Galaxy formation and evolution

Abstract:

In this thesis, I present results on the statistical properties of luminous star-forming galaxies in the early Universe, spanning the first two billion years of cosmic time. I combine deep, degree-scale optical and near-infrared imaging from the latest ground-based surveys with next-generation observatories - Euclid, which provides deep, degree-scale space-based near-infrared imaging for the first time, and JWST, which delivers unrivalled depth and resolution in the near-infrared and infrared. With this unique combination, I place strong constraints on the number densities and size-scaling relations of the first galaxies.

First, by combining imaging from the VISTA telescope with deep ground-based optical surveys, infrared imaging from Spitzer/IRAC, and early data from Euclid, I construct a sample of galaxy candidates at redshift 6.5 < z < 7.5 spanning a rest-UV absolute magnitude range of −23.5 ≤ M_uv ≤ −20.2. These sources represent some of the most luminous and massive galaxies at this epoch. After accounting for brown dwarf contamination through a careful SED-fitting analysis, I find that the rest-frame UV luminosity function at z ≃ 7 is best described by double-power law, showing an excess relative to a Schechter function at absolute rest-UV magnitudes M_uv ≲ −22.5 and evolving slowly from z ≃ 8. This suggests that luminous galaxies at this epoch are not yet significantly affected by dust obscuration or mass quenching, and that active galactic nuclei do not contribute significantly to the luminosity function until very bright magnitudes (M_uv < −24).

I then measure the size-scaling relations of 1,668 luminous galaxies at z ≃ 3 − 5 using the JWST PRIMER survey. These sources were selected from ground-based, seeing-dominated imaging, presenting an unbiased sampling of the morphology and size distributions of luminous sources. I find a build-up of large (R_e > 2 kpc) galaxies at z = 3 relative to z = 4 − 5, a redshift-dependent size evolution leading to larger mean sizes at z = 3, and an increase in the intrinsic scatter of the size-mass relations towards lower redshift. These results suggest that by z = 3, some galaxies have undergone dissipative processes such as mergers and gas accretion, allowing for the formation of rare, larger galaxies. However, the majority of galaxies remain compact over this redshift range, with a typical (modal) size of R_e = 0.7 − 0.9 kpc. Finally, I find that the size-mass and size-luminosity relations are consistent with predictions from simulations such as Illustris and FLARES, providing evidence for centrally concentrated star formation in the most massive galaxies at high redshift.

Abstract:

Accurate cosmology from upcoming weak lensing surveys relies on knowledge of the total matter power spectrum at percent level at scales $k < 10$ $h$/Mpc, for which modelling the impact of baryonic physics is crucial. We compare measurements of the total matter power spectrum from the Horizon cosmological hydrodynamical simulations: a dark matter-only run, one with full baryonic physics, and another lacking Active Galactic Nuclei (AGN) feedback. Baryons cause a suppression of power at $k\simeq 10$ $h/$Mpc of $<15\%$ at $z=0$, and an enhancement of a factor of a few at smaller scales due to the more efficient cooling and star formation. The results are sensitive to the presence of the highest mass haloes in the simulation and the distribution of dark matter is also impacted up to a few percent. The redshift evolution of the effect is non-monotonic throughout $z=0-5$ due to an interplay between AGN feedback and gas pressure, and the growth of structure. We investigate the effectiveness of the "baryonic correction model" proposed by Schneider & Teyssier (2015) in describing our results. We require a different redshift evolution and propose an alternative fitting function with $4$ free parameters that reproduces our results within $5\%$. Compared to other simulations, we find the impact of baryonic processes on the total matter power spectrum to be smaller at $z=0$. Nevertheless, our results also suggest that AGN feedback is not strong enough in the simulation. Total matter power spectra from the Horizon simulations are made publicly available at https://www.horizon-simulation.org/catalogues.html.

Abstract:

We present a comprehensive theoretical study, within a fully realistic semi-analytical framework, of the photometric properties of early-type progenitors in the redshift range 00.7) spirals have ~75-95 percent chance of being a progenitor, while the corresponding probability for large blue spirals (M_B<-21.5, B-V<0.7) is ~50-75 percent. Finally, we explore the correspondence between the true progenitor set of present-day early-types and the commonly used `red-sequence', defined as the set of galaxies within the part of the colour-magnitude space which is dominated by early-type objects. While large members (M_V<-22) of the `red sequence' trace the progenitor set accurately in terms of numbers and mass, the relationship breaks down severely at fainter luminosities (M_V>-21). Hence the red sequence is generally not a good proxy for the progenitor set of early-type galaxies.

Abstract:

Building galaxy merger trees from a state-of-the-art cosmological hydrodynamics simulation, Horizon-AGN, we perform a statistical study of how mergers and smooth accretion drive galaxy morphologic properties above $z > 1$. More specifically, we investigate how stellar densities, effective radii and shape parameters derived from the inertia tensor depend on mergers of different mass ratios. We find strong evidence that smooth accretion tends to flatten small galaxies over cosmic time, leading to the formation of disks. On the other hand, mergers, and not only the major ones, exhibit a propensity to puff up and destroy stellar disks, confirming the origin of elliptical galaxies. We also find that elliptical galaxies are more susceptible to grow in size through mergers than disc galaxies with a size-mass evolution $r \prop M^{1.2}$ instead of $r \prop M^{-0.5} - M^{0.5}$ depending on the merger mass ratio. The gas content drive the size-mass evolution due to merger with a faster size growth for gas-poor galaxies $r \prop M^2$ than for gas-rich galaxies $r \prop M$.