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 the Australia Telescope Compact Array ( 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 _ { H _ { 2 } } = ( 6 \pm 1 ) \times 10 ^ { 9 } M _ { \odot } , which is distributed in two main reservoirs separated by \sim 9 kpc . \sim 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 \pm 0.3 ) \times 10 ^ { 12 } L _ { \odot } , 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 } Â 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 Hubble Space Telescope image at the tip of the northern tidal tail .
The formation of such structures is predicted by simulations of colliding galaxies .