Molecular gas kinematics in barred spiral galaxies

To quantify the effect that bar-driven mass inflow can have on the evolution of a galaxy requires an understanding of the dynamics of the inflowing gas. In this paper we study the kinematics of the dense molecular gas in a set of seven barred spiral galaxies to determine which dynamical effects dominate. The kinematics are derived from observations of the CO J = (1-0) line made with the Berkeley-Illinois-Maryland Association (BIMA) millimeter array. We compare the observed kinematics with those predicted by ideal gas hydrodynamic and ballistic-cloud-based models of gas how in a barred potential. The hydrodynamic model is in good qualitative agreement with both the current observations of the dense gas and previous observations of the kinematics of the ionized gas. The observed kinematics indicate that the gas abruptly changes direction upon entering the dust lanes to flow directly down the dust lanes along the leading edge of the bar until the dust lanes, approach the nuclear ring. Near the location where the dust lanes intersect the nuclear ring, we see two velocity components: a low-velocity component, corresponding to gas on circular orbits, and a higher velocity component, which can be attributed to the fraction of gas flowing down the bar dust lane that sprays past the contact point toward the other half of the bar. The ballistic-cloud-based model of the ISM is not consistent with the observed kinematics. The kinematics in the dust lanes require large velocity gradients that cannot be reproduced by an ISM composed of ballistic clouds with long mean free paths. Therefore, even the dense ISM responds to hydrodynamic forces.