Rigging dark halos: why is hierarchical galaxy formation consistent with the inside-out build-up of thin discs?

TL;DR

This paper explains how the cosmic web's dynamics lead to focused, steady, and angular momentum-rich gas inflow into dark matter halos, supporting inside-out galaxy disc formation.

Contribution

It links large-scale cosmic web dynamics to the angular momentum properties of gas inflow, explaining the efficiency of cold flows in forming thin galactic discs.

Findings

Gas inflow is focused, consistent, and amplified by cosmic web dynamics.

Filamentary inflow carries high angular momentum, aiding disc formation.

Large-scale motions of voids and walls influence gas accretion patterns.

Abstract

State-of-the-art hydrodynamical simulations show that gas inflow through the virial sphere of dark matter halos is focused (i.e. has a preferred inflow direction), consistent (i.e. its orientation is steady in time) and amplified (i.e. the amplitude of its advected specific angular momentum increases with time). We explain this to be a consequence of the dynamics of the cosmic web within the neighbourhood of the halo, which produces steady, angular momentum rich, filamentary inflow of cold gas. On large scales, the dynamics within neighbouring patches drives matter out of the surrounding voids, into walls and filaments before it finally gets accreted onto virialised dark matter halos. As these walls/filaments constitute the boundaries of asymmetric voids, they acquire a net transverse motion, which explains the angular momentum rich nature of the later infall which comes from further away. We conjecture that this large-scale driven consistency explains why cold flows are so efficient at building up high redshift thin discs from the inside out.