Properties of Molecular Gas in the Bar of Maffei 2

Abstract

Abstract We investigated the physical properties of molecular gas in one of the nearest barred spiral galaxies, Maffei 2, using the C12O ($J$$=$ 1–0) emission line taken for the Nobeyama CO Atlas of Nearby Spiral Galaxies. Position–velocity diagrams perpendicular to the apparent major axis of the bar show an abrupt velocity change across the bar, which is caused by molecular gas motion of $\sim\ $100 km s$^{-1}$ along the leading edges of the bar. The distribution of the peak temperature ($T_{\rm peak}$), velocity width ($\Delta V$), and integrated intensity of the C12O spectra ($I_{\rm CO}$) in the ridges of the bar is quite different from that in the spiral arms in $T_{\rm peak}$–$\Delta V$–$I_{\rm CO}$ space, suggesting that molecular-gas properties are different in these regions. Simple model calculations assuming an ensemble of uniform and spherical molecular clouds in a large velocity-gradient approximation indicate that molecular gas in the bar ridge regions may be gravitationally unbound, which suggests that molecular gas is hard to become dense, and to form stars. Moreover, the gravitationally unbound condition makes the CO-to-H$_{2}$ conversion factor in the bar ridges smaller than in the arms. A lower star-formation efficiency in bars indicated by previous studies is caused by such a condition that molecular gas is gravitationally unbound, as well as by an overestimation of molecular gas mass in the bar regions relative to spiral arms using a constant CO-to-H$_{2}$ conversion factor.