A census of young stellar populations in the warm ULIRG PKS1345+12

Abstract

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 galaxy