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 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 (log U ~
-2.5 +- 0.03), column density (N_H ~ 10^{21} cm^{-2}), metallicity ([M/H] ~
-0.20 +- 0.15), and mass (~ 10^8 Msun) of the gaseous bridge. We simultaneously
constrain properties of the QSO-host (M_DM>8.8x 10^{11} Msun) and its companion
galaxy (M_DM>2.1 x 10^{11} Msun; M_star ~ 2 x 10^{10} Msun; stellar burst
age=300-800 Myr; SFR~6 Msun/yr; 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 passage 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 detectable AGN
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.
