Characterising filaments in cosmological simulations
Keywords: galaxy evolution
galaxy formation
large-scale structure of the universe
numerical methods

At high redshift, the cosmic web is widely expected to have a significant impact on the morphologies, dynamics and star formation rates of the galaxies embedded within it, underscoring the need for a comprehensive study of the properties of such a filamentary network. With this goal in mind, we perform an analysis of high-z gas and dark matter (DM) filaments around a Milky Waylike progenitor simulated with the RAMSES adaptive mesh refinement (AMR) code from cosmic scales (∼1 Mpc) down to the virial radius of its DM halo host (∼20 kpc at z = 4). Radial density profiles of both gas and DM filaments are found to have the same functional form, namely a plummer-like profile modified to take into account the wall within which these filaments are embedded. Measurements of the typical filament core radius r0 from the simulation are consistent with that of isothermal cylinders in hydrostatic equilibrium. Such an analytic model also predicts a redshift evolution for the core radius of filaments in fair agreement with the measured value for DM (r0 ∝ (1+z)−3.18±0.28). Gas filament cores grow as r0 ∝ (1 + z)−2.72±0.26. In both gas and DM, temperature and vorticity sharply drop at the edge of filaments, providing an excellent way to constrain the outer filament radius. When feedback is included the gas temperature and vorticity fields are strongly perturbed, hindering such a measurement in the vicinity of the galaxy. However, the core radius of the filaments as measured from the gas density field is largely unaffected by feedback, and the median central density is only reduced by about 20%. Extending the analysis to a larger cosmological sample of filaments feeding tens of galaxies with halo masses between 1010M⊙ and 1012.6M⊙, the median filament density profiles are found to be consistent at all redshifts. This is highly suggestive of the existence of a universal filament density profile. Gas and DM filaments that feed higher mass halos and galaxies are typically higher density and hotter, with the central gas temperature rising faster than the peak temperature. This confirms that cold accretion from filaments disappears at low redshifts. Galaxies are found to be rapidly accreted into the filamentary network, with halos that reside in the network typically having a higher galaxy formation efficiency. However, the density of the nearest filament, a proxy of the local density, has little effect on the galaxy properties we considered. Gas filaments exhibit the tendency to evolve away from a state where they are confined within the DM counterpart as time goes by and final galaxy properties do not correlate well with filament densities, suggesting that other processes are more important in shaping the low redshift properties of galaxies. The thesis also explored the impact of reionisation on the accretion of gas onto cosmic filaments, and the effect of this on the embedded galaxies using a set of large scale cosmological radiative transfer simulations. Reionisation is able to heat the IGM filaments to the point where the gas no longer sees the shallow potential well of the lower mass filaments. While the DM is unaffected by this, the filament gas density is reduced by 60-80% by z = 6, driven by proximity to the galaxies they feed. Filaments with shock temperatures higher than the post-reionisation IGM gas temperature continue to accrete efficiently in spite of reionisation, and feed high mass galaxies. Meanwhile halos with Mvir ≲ 108M⊙ see their gas accretion rates reduced by an order of magnitude. For even lower mass halos, with Mvir ≲ 3 × 107M⊙, gas outflow rates are enhanced due to photoevaporation, however, the reduction in inflow is even greater and thus starvation is more important in quenching the star formation in low mass halos at early epochs.