Galaxy formation and evolution

Abstract:

ABSTRACT We introduce a new method to mitigate numerical diffusion in adaptive mesh refinement (AMR) simulations of cosmological galaxy formation, and study its impact on a simulated dwarf galaxy as part of the ‘EDGE’ project. The target galaxy has a maximum circular velocity of $21\, \mathrm{km}\, \mathrm{s}^{-1}$ but evolves in a region that is moving at up to $90\, \mathrm{km}\, \mathrm{s}^{-1}$ relative to the hydrodynamic grid. In the absence of any mitigation, diffusion softens the filaments feeding our galaxy. As a result, gas is unphysically held in the circumgalactic medium around the galaxy for $320\, \mathrm{Myr}$, delaying the onset of star formation until cooling and collapse eventually triggers an initial starburst at z = 9. Using genetic modification, we produce ‘velocity-zeroed’ initial conditions in which the grid-relative streaming is strongly suppressed; by design, the change does not significantly modify the large-scale structure or dark matter accretion history. The resulting simulation recovers a more physical, gradual onset of star formation starting at z = 17. While the final stellar masses are nearly consistent ($4.8 \times 10^6\, \mathrm{M}_{\odot }$ and $4.4\times 10^6\, \mathrm{M}_{\odot }$ for unmodified and velocity-zeroed, respectively), the dynamical and morphological structure of the z = 0 dwarf galaxies are markedly different due to the contrasting histories. Our approach to diffusion suppression is suitable for any AMR zoom cosmological galaxy formation simulations, and is especially recommended for those of small galaxies at high redshift.

Abstract:

We examine how the mass assembly of central galaxies depends on their location in the cosmic web. The HORIZON-AGN simulation is analysed at z ∼ 2 using the DISPERSE code to extract multi-scale cosmic filaments. We find that the dependency of galaxy properties on large-scale environment is mostly inherited from the (large-scale) environmental dependency of their host halo mass. When adopting a residual analysis that removes the host halo mass effect, we detect a direct and non-negligible influence of cosmic filaments. Proximity to filaments enhances the build-up of stellar mass, a result in agreement with previous studies. However, our multi-scale analysis also reveals that, at the edge of filaments, star formation is suppressed. In addition, we find clues for compaction of the stellar distribution at close proximity to filaments. We suggest that gas transfer from the outside to the inside of the haloes (where galaxies reside) becomes less efficient closer to filaments, due to high angular momentum supply at the vorticity-rich edge of filaments. This quenching mechanism may partly explain the larger fraction of passive galaxies in filaments, as inferred from observations at lower redshifts.

Abstract:

Determining the spatial distribution and intrinsic physical properties of neutral hydrogen on cosmological scales is one of the key goals of next-generation radio surveys. We use the EAGLE galaxy formation simulations to assess the properties of damped Lyman α absorbers (DLAs) that are associated with galaxies and their underlying dark matter haloes between 0 ≤ z ≤ 2. We find that the covering fraction of DLAs increases at higher redshift; a significant fraction of neutral atomic hydrogen (H I) resides in the outskirts of galaxies with stellar mass ≥1010 M⊙; and the covering fraction of DLAs in the circumgalactic medium (CGM) is enhanced relative to that of the interstellar medium (ISM) with increasing halo mass. Moreover, we find that the mean density of the H I in galaxies increases with increasing stellar mass, while the DLAs in high- and low-halo mass systems have higher column densities than those in galaxies with intermediate halo masses (∼1012 M⊙ at z = 0). These high-impact CGM DLAs in high-stellar mass systems tend to be metal poor, likely tracing smooth accretion. Overall, our results point to the CGM playing an important role in DLA studies at high redshift (z ≥ 1). However, their properties are impacted both by numerical resolution and the detailed feedback prescriptions employed in cosmological simulations, particularly that of active galactic nuclei.