2,296 research outputs found
Demographics of dark-matter haloes in standard and non-standard cosmologies
This thesis explores topics related to the formation and development of the large-scale
structure in the Universe, with the focus being to compute properties of the
evolved non-linear density field in an approximate way. The first three chapters form an
introduction: Chapter 1 contains the theoretical basis of modern cosmology, Chapter
2 discusses the role of N-body simulations in the study of structure formation and
Chapter 3 considers the phenomenological halo model.
In Chapter 4 a novel method of computing the matter power spectrum is developed.
This method uses the halo model directly to make accurate predictions for the matter
spectrum. This is achieved by fitting parameters of the model to spectra from accurate
simulations. The final predictions are good to 5% up to k = 10 hMpc-1 across a range
of cosmological models at z = 0, however accuracy degrades at higher redshift and at
quasi-linear scales.
Chapter 5 is dedicated to a new method of rescaling a halo catalogue that has previously
been generated from a simulation of a specific cosmological model to a different
model; a gross rescaling of the simulation box size and redshift label takes place, then
individual halo positions are modified in accord with the large scale displacement field
and their internal structure is altered. The final power spectrum of haloes can be
matched at the 5% level up to k = 1 hMpc-1, as can the spectrum of particles within
haloes reconstituted directly from the rescaled catalogues.
Chapter 6 applies the methods of the previous two chapters to modified gravity
models. This is done in as general a way possible but tests are restricted to f(R) type
models, which have a scale-dependent linear growth rate as well as having 'chameleon
screening' - by which modifications to gravity are screened within some haloes. Taking
these effects into account leads to predictions of the matter spectrum at the 5% level
and rescaled halo distributions that are accurate to 5% in both real and redshift space.
For the spectrum of halo particles it is demonstrated that accurate results may be
obtained by taking the enhanced gravity in some haloes into account
Perturbative Gaussianizing transforms for cosmological fields
Constraints on cosmological parameters from large-scale structure have
traditionally been obtained from two-point statistics. However, non-linear
structure formation renders these statistics insufficient in capturing the full
information content available, necessitating the measurement of higher-order
moments to recover information which would otherwise be lost. We construct
quantities based on non-linear and non-local transformations of weakly
non-Gaussian fields that Gaussianize the full multivariate distribution at a
given order in perturbation theory. Our approach does not require a model of
the fields themselves and takes as input only the first few polyspectra, which
could be modelled or measured from simulations or data, making our method
particularly suited to observables lacking a robust perturbative description
such as the weak-lensing shear. We apply our method to simulated density
fields, finding a significantly reduced bispectrum and an enhanced correlation
with the initial field. We demonstrate that our method reconstructs a large
proportion of the linear baryon acoustic oscillations, improving the
information content over the raw field by 35%. We apply the transform to toy
21cm intensity maps, showing that our method still performs well in the
presence of complications such as redshift-space distortions, beam smoothing,
pixel noise, and foreground subtraction. We discuss how this method might
provide a route to constructing a perturbative model of the fully non-Gaussian
multivariate likelihood function.Comment: 15 pages, 10 figures. Minor changes to match version published in
MNRA
Magnetic fields in galaxies: I. Radio disks in local late-type galaxies
We develop an analytical model to follow the cosmological evolution of
magnetic fields in disk galaxies. Our assumption is that fields are amplified
from a small seed field via magnetohydrodynamical (MHD) turbulence. We further
assume that this process is fast compared to other relevant timescales, and
occurs principally in the cold disk gas. We follow the turbulent energy density
using the Shabala & Alexander (2009) galaxy formation and evolution model.
Three processes are important to the turbulent energy budget: infall of cool
gas onto the disk and supernova feedback increase the turbulence; while star
formation removes gas and hence turbulent energy from the cold gas. Finally, we
assume that field energy is continuously transferred from the incoherent random
field into an ordered field by differential galactic rotation. Model
predictions are compared with observations of local late type galaxies by Fitt
& Alexander (1993) and Shabala et al. (2008). The model reproduces observed
magnetic field strengths and luminosities in low and intermediate-mass
galaxies. These quantities are overpredicted in the most massive hosts,
suggesting that inclusion of gas ejection by powerful AGNs is necessary in
order to quench gas cooling and reconcile the predicted and observed magnetic
field strengths.Comment: 10 pages, 8 figures; MNRAS in pres
On constraining Cosmology and the Halo Mass Function with Weak Gravitational Lensing
The discrepancy between the weak lensing (WL) and the {\it Planck}
measurements of has been a subject of several studies. These studies tend
to show that a suppression of the amplitude of the mass power spectrum
at high could resolve it. The WL signal at small-scale is sensitive to
various effects, such as baryonic effects and intrinsic alignment. The accuracy
of depends on the modelling precision of these effects. A common
approach for calculating relies on a halo model. Amongst the various
components necessary for the construction of , the halo mass function
(HMF) is an important one. Traditionally, the HMF has been assumed to follow a
fixed model. Recent literature shows that baryonic physics, amongst several
other factors, could affect the HMF. In this study, we investigate the impact
of allowing the HMF to vary. This provides a way of testing the validity of the
halo model-HMF calibration using data. We find that the {\it Planck} cosmology
is not compatible with the vanilla HMF for both the DES-y3 and the KiDS-1000
data. When the cosmology and the HMF parameters are allowed to vary, the {\it
Planck} cosmology is no longer in tension. The modified HMF predicts a matter
power spectrum with a power loss at , in
agreement with the recent studies. We show that Stage IV surveys will be able
to measure the HMF parameters with a few percent accuracy.Comment: 16 pages (including appendixes), 10 figures, 3 tables, main results
in Figs. 5&
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