The properties of the stellar populations in ULIRGs II: the star
  formation histories and evolution
  This is the second of two papers presenting a detailed long-slit
spectroscopic study of the stellar populations in a sample of 36 ULIRGs. In the
previous paper we presented the sample, the data and the spectral synthesis
modelling while in this paper, we carry out a detailed analysis of the
modelling results. We find that the star formation histories of ULIRGs are
complex, with at least two epochs of star formation activity and that the
charcteristic timescale of the star formation acivity is <100Myr. These results
are consistent with models that predict an epoch of enhanced star formation
coinciding with the first pass of the merging nuclei, along with a further,
more intense, episode of star formation occurring as the nuclei finally merge
together. It is also found that the young stellar populations (YSPs) tend to be
younger and more reddened in the nuclear regions of the galaxies. This is in
good agreement with the merger simulations, which predict that the bulk of the
star formation activity in the final stages of mergers will occur in the
nuclear regions of the merging galaxies. In addition, our results show that
ULIRGs have total stellar masses that are similar to, or smaller than, the
break of the galaxy mass function (m* = 1.4 x 10^{11} Msolar). Finally, we find
no significant differences between the ages of the YSP in ULIRGs with and
without optically detected Seyfert nuclei, nor between those with warm and cool
mid- to far-IR colours. While this results do not entirely rule out the idea
that cool ULIRGs with HII/LINER spectra evolve into warm ULIRGs with
Seyfert-like spectra, it is clear that the AGN activity in local Seyfert-like
ULIRGs has not been triggered a substantial period (>=100 Myr) after the major
merger-induced starbursts in the nuclear regions.
