Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

The large-scale structure of the Universe encodes invaluable information about the fundamental cosmological parameters, the physics of structure formation, and the thermodynamic history of the Universe. In this thesis, we explore how secondary anisotropies of the Cosmic Microwave Background (CMB), particularly the thermal Sunyaev-Zeldovich (tSZ) effect and CMB lensing, can improve constraints on the large-scale structure. We develop a framework to cross-correlate tSZ maps from the Planck satellite with the distribution of galaxies at low redshift using tomographic bins with data from the 2MASS Photometric Redshift catalogue and WISE x SuperCOSMOS. These cross-correlations enable precise measurements of the bias-weighted gas pressure, ⟨𝑏P_e⟩, and the hydrostatic mass bias parameter, 1−b_H, as a function of redshift.

This thesis is primarily based on two complementary studies employing galaxy clustering (𝛿_𝑔 × 𝛿_𝑔), galaxy-tSZ cross-correlations (𝛿𝑔 × y), and galaxy-CMB lensing cross-correlations (𝛿𝑔 × K) to constrain cosmological and thermodynamic parameters across six redshift bins (𝑧 ∈ [0.1, 0.6]).

In the first study, we use a combination of 𝛿_𝑔 × 𝛿_𝑔 and 𝛿_𝑔 × y to improve constraints on the thermal history of the Universe. We achieve ~6 \% precision on 1-b_H across the six redshift bins, finding consistency with previous results and no evidence for significant redshift dependence. Our best-fit value of 1−𝑏_H = 0.75 ± 0.03 aligns well with joint analyses of Planck cluster counts and CMB anisotropies calibrated with CMB lensing. Additionally, our constraints on ⟨𝑏𝑃_𝑒⟩, accurate to ~10% per bin, represent the most precise measurements to date, providing a robust test of baryonic feedback and models of energy injection.

The second study incorporatess 𝛿_𝑔 × K  to enhance our tomographic analysis of structure growth and gas thermodynamics. Using CMB lensing as an additional tracer of large-scale structure, we constrain the amplitude of fluctuations of the matter power spectrum, 𝜎8, to 6% across all redshift bins, the hydrostatic mass bias, 1 - b_H to ~ 18 %, the bias-weighted average electron pressure, ⟨𝑏𝑃_𝑒⟩, to ~12%, the thermal energy density, Ω_th  to ~ 10%, as well as T_AGN, a single parameter quantifying the intensive thermodynamic properties of haloes. We perform multiple robustness checks to verify the stability of our models, and we report broad agreement with previous results in the literature.

This work builds on the use of secondary CMB anisotropies as a probe of non-linear physics, and of the interplay between dark matter and baryonic matter in haloes. We incorporate novel methods for combining tSZ data with other probes, in an attempt to refine models of halo bias and constrain 𝜎8. The inclusion of 𝛿𝑔 × K helps to break degeneracies between key parameters of the theoretical framework used. Hence, we highlight the importance of secondary CMB anisotropies as a complementary tool for understanding the large-scale structure, offering new insights into the thermal evolution of the Universe, as well as the growth of structure.

Abstract:

Abstract:

Our aim is to quantify the impact of systematic effects on the inference of cosmological parameters from cosmic shear. We present an end-to-end approach that introduces sources of bias in a modelled weak lensing survey on a galaxy-by-galaxy level. Residual biases are propagated through a pipeline from galaxy properties (one end) through to cosmic shear power spectra and cosmological parameter estimates (the other end), to quantify how imperfect knowledge of the pipeline changes the maximum likelihood values of dark energy parameters. We quantify the impact of an imperfect correction for charge transfer inefficiency (CTI) and modelling uncertainties of the point spread function (PSF) for Euclid, and find that the biases introduced can be corrected to acceptable levels.

Abstract:

Ever since the thick disk was proposed to explain the vertical distribution of the Milky Way disk stars, its origin has been a recurrent question. We aim to answer this question by inspecting 19 disk galaxies with stellar mass greater than $10^{10}\,\rm M_\odot$ in recent cosmological high-resolution zoom-in simulations: Galactica and NewHorizon. The thin and thick disks are reproduced by the simulations with scale heights and luminosity ratios that are in reasonable agreement with observations. When we spatially classify the disk stars into thin and thick disks by their heights from the galactic plane, the "thick" disk stars are older, less metal-rich, kinematically hotter, and higher in accreted star fraction than the "thin" disk counterparts. However, both disks are dominated by stellar particles formed in situ. We find that approximately half of the in-situ stars in the thick disks are formed even before the galaxies develop their disks, and the other half are formed in spatially and kinematically thinner disks and then thickened with time by heating. We thus conclude from our simulations that the thin and thick disk components are not entirely distinct in terms of formation processes, but rather markers of the evolution of galactic disks. Moreover, as the combined result of the thickening of the existing disk stars and the continued formation of young thin-disk stars, the vertical distribution of stars does not change much after the disks settle, pointing to the modulation of both orbital diffusion and star formation by the same confounding factor: the proximity of galaxies to marginal stability.

Abstract:

Galactic-scale tests have proven to be powerful tools in constraining fundamental physics in previously under-explored regions of parameter space. In this thesis we use astrophysical systems to test some of the fundamental principles governing our current theories of the Universe, through the development of source-by-source, Monte Carlo-based forward models.

We consider modifications to the propagation of light by one of three effects: quantum gravity (QG), a non-zero photon mass and a violation of the Weak Equivalence Principle (WEP). We use spectral lag data of Gamma Ray Bursts from the BATSE satellite to constrain the photon mass to be $m_\gamma < 4.0 \times 10^{-5} \, h \, {\rm eV}/c^2$ and the QG length scale to be $\ell_{\rm QG} < 5.3 \times 10^{-18} \, h \, {\rm \, GeV^{-1}}$ at 95\% confidence, WEP to $\Delta \gamma < 2.1 \times 10^{-15}$ at $1 \sigma$ confidence between photon energies of $25 {\rm \, keV}$ and $325 {\rm \, keV}$, and we demonstrate that these constraints are robust to how one models other contributions to the signal.

We investigate Galileon modified gravity theories by studying the offsets between the centre of a galaxy and its host supermassive black hole (BH). We constrain the Galileon coupling to be $\Delta G / G_{\rm N} < 0.16$ at $1\sigma$ confidence for Galileons with crossover scale $r_{\rm C} \gtrsim H_0^{-1}$. Inspired by the aforementioned test of modified gravity, we study spatially offset BHs in the Horizon-AGN simulation and compare these to observations, finding i) the fraction of spatially offset BHs increases with cosmic time, ii) BHs live on prograde orbits in the plane of the galaxy with an orbital radius that decays with time but stalls near $z=0$, and iii) the magnitudes of offsets from the galaxy centres are substantially larger in the simulation than in observations.

By cross-correlating dark matter density fields inferred from the spatial distribution of galaxies with gamma ray data from the \textit{Fermi} Large Area Telescope, marginalising over uncertainties in this reconstruction, small-scale structure and parameters describing astrophysical contributions to the observed gamma ray sky, we place constraints on the dark matter annihilation cross-sections and decay rates. We rule out the thermal relic cross-section or $s$-wave annihilation for all $m_\chi \lesssim 7 {\rm \, GeV}/c^2$ at 95\% confidence if the annihilation produces $Z$ bosons, gluons or quarks less massive than the bottom quark. We infer a contribution to the gamma ray sky with the same spatial distribution as dark matter decay at $3.3\sigma$. Although this could be due to dark matter decay via these channels with a decay rate $\Gamma \approx 3 \times 10^{-28} {\rm \, s^{-1}}$, we find that a power-law spectrum of index $p=-2.75^{+0.71}_{-0.46}$ is preferred by the data.

Finally, we outline a framework for assessing the reliability of the methods used in this thesis by constructing and testing more advanced models using cosmological hydrodynamical simulations. As a case study, we use the Horizon-AGN simulation to investigate warping of stellar disks and offsets between gas and stars within galaxies, which are powerful probes of screened fifth-forces.