title: Exploring the Diversity of Dwarf Galaxy Populations with Hydrodynamical Simulations of Targeted Cosmological Environments
Creators: name: Briggs, J
Publisher: name: Liverpool John Moores University
Year: 2025

description: The standard Λ-Cold Dark Matter (CDM) model of cosmology has provided accurate predictions and explanations for the large-scale structure of the Universe and its evolution with time, however, on length and halo mass scales below ∼1Mpc and ∼ 1011M⊙ discrepancies between theory and observation have propelled the study of the low mass regime to the forefront of cosmology and astrophysics (see e.g. Bullock &amp; Boylan-Kolchin, 2017a). Dwarf galaxies provide unique insight to galaxy formation and evolution: their early formation times and shallow gravitational potentials make them equally sensitive to internal astrophysical processes, such as feedback from young stars and supernovae (SNe), and external environmental influences, such as the ram pressure stripping of gas and the tidal stripping of stars. Furthermore, isolated dwarf galaxies are shielded from the extreme environmental effects that occur in high density environments and provide interesting ‘laboratories’ for examining galaxy growth in the ΛCDM framework, in particular, how low mass galaxies occupy haloes and how their large scale environment influences key properties.

I introduce the Columba suite of simulations of galaxy formation in the ΛCDM cosmogony, which are designed primarily to examine the influence of large-scale cosmic environment on the evolution of isolated dwarf galaxies, i.e. those not in close proximity to larger structures at the present day. I focus on this subset of dwarf galaxies in order to explore the influence of large-scale overdensity and the intrinsic properties of their host DM haloes on galaxy evolution in the low-mass regime, in the absence of direct environmental influences driven by tidal and ram pressure forces exerted by near neighbours. I describe the selection of the zoom-in regions of the Columba suite which are selected from a L = 400Mpc parent simulation and span a range of underdense environments, from voids to filamentary structures, and host a diverse population of dwarf galaxies. The Columba simulations incorporate a galaxy formation model that represents an evolution of that used for EAGLE which explicitly follows the cold, dense, neutral gas of the ISM, adopts a star formation model that accounts for turbulence, a treatment of pre-supernova feedback from massive stars, more sophisticated methods of distributing the energy liberated by SNe and active galactic nuclei (AGN), and updated treatments of BH growth and dynamics.

The Columba simulations give an insight into the formation and evolution of isolated dwarf galaxies in diverse cosmic environments. I utilise the simulations to examine the effect of large scale environment on key properties of field dwarf galaxies quantified by their spherical overdensity in r = 5Mpc. While the amplitudes of the halo mass functions (HMF) and stellar mass functions (SMF) exhibit a dependence on the region’s density, their slopes, when normalized to the cosmic mean, do not show an environmental dependence. Similarly, the stellar mass halo mass (SMHM) relation does not depend on the region’s environment, maintaining consistent slopes despite density variations. The luminous fraction of galaxies is not significantly affected by their large scale environment but show a correlation with halo concentration. Halo concentration has been shown to be strongly indicative of a halo’s formation time and studies have shown that the scatter in the SMHM relation is strongly correlated with the concentration of their DM halo. I further investigate the influence of halo concentration on properties of isolated field dwarf galaxies. At a fixed mass, high concentration haloes host larger dwarf galaxies, quantified by a peak Spearman rank correlation coefficient of ρ = 0.85. This effect propagates through their cumulative star formation histories which show that high concentration, and hence earlier forming, haloes assemble their stellar mass earlier across the full mass range examined, and is reflected by shorter t90, the time it takes for 90% of the total stellar mass to assemble. The central galaxies in the Columba suite show a diversity in morphology from disc to elliptical structures, and though halo concentration shows a strong influence on the evolution of isolated dwarf galaxies, it does not demonstrate a strong correlation with their kinematic morphology.