We present a detailed investigation of the young stellar populations(YSP) in
the radio-loud ultra luminous infrared galaxy (ULIRG) PKS1345+12, based on high
resolution HST imaging and long slit spectra taken with the WHT. While the
images clearly show bright knots suggestive of super star clusters(SSC), the
spectra reveal the presence of YSP in the diffuse light across the full extent
of the halo of the merging-double nucleus system. Spectral synthesis modelling
has been used to estimate the ages of the YSP for both the SSC and the diffuse
light sampled by the spectra. For the SSC we find ages t{SSC} < 6 Myr with
reddenings 0.2 < E(B-V) < 0.5 and masses 10e6 < M{SSC} < 10e7 M{solar}.
However, in some regions of the galaxy we find that the spectra of the diffuse
light component can only be modelled with a relatively old post-starburst YSP
(0.04 - 1.0 Gyr) or with a disk galaxy template spectrum. The results
demonstrate the importance of accounting for reddening in photometric studies
of SSC, and highlight the dangers of focussing on the highest surface
brightness regions when trying to obtain a general impression of the star
formation activity in the host galaxies of ULIRGs. The case of PKS1345+12
provides clear evidence that the star formation histories of the YSP in ULIRGs
are complex. Intriguingly, our long-slit spectra show line splitting at the
locations of the SSC, indicating that they are moving at up to 450km s-1 with
respect to the local ambient gas. Given their kinematics, it is plausible that
the SSC have been formed either in fast moving gas streams/tidal tails that are
falling back into the nuclear regions as part of the merger process, or as a
consequence of jet-induced star formation linked to the extended, diffuse radio
emission detected in the halo of the galaxyComment: accepted for publication in MNRA
