Stellar Populations in the Host Galaxies of Markarian 1014, IRAS 07598+6508, and Markarian 231

Abstract

We present deep spectroscopic and imaging data of the host galaxies of Mrk 1014, IRAS 07598 + 6508, and Mrk 231. These objects form part of both the QSO and the ultraluminous infrared galaxy (ULIG) families and may represent a transition stage in an evolutionary scenario. Our imaging shows that all three objects have highly perturbed hosts with tidal tails and destroyed disks and appear to be in the final stages of major mergers. The host galaxies of the three objects have spectra typical of E +A galaxies, showing simultaneously features from an old and a young stellar component. We model spectra from different regions of the host galaxies using Bruzual & Chariot spectral synthesis models using two-component models including an old underlying population and recent superposed starbursts. Mrk 1014 has intense star formation concentrated in a large knot less than 2 kpc from the nucleus, along the leading edge of the tidal tail, and in several knots scattered around the host. The starburst ages in these regions range from 180 to 290 Myr. IRAS 07598 + 6508 has multiple knots of star formation concentrated in two regions within 16 kpc of the QSO nucleus, with ages ranging from 30 to 70 Myr; the host galaxy shows an older population in other regions. Mrk 231 shows a wider range of starburst ages, ranging from 42 Myr in the arc 3 kpc south of the nucleus to ∼300 Myr spread on a "plateau" ~20 kpc across around the nucleus, as well as a UV-bright region 12 kpc south of the nucleus, which is apparently a region of currently active star formation. Our results indicate a strong connection between interactions and vigorous bursts of star formation in these objects. We propose that the starburst ages found are indicative of young ages for the QSO activity. The young starburst ages found are also consistent with the intermediate position of these objects in the far infrared color-color diagram.