Dynamical masses in luminous infrared galaxies

We have studied the dynamics and masses of a sample of 10 nearby luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs, respectively) using 2.3 μm ¹²CO-absorption line spectroscopy and near-infrared H- and K_s-band imaging. By combining velocity dispersions derived from the spectroscopy, disk scale lengths obtained from the imaging, and a set of likely model density profiles, we calculate dynamical masses for each LIRG. For the majority of the sample, it is difficult to reconcile our mass estimates with the large amounts of gas derived from millimeter observations and from a standard conversion between ¹²CO emission and H₂ mass. Our results imply that LIRGs do not have huge amounts of molecular gas (10 ¹⁰-10¹¹ M_⊙) at their centers and support previous indications that the standard conversion of ¹²CO to H ₂ probably overestimates the gas masses and cannot be used in these environments. This in turn suggests much more modest levels of extinction in the near-infrared for LIRGs than previously predicted (A_v ∼ 10-20 vs. A_v ∼ 100-1000). The lower gas mass estimates indicated by our observations imply that the star formation efficiency in these systems is very high and is triggered by cloud-cloud collisions, shocks, and winds rather than by gravitational instabilities in circumnuclear gas disks.