The molecular interstellar medium of dwarf galaxies on kiloparsec scales:: A new survey for CO in northern, IRAS-detected dwarf galaxies

We present a new survey for CO in dwarf galaxies using the ARO Kitt Peak 12 m telescope. This survey consists of observations of the central regions of 121 northern dwarfs with IRAS detections and no known CO emission. We detect CO in 28 of these galaxies and marginally detect another 16, increasing by about 50% the number of such galaxies known to have significant CO emission. The galaxies we detect are comparable in stellar and dynamical mass to the Large Magellanic Cloud, although somewhat brighter in CO and fainter in the far-IR. Within dwarfs, we find that the CO luminosity LCO is most strongly correlated with the K-band and the far-infrared luminosities. There are also strong correlations with the radio continuum (RC) and B-band luminosities and linear diameter. Conversely, we find that far-IR dust temperature is a poor predictor of CO emission within the dwarfs alone, although a good predictor of normalized CO content among a larger sample of galaxies. We suggest that L-CO and L-K correlate well because the stellar component of a galaxy dominates the midplane gravitational field and thus sets the pressure and density of the atomic gas, which control the formation of H-2 from Hi. We compare our sample with more massive galaxies and find that dwarfs and large galaxies obey the same relationship between CO and the 1.4 GHz RC surface brightness. his relationship is well described by a Schmidt law with Sigma(RC)/Sigma(CO)(1:3). CO. Therefore, dwarf galaxies and large spirals exhibit the same relationship between molecular gas and star formation rate (SFR). We find that this result is robust to moderate changes in the RC-to-SFR and CO-to-H-2 conversion factors. Our data appear to be inconsistent with large ( order of magnitude) variations in the CO-to-H-2 conversion factor in the star- forming molecular gas.