Spectroscopy of close companions to quasi-stellar objects and the ages of interaction-induced starbursts

We present low-resolution absorption-line spectra of three candidate close (<3 degrees) companions to the low-redshift QSOs 3CR 323.1, PG 1700+518, and PKS 2135-147. The spectra were obtained with the Low-Resolution Imaging Spectrometer (LRIS) on the Keck telescopes and with the Faint Object Spectrograph on the University of Hawaii 2.2 m telescope. For 3CR 323.1 and PG 1700+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 3CR 323.1 indicate a stellar population of intermediate age (similar to 2.3 Gyr). In contrast, the spectrum of the companion object to PG 1700+518 shows strong Balmer absorption lines from a relatively young stellar population, along with the Mg I b absorption feature and the 4000 Angstrom 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 approximately 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 PKS 2135-147 is actually a projected Galactic G star.