Cosmology

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:

Anomalous Microwave Emission (AME) is a significant component of Galactic diffuse emission in the frequency range $10$-$60\,$GHz and a new window into the properties of sub-nanometre-sized grains in the interstellar medium. We investigate the morphology of AME in the $\approx10^{\circ}$ diameter $\lambda$ Orionis ring by combining intensity data from the QUIJOTE experiment at $11$, $13$, $17$ and $19\,$GHz and the C-Band All Sky Survey (C-BASS) at $4.76\,$GHz, together with 19 ancillary datasets between $1.42$ and $3000\,$GHz. Maps of physical parameters at $1^{\circ}$ resolution are produced through Markov Chain Monte Carlo (MCMC) fits of spectral energy distributions (SEDs), approximating the AME component with a log-normal distribution. AME is detected in excess of $20\,\sigma$ at degree-scales around the entirety of the ring along photodissociation regions (PDRs), with three primary bright regions containing dark clouds. A radial decrease is observed in the AME peak frequency from $\approx35\,$GHz near the free-free region to $\approx21\,$GHz in the outer regions of the ring, which is the first detection of AME spectral variations across a single region. A strong correlation between AME peak frequency, emission measure and dust temperature is an indication for the dependence of the AME peak frequency on the local radiation field. The AME amplitude normalised by the optical depth is also strongly correlated with the radiation field, giving an overall picture consistent with spinning dust where the local radiation field plays a key role.

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.

Abstract:

Cosmic filaments are the channel through which galaxy groups assemble their mass. Cosmic connectivity, namely the number of filaments connected to a given group, is therefore expected to be an important ingredient in shaping group properties. The local connectivity is measured in COSMOS around X-Ray detected groups between redshift 0.5 and 1.2. To this end, large-scale filaments are extracted using the accurate photometric redshifts of the COSMOS2015 catalogue in two-dimensional slices of thickness 120 comoving Mpc centred on the group's redshift. The link between connectivity, group mass and the properties of the brightest group galaxy (BGG) is investigated. The same measurement is carried out on mocks extracted from the lightcone of the hydrodynamical simulation Horizon-AGN in order to control systematics. More massive groups are on average more connected. At fixed group mass in low-mass groups, BGG mass is slightly enhanced at high connectivity, while in high mass groups BGG mass is lower at higher connectivity. Groups with a star-forming BGG have on average a lower connectivity at given mass. From the analysis of the Horizon-AGN simulation, we postulate that different connectivities trace different paths of group mass assembly: at high group mass, groups with higher connectivity are more likely to have grown through a recent major merger, which might be in turn the reason for the quenching of the BGG. Future large-field photometric surveys, such as Euclid and LSST, will be able to confirm and extend these results by probing a wider mass range and a larger variety of environment.