the zero-point has negligible impact on the velocity power
spectrum.Comment: 24 pages, 11 figures, 7 tables. Accepted for publication in MNRA

### Standard siren speeds: improving velocities in gravitational-wave measurements of H0

We re-analyse data from the gravitational-wave event GW170817 and its host galaxy NGC 4993 to demonstrate the importance of accurate total and peculiar velocities when measuring the Hubble constant using this nearby standard siren. We show that a number of reasonable choices can be made to estimate the velocities for this event, but that systematic differences remain between these measurements depending on the data used. This leads to significant changes in the Hubble constant inferred from GW170817. We present Bayesian model averaging as one way to account for these differences, and obtain H-0 = 66.8(-9.2)(+13.4) km s(-1)Mpc(-1). Adding additional information on the viewing angle from high-resolution imaging of the radio counterpart refines this to H-0 = 64.8(-7.2)(+7.3) km s(-1) Mpc(-1). During this analysis, we also present an alternative Bayesian model for the posterior on H-0 from standard sirens that works more closely with observed quantities from redshift and peculiar velocity surveys. Our results more accurately capture the true uncertainty on the total and peculiar velocities of NGC 4993 and show that exploring how well different data sets characterize galaxy groups and the velocity field in the local Universe could improve this measurement further. These considerations impact any low-redshift distance measurement, and the improvements we suggest here can also be applied to standard candles like Type Ia supernovae. GW170817 is particularly sensitive to peculiar velocity uncertainties because it is so close. For future standard siren measurements, the importance of this error will decrease as (i) we will measure more distant standard sirens and (ii) the random direction of peculiar velocities will average out with more detections

### Bulk flow and shear in the local Universe: 2MTF and COSMICFLOWS-3

The low-order kinematic moments of galaxies, namely bulk flow and shear,
enables us to test whether theoretical models can accurately describe the
evolution of the mass density field in the nearby Universe. We use the
so-called etaMLE maximum likelihood estimator in logdistance space to measure
thesemoments from a combined sample of the 2MASS Tully-Fisher (2MTF) survey and
the COSMICFLOWS-3 (CF3) compilation. Galaxies common between 2MTF and CF3
demonstrate a small zero-point difference of -0.016+-0.002 dex.We test the
etaMLE on 16 mock 2MTF survey catalogues in order to explore how well the
etaMLE recovers the true moments, and the effect of sample anisotropy. On the
scale size of 37 Mpc/h, we find that the bulk flow of the local Universe is 259
+- 15 km/h in the direction is (l,b)=(300+-4, 23+-3) (Galactic coordinates).
The average shear amplitude is 1.7+-0.4 h km/s/Mpc. We use a variable window
function to explore the bulk and shear moments as a function of depth. In all
cases, the measurements are consistent with the predictions of the L cold dark
matter (LCDM) model.Comment: 11 pages, 10+2 figures, published in MNRAS, Oct/201

### Constraining the growth rate of structure with phase correlations

We show that correlations between the phases of the galaxy density field in
redshift space provide additional information about the growth rate of
large-scale structure that is complementary to the power spectrum multipoles.
In particular, we consider the multipoles of the line correlation function
(LCF), which correlates phases between three collinear points, and use the
Fisher forecasting method to show that the LCF multipoles can break the
degeneracy between the measurement of the growth rate of structure $f$ and the
amplitude of perturbations $\sigma_8$ that is present in the power spectrum
multipoles at large scales. This leads to an improvement in the measurement of
$f$ and $\sigma_8$ by up to 220 per cent for $k_{\rm max} = 0.15 \,
h\mathrm{Mpc}^{-1}$ and up to 50 per cent for $k_{\rm max} = 0.30 \,
h\mathrm{Mpc}^{-1}$ at redshift $z=0.25$, with respect to power spectrum
measurements alone for the upcoming generation of galaxy surveys like DESI and
Euclid. The average improvements in the constraints on $f$ and $\sigma_8$ for
$k_{\rm max} = 0.15 \, h\mathrm{Mpc}^{-1}$ are $\sim 90$ per cent for the DESI
BGS sample with mean redshift $\overline{z}=0.25$, $\sim 40$ per cent for the
DESI ELG sample with $\overline{z}=1.25$, and $\sim 40$ per cent for the Euclid
H$\alpha$ galaxies with $\overline{z}=1.3$. For $k_{\rm max} = 0.30 \,
h\mathrm{Mpc}^{-1}$, the average improvements are $\sim 40$ per cent for the
DESI BGS sample and $\sim 20$ per cent for both the DESI ELG and Euclid
H$\alpha$ galaxies.Comment: 28 pages, 13 figures, 2 tables. v2 has additional discussion on
model-independence of the forecasts. v3 matches the MNRAS accepted versio

### Faster cosmological analysis with power spectrum without simulations

Future surveys could obtain tighter constraints for the cosmological
parameters with the galaxy power spectrum than with the Cosmic Microwave
Background. However, the inclusion of multiple overlapping tracers, redshift
bins, and more non-linear scales means that generating the necessary ensemble
of simulations for model-fitting presents a computational burden. In this work,
we combine full-shape fitting of galaxy power spectra, analytical covariance
matrix estimates, and the MOPED compression for the first time to constrain the
cosmological parameters directly from a state-of-the-art set of galaxy
clustering measurements. We find it takes less than a day to compute the
analytical covariance and compression matrices needed for this analysis while
it takes several months to calculate the simulated ones. Additionally, the
MOPED compression reduces the bias in the covariance matrix and speeds up the
likelihood analysis. In combination, we find that even without a priori
knowledge of the best-fit cosmological or galaxy bias parameters, the
analytical covariance matrix with the MOPED compression still gives
cosmological constraints consistent, to within $0.2\sigma$, with the ones
obtained using the simulated covariance matrices. The pipeline we have
developed here can hence significantly speed up the analysis for future surveys
such as DESI and Euclid.Comment: 11 pages, 4 figures, and 1 table. To be submitted to MNRAS. Comments
are welcom

### Can Einstein (rings) surf Gravitational Waves?

How does the appearance of a strongly lensed system change if a gravitational
wave is produced by the lens? In this work we address this question by
considering a supermassive black hole binary at the center of the lens emitting
gravitational waves propagating either colinearly or orthogonally to the line
of sight. Specializing to an Einstein ring configuration (where the source, the
lens and the observer are aligned), we show that the gravitational wave induces
changes on the ring's angular size and on the optical path of photons. The
changes are the same for a given pair of antipodal points on the ring, but
maximally different for any pair separated by $90^{\circ}$. For realistic
lenses and binaries, we find that the change in the angular size of the
Einstein ring is dozens of orders of magnitude smaller than the precision of
current experiments. On the other hand, the difference in the optical path
induced on a photon by a gravitational wave propagating \textit{orthogonally}
to the line of sight triggers, at peak strain, time delays in the range $\sim
0.01 - 1$ seconds, making the chance of their detection (and thus the use of
Einstein rings as gravitational wave detectors) less hopeless.Comment: v2. Version accepted for publication in the Open Journal of
Astrophysics. 8 pages, four figures. Comments are welcome

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