Spectroscopy of Close Companions to QSOs and the Ages of Interaction-Induced Starbursts

We present low-resolution absorption-line spectra of three candidate close (< 3) companions to the low redshift QSOs 3CR323.1, PG1700+518 and PKS2135−147. The spectra were obtained with LRIS on the Keck telescopes and with the Faint Object Spectrograph on the University of Hawaii 2.2m telescope. For 3CR323.1 and PG1700+518, we measure relative velocities that are consistent with an association between the QSOs and their companion galaxies. The spectral features of the companion galaxy to 3CR323.1 indicate a stellar population of intermediate age (∼ 2.3 Gyr). In contrast, the spectrum of the companion object to PG1700+518 shows strong Balmer absorption lines from a relatively young stellar population, along with the Mg Ib absorption feature and the 4000 Å break from an older population. By modeling the two stellar components of this spectrum, it is possible to estimate the time that has elapsed since the end of the most recent major starburst event: we obtain ∼ 0.1 Gyr. This event may have coincided with an interaction that triggered the QSO activity. Finally, our spectroscopy shows conclusively that the supposed companion to PKS2135−147 is actually a projected Galactic G star. Subject headings: galaxies: evolution—galaxies: interactions—quasars: individual (3C323.1, PG1700+518, PKS2135−147) Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. Visiting Astronomer, W.M. Keck Observatory, jointly operated by the California Institute of Technology and the University of California.