Bar‐driven Mass Inflow: How Bar Characteristics Affect the Inflow

Abstract

If the interstellar medium (ISM) in a galaxy is driven into the nuclear region, it will change the evolution of the galaxy. Although it is generally assumed that bars are efficient at driving the ISM to smaller radii, how the characteristics of a bar affect the inflow has never been examined in detail. In this paper we investigate how various bar characteristics affect the ability of the bar to drive the ISM to smaller radii. We model the ISM using high-resolution hydrodynamic simulations and are able to show that weak bars have almost no effect on the radial distribution of the ISM. We also find that for all bar strengths, bars are only able to drive gas down to a radius where a ring forms. Inside of this ring there is very little net inflow. When a nuclear ring forms the bar becomes very efficient at driving gas down to the inner kiloparsec. Alternatively, when no nuclear ring forms, gas forms a ring at the largest nonlooping orbit whose major axis is parallel to the major axis of the bar (x1 orbit). For all but the thinnest bars, this ring is far from the nuclear region. We term this type of ring an x1 ring and show that inner rings are a subset of x1 rings and form when there are no looping x1 orbits. In this case, the gas accumulates at the largest x1 orbit inside of the looping 4 : 1 orbits. We also show that the one-dimensional measure of bar strength based on the maximum nonaxisymmetric force, Qb, is degenerate with several bar characteristics that control orbit family transitions. Because these orbit transitions determine where and whether rings form, and it is ring formation that is the ultimate result of bar-driven inflow, Qb is not a useful metric for determining the effect of a bar on the evolution of a galaxy.