The interstellar medium of the hot-spot galaxy NGC 2903

Abstract

We report 21 cm radio continuum, H-alpha, IRAS, and J = 1 --> 0 and 2 --> 1 CO observations of the hot-spot galaxy NGC 2903. The radio continuum, which traces synchrotron emission, H-alpha, which traces thermal H II regions, and CO, which traces neutral molecular gas, are distributed almost identically in a central bar. The CO data are consistent with emission from giant molecular clouds with properties typical of Milky Way clouds; that is, with large optical depths, excitation temperatures near 10 K, and sizes about 100 pc. If the standard CO luminosity to H-2 mass conversion factor holds in NGC 2903, then the central 770 pc contains about 3 x 10(8) M. in molecular gas, comparable to the central 500 pc of the Galaxy, and a mass surface density of approximately 240 M. pc-2, comparable to Orion and the Galactic center. The star formation rate per unit area is enhanced by an order of magnitude in the nuclear region. The global far-infrared-to-CO luminosity ratio, which is often interpreted as a measure of star formation efficiency, is typical of that for normal spirals. However, the local FIR/CO ratio varies considerably. In particular, 100-mu-m emission extends well outside the molecular bar into a region of H I emission. We associate this emission with cirrus, which contributes about 30% of the global 100-mu-m flux. Where cirrus emission dominates, the FIR/CO ratio does not reflect the local star formation efficiency. The nuclear starburst in NGC 2903 may result from a large amount of molecular gas with normal star formation efficiency collected into a small nuclear region, due to streaming motions induced by the bar.