The nuclear gas dynamics and star formation of Markarian 231

We report adaptive optics H- and K-band spectroscopy of the inner few arcseconds of the luminous merger/ ultraluminous infrared galaxy (ULIRG)/QSO Mrk 231, at spatial resolutions as small as 0."085. For the first time we have been able to resolve the active star-forming region close to the active galactic nucleus (AGN) using stellar absorption features, finding that its luminosity profile is well represented by an exponential function with a disk scale length 0."18-0."24 ( 150 - 200 pc), and implying that the stars exist in a disk rather than a spheroid. The stars in this region are also young ( 10 - 100 Myr), and it therefore seems likely that they have formed in situ in the gas disk, which itself resulted from the merger. The value of the stellar velocity dispersion (similar to100 km s(-1) rather than the usual few times 10 km s(-1) in large-scale disks) is a result of the large mass surface density of the disk. The stars in this region have a combined mass of at least 1.6 x 10(9) M-., and account for 25% - 40% of the bolometric luminosity of the entire galaxy. At our spatial resolution the stellar light in the core is diluted by more than a factor of 10 even in the H band by continuum emission from hot dust around the AGN. We have detected the 2.12 mum 1 - 0 S(1) H-2 and 1.64 mum [Fe II] lines out to radii exceeding 0."5. The kinematics for the two lines are very similar to each other as well as to the stellar kinematics, and broadly consistent with the nearly face-on rotating disk reported in the literature and based on interferometric CO 1 - 0 and CO 2 - 1 measurements of the cold gas. However, they suggest a more complex situation in which the inner 0."2 - 0."3 (200 pc) is warped out of its original disk plane. Such a scenario is supported by the projected shape of the nuclear stellar disk, the major axis of which is significantly offset from the nominal direction, and by the pronounced shift on very small scales in the direction of the radio jet axis, which has been reported in the literature.