Rodríguez Zaurín, J.; Holt, J.; Tadhunter, C. N.; González Delgado, R. M.
Referencia bibliográfica
Monthly Notices of the Royal Astronomical Society, Volume 375, Issue 4, pp. 1133-1145.
Fecha de publicación:
3
2007
Número de citas
23
Número de citas referidas
23
Descripción
We present a detailed investigation of the young stellar populations
(YSP) in the radio-loud ultraluminous infrared galaxy (ULIRG),
PKS1345+12 (z = 0.12), based on high-resolution Hubble Space Telescope
(HST) imaging and long-slit spectra taken with the William Herschel
Telescope (WHT) at La Palma. While the images clearly show bright knots
suggestive of super star clusters (SSCs), 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
tSSC < 6Myr with reddenings 0.2 < E(B - V) < 0.5 and
masses 106 < MYSPSSC <
107Msolar. In the region to the south of the
western nucleus that contains the SSC our modelling of the spectrum of
the diffuse light is also consistent with a relatively young age for the
YSP (~5Myr), although older YSP ages cannot be ruled out. However, in
other 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 (0.04-1.0Gyr) or with a disc galaxy template
spectrum. The results demonstrate the importance of accounting for
reddening in photometric studies of SSC and highlight the dangers of
focusing 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. While the SSC
represent the vigorous phase of star formation associated with the final
stages of the merger, the YSP in the diffuse light are likely to
represent star formation in one or more of the merging galaxies at an
earlier stage or prior to the start of the merger.
Intriguingly, our long-slit spectra show line splitting at the locations
of the SSC, indicating that they are moving at up to 450kms-1
with respect to the local ambient gas. Given their kinematics, it is
plausible that the SSCs 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.