Molecular Gas and Star Formation in Bars of Nearby Spiral Galaxies

We compare the distribution of molecular gas and star formation activity in the bar region of six spirals (NGC 2903, 3627, 4321, 5457, 6946, and IC 342) from the BIMA Survey of Nearby Galaxies. The molecular gas, traced using the CO (J = 1–0) emission line, is brightest along the leading edge of the stellar bar in the bar dust lanes. The star formation activity, traced using the Hα emission line, is offset toward the leading side of the CO emission. A cross-correlation analysis shows that (1) the H II regions are offset 0–800 pc on the leading side of the CO emission, (2) the largest offsets are found in the strongest bars, and (3) there is a wide range in offsets in a single bar with no systematic pattern as a function of the galactocentric radius. The CO-Hα offset constrains how stars may form depending on the gas flow. We examine possible star formation scenarios in context of the two main classes of bar gas-flow simulations, the N-body/sticky particle and hydrodynamic models. Though both model gas flows are generally consistent with the observed offsets, we suggest the inclusion of a two-phase or multiphase medium to improve the agreement between models and observations.