Hierarchical formation of bulgeless galaxies: Why outflows have low angular momentum

TL;DR

This study uses high-resolution cosmological simulations to show that galactic outflows preferentially remove low angular momentum gas, facilitating the formation of bulgeless disc galaxies in a cold dark matter universe.

Contribution

It demonstrates how outflows driven by supernovae and stellar winds naturally remove low angular momentum material, explaining the formation of bulgeless galaxies.

Findings

Outflows preferentially expel low angular momentum gas.

Outflows are enhanced during mergers, preventing classical bulge formation.

High angular momentum gas remains, enabling disc formation.

Abstract

Using high resolution, fully cosmological smoothed particle hydro-dynamical simulations of dwarf galaxies in a Lambda cold dark matter Universe, we show how baryons attain a final angular momentum distribution which allows pure disc galaxies to form. Blowing out substantial amounts of gas through supernovae and stellar winds, which is well supported observationally, is a key ingredient in forming bulgeless discs. We outline why galactic outflows preferentially remove low angular momentum material, and show that this is a natural result when structure forms in a cold dark matter cosmology. The driving factors are a) the mean angular momentum of accreted material increases with time, b) lower potentials at early times, c) the existence of an extended reservoir of high angular momentum gas which is not within star forming regions, meaning that only gas from the inner region (low angular momentum gas) is expelled and d) the tendency for outflows to follow the path of least resistance which is perpendicular to the disc. We also show that outflows are enhanced during mergers, thus expelling much of the gas which has lost its angular momentum during these events, and preventing the formation of "classical", merger driven bulges in low mass systems. Stars formed prior to such mergers form a diffuse, extended stellar halo component.