Beyond the nuclear starburst? Clustered star formation in major mergers
  Recent simulation work has successfully captured the formation of the star
clusters that have been observed in merging galaxies. These studies, however,
tend to focus on studying extreme starbursts, such as the Antennae galaxies. We
aim to establish whether there is something special occurring in these extreme
systems or whether the mechanism for cluster formation is present in all
mergers to a greater or lesser degree. We undertake a general study of
merger-induced star formation in a sample of 5 pc resolution adaptive mesh
refinement simulations of low redshift equal-mass mergers with randomly-chosen
orbital parameters. We find that there is an enhanced mass fraction of very
dense gas that appears as the gas density probability density function evolves
during the merger. This finding has implications for the interpretation of some
observations; a larger mass fraction of dense gas could account for the
enhanced HCN/CO ratios seen in ULIRGs and predicts that alpha_CO is lower in
mergers, as for a given mass of H_2, CO emission will increase in a denser
environment. We also find that as the star formation rate increases, there is a
correlated peak in the velocity dispersion of the gas, which we attribute to
increasing turbulence driven by the interaction itself. Star formation tends to
be clumpy: in some cases there is extended clumpy star formation, but even when
star formation is concentrated within the inner kpc (i.e. what may be
considered a nuclear starburst) it still often has a clumpy, rather than a
smooth, distribution. We find no strong evidence for a clear bimodality in the
Kennicutt-Schmidt relation for the average mergers simulated here. Instead,
they are typically somewhat offset above the predicted quiescent relation
during their starbursts.
