Within LCDM, galaxies are regulated by both large-scale processes such as mergers, tidal torques and smooth accretion via filaments, as well as by small-scale processes such as star formation and feedback. These mechanisms leave characteristic signatures imprinted in the stellar, gaseous, and dark matter halos of the galaxy. We use numerical simulations to study these halos and focus on establishing the link between halo observables today and the past evolution of galaxies. We find that stellar halos in Milky Way (MW)-like objects have a wide variety of masses and shapes that strongly correlate with their assembly histories. In particular, the low stellar halo fraction of our own Galaxy indicates an overall early and quiet assembly history. The recent detection of a major merger event ~10 Gyr ago leading to a massive and radial stellar halo component seems plausible, although its compactness is difficult to reproduce within LCDM. We predict that a significant fraction of the mass associated with this merger may lie undiscovered in the outskirts of the MW's stellar halo. Further clues on galaxy assembly histories may lurk in their gaseous halos, whose observational study has only recently become possible for large galaxies. Using intermediate-volume simulations, we investigate the potential of MUSE to detect the gaseous filaments feeding galaxy halos and tracing the cosmic web. We find that including feedback effects and degrading pixel resolution may increase the detectability of these flows in realistic mock images. Furthermore, filaments are affected by feedback, providing powerful observational constraints to theoretical models. The phenomenon of feedback remains, however, too simplistically modeled within cosmological simulations and is expected to have its maximum impact on low-mass galaxies. We use idealized numerical simulations of a dwarf halo to understand the interplay between two possible feedback sources: stars and black holes. We find that although black holes negligibly affect the star formation history of the dwarf, they may leave strong signatures on outflow properties. The future observational detection of these outflows promises novel constraints on black hole growth as well as stellar and black hole feedback models in cosmological numerical simulations.

We have developed a post-processing tagging technique to model globular clusters (GCs) in cosmological hydrodynamical simulations. We have applied our method to the Illustris and TNG50 simulations to study several aspects of GCs in galaxy groups and clusters, regimes where no other theoretical technique is available to link galaxies, halos and GCs. We find that GCs are good tracers of dark matter–both in terms of their radial distribution and shape to trace the host halos in groups and clusters, but also through their kinematics to constrain the dynamical mass of dwarf galaxies. We have used our catalogs to establish one-to-one comparison to observational determinations of dark matter mass in dwarfs, finding that while systems with more than 10 GC may recover the right dark matter mass content via Jeans modeling and other derived mass estimators, for dwarfs with less than 10 GCs, assumptions in the prior and different methods to estimate the velocity dispersion may heavily bias the ability to infer dark matter mass from kinematics. We find that dwarf galaxies are consistent with populating an extrapolation of a single power-law relation between GC mass and halo mass observed for more massive systems, at least all the way down to dwarfs comparable to dSphs in the Local Group (M∗ = 5×106M⊙). Lastly, we explore the GC systems of the set of morphologically defined ultra-diffuse galaxies (UDGs) within the TNG50 simulation. Observationally, the kinematics of the GC systems of UDGs show a large diversity, with systems ranging from apparent “failed galaxies” living in overly- massive dark matter halos to the opposite extreme, where UDGs are seemingly dark-matter free. We use our GC catalog to demonstrate that much of this diversity, in particular towards low GC numbers, might arise as a combination of a low number of dynamical tracers coupled to ongoing tidal disruption—in agreement with evidence of stellar streams in some of the UDGs with low velocity dispersion—as well as contamination from GCs in the intracluster medium.

The standard model of cosmology assumes that dark matter (DM) is cold and collisionless. This collisionless Cold Dark Matter (CDM) model has been extremely successful in explaining various observational phenomena on mass scales larger than galaxies. Despite the successes on large scales, the CDM model faces several challenges on smaller scales that have remained unanswered for several years. One of the most intriguing discrepancies between CDM predictions and observations stems from the DM content in dwarf and spiral galaxies. While CDM predicts radially divergent DM density (cusps) at the halo centers, many observations suggest constant DM density cores. Abundance of the observed Milky Way satellites has also been at odds with the predictions from CDM. While observations suggest ≤ 100 luminous Milky Way satellites, CDM N-body simulations predict 1-2 orders of magnitude more subhalos for a Milky Way-like galaxy.

As an alternative to CDM, self-interacting dark matter (SIDM) models are proposed as potential solutions to some of these cosmological issues. In this class of DM models, DM-DM scatter- ing leads to a re-distribution of energy at the center of the halo such that after a few dynamical times, the halo reaches in-equilibrium isothermal state. This results in the re-distribution of DM particles, which, in principle, can form constant density cores opposite to the cuspy profiles predicted by the CDM model. The interaction between dark matter particles may be extended to the scenarios where DM scatters off from some other relativistic dark sector particles (e.g. dark photons), leading to suppressions in the matter power spectrum. This results in the depletion of DM halos in the regime corresponding to this cutoff in the matter power spectrum, which pro- vides another channel to address the tension between theoretical predictions and observations.

In this dissertation, I study the impact of non-gravitational interactions in the dark sector on the distribution and evolution of DM halos. I find that when entangled with the baryonic physics, SIDM models possess rich phenomenology for the formation of structures in the uni- verse. For the dark matter halos in isolation with low baryonic content, DM-DM interactions lead to a cored DM density profile (e.g. dwarf-sized galaxies), as expected from the isothermal behavior of SIDM. On the other hand, in isolated halos with a more significant contribution of baryons (e.g. closer to the mass scale of the Milky Way), a phase of core-contraction is triggered, leading to a high central density. Furthermore, the environmental effects, such as tidal stripping and tidal shocking, can dramatically change the fate of these objects. While gravitational tides remove mass from outskirts of the satellite halos, the heat transport due to the DM self-interactions results in a faster “core-collapse.” The transition from core-expansion to core-collapse is controlled by the orbit and pre-infall halo parameters.

In the DM models where DM experiences interactions with dark photons, the suppression in the matter power spectrum causes a depletion of DM halos in the regime of dwarf galaxies. In this dissertation, I develop an analytical Press-Schechter approach to compute halo abundance for SIDM models in a computationally efficient way that is not possible for the conventional numerical simulations. I calibrate and test this analytical model with cosmological simulations, and show that it is robust over different SIDM models, halo mass regimes, and cosmological time-scales. I use this formalism to put a constraint on the parameter space of DM-dark photon interactions that is consistent with previous measurements.