High-Resolution CO Observations of Luminous Infrared Galaxies

Abstract

We have performed a high-resolution imaging survey of the CO J = 1 → 0 emission in seven galaxies with infrared (IR) luminosities exceeding 3 × 10¹¹ L_☉—five of which are mergers. The resultant maps show that the molecular gas is very highly concentrated towards the cores of the mergers, with gas surface densities approaching or exceeding 10⁴ M_☉ pc^-2 within 300–400 pc of the nuclei in three cases. This result supports earlier findings based on data from a smaller sample of luminous mergers. In the two mergers that show closely spaced double IR (stellar) nuclei, CO emission peaks between the nuclei and shows an extent roughly equal to the nuclear separation. The gas cores of the individual merging galaxies appear to be coalescing, while the stellar cores remain distinct. In the three single nucleus mergers, the CO peaks are coincident with the stellar nuclei, consistent with the hypothesis that these are relatively evolved merger remnants. In two of the three mergers with the most compact CO emission (Mrk 231 and NGC 6240), the empirical Galactic conversion factor from CO luminosity to molecular gas mass appears to overestimate the nuclear gas mass by a factor of more than 2 (3.6 in the case of Mrk 231). For Mrk 231, the high brightness temperature of the CO emission (T_b > 34 K) is the likeliest explanation for this overestimate. In the third such merger (NGC 2623), however, the geometry and kinematics suggest that the molecular gas mass is within a factor of 2 of the value given by using the Galactic conversion factor. Nonetheless, in all three of these objects, the molecular gas probably dominates the nuclear gravitational potential. We suggest that the molecular gas in objects with such high gas mass surface densities (~10⁴ M_☉ pc^-2) is distributed in nuclear disks. These disks must be thin because of their self-gravity, with a full width of 30–40 pc (compared to radii of 300–400 pc) for a vertical velocity dispersion of 90 km s^-1. The mean volume density of molecular hydrogen in such disks must be over 10⁴ cm^-3. The trend of increasing L_FIR/L_CO with increasing CO surface brightness is confirmed. The high concentrations of molecular gas thus appear intimately related to the high luminosities of these systems and probably serve as the fuel.