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Hierarchical formation of bulgeless galaxies

Chris Brook (Jeremiah Horrocks Inst. - Univ. of Central Lancashire Preston)

Within a cosmological hydrodynamical simulations, we form bulgeless dwarf and disc galaxies. We show that ejection of low angular momentum gas is able to resolve the inconsistencies between the distribution of angular momentum predicted in cold dark matter theories, with observations of bulgeless disc galaxies. We show why outflows are strongly biased to have low angular momentum : (a) star formation peaks at high redshift in shallow dark matter potentials, when accreted gas has relatively low angular momentum, (b) an extended reservoir of high angular momentum gas in the outer disc provides material for prolonged SF at later times (c) outflows follow the path of least resistance which is perpendicular to the disc, and (d) 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.

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. This mechanism of redistribution of angular momentum via a galactic fountain, when coupled with the bias for gas outflows to have low angular momentum, can solve the angular momentum/bulgeless disc problem of the cold dark matter paradigm.