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In this thesis I present a novel method for constructing large scale mock galaxy and halo catalogues and apply this model to a number of important topics in modern cosmology. Traditionally such mocks are created through first evolving a high resolution particle simulation from a set of initial conditions to the present epoch, identifying bound structures and their evolution, and finally applying a semi-analytic prescription for galaxy formation. In contrast to this computationally expensive procedure, I use low resolution simulations to obtain a density field that traces large scale modes. From this background I sample the population statistics of halos: the number of halos which are typically found within a region of a given overdensity, to produce a halo catalogue. From the halo catalogue I then produce galaxies by appealing to the halo model. In this model the expected number of galaxies within a halo and the distribution of their properties is dependent on halo mass alone. By sampling conditional luminosity functions for a number of populations of galaxies, I produce a galaxy catalogue with luminosity and colour properties. The aim of developing algorithm is not to probe the mechanics of galaxy formation in great detail. It is instead intended as a method of rapidly producing mock galaxy and halo catalogues rapidly on modern desktop computers. The approach we will take is to try to distill the minimal algorithm required to achieve this and still provide useful catalogues for observational cosmologists. Both the conditional mass function and conditional luminosity functions required for the algorithm are calibrated from the Millennium Simulation, one of the highest resolution cosmology simulations to date, and its associated semi-analytic catalogues. In Chapter 2 I examine these statistics and provide fits to the quantities of interest. As a test of the method, in Chapter 3 I produce a halo and galaxy catalogue from the same large scale modes as the Millennium Simulation. The clustering statistics of galaxies and halos within this re-simulation are calculated and compared with those of the original. Con dent of the accuracy of the method, in Chapter 4 I populate a number of simulations, each 8 times the volume of the Millennium Simulation, and study the evolution of the Baryon Acoustic Oscillation signal. For each population (dark matter, halos and galaxies) I fit the BAO in the power spectrum to obtain the shift in the BAO peak. In Chapter 5 I extend the algorithm to produce lightcones: simulated skies in which the evolution of the Universe along the line of sight is accounted for. I simulate the geometry and limitations of a major pending survey and calculate the expected clustering signature I expect to see in both. The redshift space distortions induced by peculiar velocities of galaxies along the line of sight are determined and their ability to distinguish between gravity models is also explored. In Chapter 6 I detail a further extension to the algorithm for simulating weak gravitational lensing surveys. I use the analytic 2D surface density pro files of NFW profiles to dress each dark matter halo on a lightcone. The sum of these pro files over the entire population can be used to construct high resolution maps of the convergence. From these maps I calculate the spectrum of the convergence and compare with theoretical predictions. Finally in Chapter 7 I discuss further possible applications and extensions of the algorithm I have developed in this thesis.

>Magister Scientiae - MSc / Next-generation surveys of the large-scale structure of the Universe will be of great importance in allowing us to extract invaluable information about the nature of the Universe and the physical laws that govern it, at a higher precision than previously possible. In particular, they will allow us to more closely study primordial non-Gaussianity, a feature which leaves an imprint on the power spectrum of galaxies on the ultra-large scales and which acts as a powerful probe of the physics of the early Universe. To investigate the extent to which upcoming surveys will be able to improve our knowledge of primordial non-Gaussianity, we perform a forecast to predict the observational constraints on local-type primordial non-Gaussianity, as well as an extension that includes a scale dependence. We study the constraining power of a multi-tracer approach, where information from different surveys is combined to help suppress cosmic variance and break parameter degeneracies. More specifically, we consider the combination of a 21cm intensity mapping survey with each of two different photometric galaxy surveys, and also examine the effect of including CMB lensing as an additional probe. The forecast constraint from a combination of SKA1, a Euclid-like (LSST-like) survey and a CMB Stage 4 lensing experiment is (fNL) ' 0:9 (1:4) which displays a factor of 2 improvement over the case without CMB lensing, indicating that the surveys considered are indeed complementary. The constraints on the running index of the scale-dependent model are forecast as (nNL) ' 0:12 (0:22) from the same combination of surveys.

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Galaxy redshift surveys are powerful probes of cosmology. Yet, in order to fully exploit the information contained in galaxy surveys, we need to improve upon our understanding of the structure formation in the Universe. Galaxies are formed/observed at late times when the density field is no longer linear so that understanding non-linearities is essential. In this thesis, we show that, at high redshifts, we can accurately model the galaxy power spectrum in redshift space by using the standard cosmological perturbation theory. Going beyond the power spectrum, we can use the three-point function, or the bispectrum, to gain important information on the early universe as well as on the galaxy formation via measurements of primordial non-Gaussianity and galaxy bias. We show that the galaxy bispectrum is more sensitive to primordial non-Gaussianities than previously recognized, making high-redshift galaxy surveys a particularly potent probe of the physics of inflation. Weak lensing offers yet another way of probing cosmology. By cross correlating the angular position of galaxies with the shear measurement from galaxy lensing or CMB lensing, we also show that one can obtain the information on cosmological distance scale, the galaxy bias, and the primordial non Gaussianity from weak lensing method. / text

Within the currently well-established ΛCDM cosmological framework we still lack a satisfactory un- derstanding of the processes that trigger, regulate and eventually quench star formation on galactic scales. Gas flows (including inflows from the cosmic web and supernovae-driven outflows) are con- sidered to act as self-regulatory mechanisms, generating the scaling relations between stellar mass, star formation rate and metallicity observed in the local Universe by large spectroscopic surveys. These surveys, however, have so far been limited by the availability of only one spectrum per galaxy. The aim of this dissertation is to expand the study of star formation and chemical abundances to resolved scales within galaxies by using integral field spectroscopy (IFS) data, mostly from the ongoing SDSS- IV MaNGA survey. In the first part of this thesis I demonstrate the ubiquitous presence of extended low ionisation emission-line regions (LIERs) in both late- and early-type galaxies. By studying the Hα equivalent width and diagnostic line ratios radial profiles, together with tracers of the underlying stellar popula- tion, I show that LIERs are not due to a central point source but to hot evolved (post-asymptotic giant branch) stars. In light of this, I suggest a new classification scheme for galaxies based on their line emission. By analysing the colours, star formation rates, morphologies, gas and stellar kinematics and environmental properties of galaxies with substantial LIER emission, I identify two distinct popula- tions. Galaxies where the central regions are LIER-like, but show star formation at larger radii are late types in which star formation is slowly quenched inside-out. This transformation is associated with massive bulges. Galaxies dominated by LIER emission at all radii, on the other hand, are red-sequence galaxies harbouring a residual cold gas component, acquired mostly via external accretion. Quiescent galaxies devoid of line emission reside in denser environments, which suggests environmental effects as a likely cause for the existence of line-less galaxies on the red sequence. In the second part of this dissertation I focus on the study of resolved chemical abundances by characterising the gas phase oxygen and nitrogen abundance gradients in a large sample of star forming galaxies. I analyse the deviations from an exponential profile at small and large radii and the dependence of the gradients on stellar mass. These findings are interpreted in the context of the inside-out paradigm of disc growth. I then demonstrate the necessity of gas flows, which are responsible for the observed flattening of the metallicity and N/O ratio gradients at large radii. Finally, I present a case study based on one nearby galaxy (NGC 628), in which I combine IFS and cold gas data to derive a spatially resolved metal budget and estimate the mass of metals lost by the galaxy throughout its life- time. By using simple physically-motivated models of chemical evolution I infer the average outflow loading factor to be of order unity.

abstract: Ultraviolet and optical light from stars is reddened and attenuated by interstellar dust, where different sightlines across a galaxy suffer varying amounts of extinction. Tamura et al. (2009) developed an approximate method to correct for dust extinction, dubbed the “βV method,” by comparing the observed to an empirical estimate of the intrinsic flux ratio of visible and ∼3.5 μm emission. Moving beyond that empirical approach, through extensive modeling, I calibrated the βV -method for various filters spanning the visible through near infrared wavelength range, for a wide variety of simple stellar populations (SSP) and composite stellar populations (CSP). Combining Starburst99 and BC03 models, I built spectral energy distributions of SSP and CSP for various realistic star formation histories, while taking metallicity evolution into account. I convolved various 0.44–1.65 μm filter throughput curves with each model spectral energy distribution (SED) to obtain intrinsic flux ratios, βλ,0. To validate the modeling, I analyzed spatially resolved maps for the observed V- and g-band to 3.6 μm flux ratios and the inferred dust-extinction values AV for a sample of 257 nearby galaxies. Flux ratio maps are constructed using point-spread function-matched mosaics of Sloan Digitial Sky Survey g- and r-band images and Spitzer/InfraRed Array Camera 3.6μm mosaics, with all of the pixels contaminated by foreground stars or background objects masked out. Dust-extinction maps for each galaxy were created by applying the βV -method. The typical 1σ scatter in βV around the average, both within a galaxy and in each morphological type bin, is ∼20%. Combined, these result in a ∼0.4 mag scatter in AV. βV becomes insensitive to small-scale variations in stellar populations once resolution elements subtend an area larger than 10 times that of a typical giant molecular cloud. I find noticeably redder V−3.6 μm colors in the center of star-forming galaxies and galaxies with a weak AGN. The derived intrinsic V −3.6 μm colors for each Hubble type are generally consistent with the modeling. Finally, I discuss the applicability of the βV dust-correction method to more distant galaxies, for which large samples of well-matched Hubble Space Telescope rest-frame visible and James Webb Space Telescope rest-frame ∼3.5μm images will become available in the near future. / Dissertation/Thesis / Doctoral Dissertation Astrophysics 2019

I have investigated the properties of the large scale structure of the nearby Universe using data from the Galaxy and Mass Assembly survey (GAMA). I generated complementary halo mass estimates for all groups in the GAMA Galaxy Group Catalogue (G³C) using a modified caustic mass estimation algorithm. On average, the caustic mass estimates agree with dynamical mass estimates within a factor of 2 in 90% of groups. A volume limited sample of these groups and galaxies are used to generate the large scale structure catalogue. An adapted minimal spanning tree algorithm is used to identify and classify structures, detecting 643 filaments that measure up to 200 Mpc/h, each containing 8 groups on average. A secondary population of smaller coherent structures, dubbed `tendrils,' that link filaments together or penetrate into voids are also detected. On average, tendrils measure around 10 Mpc/h and contain 6 galaxies. The so-called line correlation function is used to prove that tendrils are real structures rather than accidental alignments. A population of isolated void galaxies are also identified. The properties of filaments and tendrils in observed and mock GAMA galaxy catalogues agree well. I go on to show that voids from other surveys that overlap with GAMA regions contain a large number of galaxies, primarily belonging to tendrils. This implies that void sizes are strongly dependent on the number density and sensitivity limits of the galaxies observed by a survey. Finally, I examine the properties of galaxies in different environments, finding that galaxies in filaments tend to be early-type, bright, spheroidal, and red whilst those in voids are typically the opposite: blue, late-type, and more faint. I show that group mass does not correlate with the brightness and morphologies of galaxies and that the primary driver of galaxy evolution is stellar mass.

Early Universe physics leaves distinct imprints on the Cosmic Microwave Background (CMB) and Large-Scale Structure (LSS). The current cosmological paradigm to explain the origin of the structures we see in the Universe today (CMB and LSS), named Inflation, says that the Universe went through a period of accelerated expansion. Density fluctuations that eventually have grown into the temperature fluctuations of the CMB and the galaxies and other structures we see in the LSS come from the quantization of the scalar field (inflaton) which provokes the accelerated expansion. The most simple inflationary model, which contains only one slowly-rolling scalar field with canonical kinetic term in the action, produces a power-spectrum (Fourier transform of the two-point correlation function) approximately scale invariant and an almost null bispectrum (Fourier transform of the three-point correlation function). This characteristic is called Gaussianity, once random fields that follow a normal distribution have all the odd moments null. Yet, more complex inflationary models (with more scalar fields and/or non-trivial kinetic terms in the action, etc) and possible alternatives to inflation have a non-vanishing bispectrum which can be parametrized by a non-linearity parameter f_NL, whose value differs from model to model. In this work we studied the basic ingredients to understand such statements and focused on the observational evidences of this parameters and how the current and upcoming galaxy surveys are able to impose constraints to the value of f_NL with a better accuracy, through the multi-tracer technique, than those obtained by means of CMB measurements. / A física do Universo primordial deixa sinais distintos na Radiação Cósmica de Fundo (CMB) e Estrutura em Larga Escala (LSS). O paradigma atual da cosmologia explica a origem das estruturas que vemos hoje (CMB e LSS) através da inflação, teoria que diz que o Universo passou por um período de expansão acelerada. As flutuações de densidade que eventualmente crescem, dando origem às flutuações de temperatura da CMB, às galáxias e outras estruturas que vemos na LSS, provém da quantização do campo escalar (inflaton) que provoca a tal expansão acelerada. O modelo inflacionário mais simples, o qual contém um único campo escalar nas condições de rolamento lento e termo cinético canônico da ação, possui o espectro de potências (transformada de Fourier da função de correlação de dois pontos) aproximadamente invariante de escala e o bispectro (transformada de Fourier da função de correlação de três pontos) aproximadamente nulo. Tal característica é conhecida por Gaussianidade, uma vez que campos aleatórios cuja distribuição é uma normal tem todas as funções de correlação de ordem ímpar nulas. Contudo, modelos inflacionários mais complexos (mais campos escalares, termos cinéticos não-triviais na ação, etc) e alternativas possíveis à inflação possuem um bispectro não nulo, o qual pode ser parametrizado através do parâmetro de não-linearidade f_NL, cujo valor difere de modelo para modelo. Neste trabalho estudamos os ingredientes básicos para entender tais afirmações e focamos nas evidências observacionais desse parâmetro e como os levantamentos de galáxias atuais e futuros podem impor restrições ao valor de f_NL com uma precisão maior, através da técnica de múltiplos traçadores, do que aquelas obtidas com medidas da CMB.