Paper detail

The properties of the stellar populations in ULIRGs I: sample, data and spectral synthesis modelling

We present deep long-slit optical spectra for a sample of 36 Ultraluminous Infrared Galaxies (ULIRGs), taken with the William Herschel Telescope (WHT) on La Palma with the aim of investigating the star formation histories and testing evolutionary scenarios for such objects. Here we present the sample, the analysis techniques and a general overview of the properties of the stellar populations. Spectral synthesis modelling has been used in order to estimate the ages of the stellar populations found in the diffuse light sampled by the spectra in both the nuclear and extended regions of the target galaxies. We find that adequate fits can be obtained using combinations of young stellar populations (YSPs,t_YSP<=2 Gyr), with ages divided into two groups: very young stellar populations (VYSPs, t_VYSP <=100 Myr) and intermediate-young stellar populations (IYSPs, 0.1 < t_IYSP <= 2 Gyr). Our results show that YSPs are present at all locations of the galaxies covered by our slit positions, with the exception of the northern nuclear region of the ULIRG IRAS 23327+2913. Furthermore, VYSPs are presents in at least 85% of the 133 extraction apertures used for this study. Old stellar populations (OSPs, t_{OSP} > 2 Gyr) do not make a major contribution to the optical light in the majority of the apertures extracted. In fact they are essential for fitting the spectra in only 5% (7) of the extracted apertures. The estimated total masses for the YSPs (VYSPs+IYSPs) are in the range 0.18 x 10^{10} <= M_YSP <= 50 x 10^{10} Msun. We have also estimated the bolometric luminosities associated with the stellar populations detected at optical wavelengths, finding that they fall in the range 0.07 x 10^{12} < L_bol < 2.2 x 10^{12} Lsun. In addition, we find that reddening is significant at all locations in the galaxies.