Stellar populations in merging galaxies

Abstract

To examine the stellar populations of interacting or merging galaxies, a sample of 28 objects with disturbed morphology was selected. Integrated spectra of these galaxies were obtained, to study their global star formation histories and provide a database for comparison with morphologically disturbed galaxies at high redshift. Quantitative star formation histories were determined using evolutionary population synthesis models. Special emphasis was placed on observational and systematic uncertainties, e.g., IMF, metallicity, and reddening. The merger sample was divided into two subsamples for comparison with morphologically normal galaxies. The red subsample consists of galaxies whose spectra resemble those of early-type galaxies, while the blue subsample has moderate to strong Halpha emission. The model fits to the spectra of the red merger sample are indistinguishable from those in a control sample of S0 galaxies. Differences in the upper limits on recent star formation between these mergers and a sample of elliptical galaxies may be due to metallicity effects. The minimum amount of star formation required in the last Gyr is consistent with zero for the red merger and the E/S0 samples. The maximum amount of new star formation ranges from 0.2-3.2% by mass in the merger sample and 0.0-2.7% in the E/S0 control sample. Reddening contributes the largest source of uncertainty in determining the mass of a starburst in the blue merger subsample, while burst ages are relatively unaffected. We put limits on the quantitative star formations histories in these galaxies, although the uncertainties tend to be large. We find starbursts ranging in age from 10⁷ to 10⁹ years, and burst masses from 0 to more than 20% of the total stellar mass. We find higher recent rates of star formation in the merger sample based on far-infrared luminosities and Balmer absorption strengths, respectively. We cannot distinguish between truncated star formation followed by a starburst, and alternate star formation histories, such as those appropriate to spiral-type star formation, based on our model fits alone. A Salpeter IMF appears to be an adequate one to describe star formation in these galaxies.