Hierarchical formation of bulgeless galaxies II: Redistribution of angular momentum via galactic fountains

TL;DR

This study uses cosmological simulations to show how galactic fountains redistribute angular momentum, preventing bulge formation and leading to bulgeless disc galaxies consistent with observations.

Contribution

It demonstrates that galactic fountains can effectively transfer angular momentum, explaining the absence of classical bulges in simulated galaxies within the cold dark matter framework.

Findings

Galaxies form without classical bulges after major mergers.

Galactic fountains return gas to the disc, aiding in angular momentum retention.

Simulated galaxies resemble observed bulgeless disc galaxies.

Abstract

Within a fully cosmological hydrodynamical simulation, we form a galaxy which rotates at 140 km/s, and is characterised by two loose spiral arms and a bar, indicative of a Hubble Type SBc/d galaxy. We show that our simulated galaxy has no classical bulge, with a pure disc profile at z=1, well after the major merging activity has ended. A long-lived bar subsequently forms, resulting in the formation of a secularly-formed "pseudo" bulge, with the final bulge-to-total light ratio B/T=0.21. We show that the majority of gas which loses angular momentum and falls to the central region of the galaxy during the merging epoch is blown back into the hot halo, with much of it returning later to form stars in the disc. We propose that this mechanism of redistribution of angular momentum via a galactic fountain, when coupled with the results from our previous study which showed why gas outflows are biased to have low angular momentum, can solve the angular momentum/bulgeless disc problem of the cold dark matter paradigm.