SHINING LIGHT ON MERGING GALAXIES. I. THE ONGOING MERGER OF A QUASAR WITH A "GREEN VALLEY" GALAXY

Serendipitous observations of a pair z = 0.37 interacting galaxies (one hosting a quasar) show a massive gaseous bridge of material connecting the two objects. This bridge is photoionized by the quasar (QSO), revealing gas along the entire projected 38 h(70)(-1) kpc sightline connecting the two galaxies. The emission lines that result give an unprecedented opportunity to study the merger process at this redshift. We determine the kinematics, ionization parameter (logU approximate to -2.5 +/- 0.03), column density (N-H,N-perpendicular to approximate to 10(21) cm(-2)), metallicity ([M/H]approximate to -0.20 +/- 0.15), and mass (approximate to 10(8) M-circle dot) of the gaseous bridge. We simultaneously constrain properties of the QSO host (M-DM > 8.8 x 10(11) M-circle dot) and its companion galaxy (M-DM > 2.1x10(11) M-circle dot; M-* similar to 2x10(10) M-circle dot; stellar burst age = 300-800 Myr; SFR similar to 6 M-circle dot yr(-1); and metallicity 12 + log(O/H) = 8.64 +/- 0.2). The general properties of this system match the standard paradigm of a galaxy-galaxy merger caught between first and second passages while one of the galaxies hosts an active quasar. The companion galaxy lies in the so-called green valley, with a stellar population consistent with a recent starburst triggered during the first passage of the merger and has no discernible active galactic nucleus activity. In addition to providing case studies of quasars associated with galaxy mergers, quasar/galaxy pairs with QSO-photoionized tidal bridges such as this one offer unique insights into the galaxy properties while also distinguishing an important and inadequately understood phase of galaxy evolution.