Observations of Non-Circular Cold Gas Motions in the Circumnuclear Regions of Galaxies (Invited paper)

Recent observations of non-circular cold gas motions in the central 100 pc — 1 kpc regions of galaxies are summarized, with an emphasis on relating the observed local motions to net inflow and outflow rates. INTRODUCTION Non-circular gas motions have been detected in many galaxies, providing empirical evidence that radial gas flows continue to affect the evolution of galaxies long after the initial epoch of galaxy formation. This paper reviews observations of non-circular motions of the cold (molecular and atomic) gas in the 100 pc – 1 kpc circumnuclear regions of disk galaxies. This is typically the region of galaxies where bulges begin to dominate the potential, where rotation curves change from steeply rising to flat, and where circumnuclear starbursts occur. Molecular gas is generally the dominant phase of the interstellar medium by mass in the central regions of gas-rich disk galaxies, and for this reason many of the observations discussed in this review are of CO or other tracers of molecular gas. The weakness of HI emission in circumnuclear regions, and the related difficulty of determining HI kinematics at high resolution, make HI results less common but valuable. Although on average inflowing gas is probably colder than outflowing gas, there are examples of both outflowing molecular gas and inflowing ionized gas. The relationships between the observed local non-circular motions and the net radial flow rates vary considerably, and in several important cases the net flow rates are much smaller than the local non-circular motions.