Dynamical Masses in Luminous Infrared Galaxies

We have studied the dynamics and masses of a sample of ten nearby luminousand ultraluminous infrared galaxies (LIRGS and ULIRGs), using 2.3 micron COabsorption line spectroscopy and near-infrared H- and Ks-band imaging. Bycombining velocity dispersions derived from the spectroscopy, diskscale-lengths obtained from the imaging, and a set of likely model densityprofiles, we calculate dynamical masses for each LIRG. For the majority of thesample, it is difficult to reconcile our mass estimates with the large amountsof gas derived from millimeter observations and from a standard conversionbetween CO emission and H_2 mass. Our results imply that LIRGs do not have hugeamounts of molecular gas (10^10-10^11 Msolar) at their centers, and supportprevious indications that the standard conversion of CO to H_2 probablyoverestimates the gas masses and cannot be used in these environments. This inturn suggests much more modest levels of extinction in the near-infrared forLIRGs than previously predicted (A_V~10-20 versus A_V~100-1000). The lower gasmass estimates indicated by our observations imply that the star formationefficiency in these systems is very high and is triggered by cloud-cloudcollisions, shocks, and winds rather than by gravitational instabilities incircumnuclear gas disks.Comment: 14 pages, 2 figures, accepted to Ap