The BIMA survey of nearby galaxies. I. The radial distribution of CO emission in spiral galaxies

We present the first results of the Berkeley-Illinois-Maryland Association Survey of Nearby Galaxies (BIMA SONG), an imaging survey of the CO J = 1-0 emission in 44 nearby spiral galaxies at a typical resolution of 6". BIMA SONG differs from previous high-resolution CO surveys in that (1) CO brightness was not an explicit selection criterion, (2) a larger area (200" diameter for most galaxies) of each galaxy was imaged, and (3) fully sampled single-dish CO data (55" resolution) were obtained for over half of the sample galaxies, so all of the CO flux is imaged in these galaxies. Here we present CO maps for a subsample of 15 BIMA SONG galaxies for which we have also obtained near-infrared or optical broadband data. The CO maps display a remarkable variety of molecular gas morphologies, and, as expected, the CO surface brightness distributions show considerably more substructure than the stellar light distributions, even when averaged over kiloparsec scales. The radial distribution of stellar light in galactic disks is generally characterized as an exponential. It is, therefore, of interest to investigate whether the molecular gas, which is the star-forming medium, has a similar distribution. Though our low-resolution single-dish radial profiles of CO emission can be described by simple exponentials, this is not true for the emission at our full 6" resolution. The scale lengths of the CO disks are correlated with the scale lengths of the stellar disks, with a mean ratio of the scale lengths of about 1. There is, however, considerable intrinsic scatter in the correlation. We also find that (1) there is also a weak correlation between the ratio of K-band to CO luminosity and Hubble type; (2) in half of the galaxies presented here, CO emission does not peak at the location of the stellar nucleus; (3) averaged over the inner kiloparsec, the CO emission in one-half of the galaxies exhibits an excess over that expected from an exponential disk, which is similar to the excess in stellar light caused by the bulge stars; and (4) this excess CO emission may be due to an increase in the total molecular gas content in the bulge region, or alternatively, to an increase in the CO emissivity caused by the increased pressure of the bulge region.