Kinematics of Low‐ z Ultraluminous Infrared Galaxies and Implications for Dynamical Mass Derivations in High‐ z Star‐forming Galaxies

Abstract

The kinematic properties of the gaseous and stellar components of 11 ultraluminous infrared galaxies (ULIRGs; 14 nuclei) are investigated by means of integral field spectroscopy (IFS) with the INTEGRAL system and available IR and CO millimeter spectroscopy. The sample of ULIRGs cover different phases of the merging process and span all levels of activity from pure starbursts to Seyfert nuclei. The IFS data show that the ionized gas has a complex velocity structure with peak-to-peak velocity differences of a few to several hundred km s-1, detected in tidal tails or extranuclear star-forming regions. The velocity field of the ionized gas on scales of a few to several kiloparsecs is dominated by tidally induced flows and does not, in general, correspond to rotationally supported systems with a privileged orientation along the major rotating axis. The central velocity amplitude of the ionized gas and stars shows discrepancies in some galaxies but has, on average, a similar value (ratio of 0.92+/-0.37) , while the velocity amplitude of the molecular gas is a factor of 2 larger (ratio of 1.9+/-0.6) than that of the stars and ionized gas. The central velocity amplitude measured using different kinematic tracers should therefore not be used in ULIRGs as a reliable tracer of mass, in general. The IFS data also show that the velocity dispersion of the ionized gas maps the large-scale motions associated with tidal tails and extranuclear regions, with often the highest velocity dispersion not being associated with the nucleus galaxies. There is, however, a good agreement between the central ionized gas and stellar velocity dispersions (ratio of 1.01+/-0.13), while the cold molecular gas velocity dispersion has lower values (average of about 0.8 that of the stellar and ionized gas). The central ionized gas velocity dispersion is therefore a robust and homogeneous observable and a good tracer of the dynamical mass in these systems. The IFS-based central ionized gas velocity dispersion measurements confirm that ULIRGs' hosts are moderate-mass (