SDSS J0025-10 at z=0.30: a (U)LIRG to optical QSO transition candidate
  We have characterized the amount, spatial distribution and kinematics of the
molecular gas in the merging, double nucleus type 2 quasar SDSS J0025-10 at
z=0.30 using the CO(1-0) transition, based on data obtained with ATCA. This is
one of the scarce examples of quasar host galaxies where the CO emission has
been resolved spatially at any redshift. We infer a molecular gas mass M(H2) =
(6 +/- 1) x 1e9 Msun, which is distributed in two main reservoirs separated by
~9 kpc. ~60% of the gas is in the central region, associated with the QSO
nucleus and/or the intermediate region between the two nuclei. The other 40% is
associated with the northern tidal tail and is therefore unsettled.
  With its high infrared luminosity L(IR) = (1.1 +/- 0.3) x 1e12 Lsun, SDSS
J0025-10 is an analogue of local luminous LIRGs and ULIRGs. On the other hand,
the clear evidence for an ongoing major merger of two gas rich progenitors, the
high L(IR) dominated by a starburst, the massive reservoir of molecular gas
with a large fraction still unsettled, and the quasar activity are all
properties consistent with a transition phase in the (U)LIRG-optical QSO
evolutionary scenario. We propose that we are observing the system during a
particular transient phase, prior to more advanced mergers where the nuclei
have already coalesced.
  We argue that a fraction of the molecular gas reservoir is associated with a
tidal dwarf galaxy identified in the optical HST image at the tip of the
northern tidal tail. The formation of such structures is predicted by
simulations of colliding galaxies.
