The BIMA Survey of Nearby Galaxies (BIMA SONG). II. The CO Data

Abstract

The BIMA Survey of Nearby Galaxies is a systematic imaging study of the 3 mm CO J = 1-0 molecular emission within the centers and disks of 44 nearby spiral galaxies. The typical spatial resolution of the survey is 6'' or 360 pc at the average distance (12 Mpc) of the sample. The velocity resolution of the CO observations is 4 km s -1 , though most maps are smoothed to 10 km s -1 resolution. For 33 galaxies, multifield observations ensured that a region ≳190'' (〈 D 〉 = 10 kpc) in diameter was imaged. For the remaining 11 galaxies, which had smaller optical diameters and were on average farther away, single-pointing observations imaged a 100'' diameter (〈 D 〉 = 11 kpc) region. The sample was not chosen based on CO or infrared brightness; instead, all spirals were included that met the selection criteria of v ⊙ ≤ 2000 km s -1 , δ ≥ -20°, i ≤ 70°, D 25 < 70', and B T < 11.0. The detection rate was 41/44 sources or 93%; of the three nondetections, one (M81) is known to have CO emission at locations outside the survey field of view. Fully sampled single-dish CO data were incorporated into the maps for 24 galaxies; these single-dish data comprise the most extensive collection of fully sampled, two-dimensional single-dish CO maps of external galaxies to date. We also tabulate direct measurements of the global CO flux densities for these 24 sources. For the remaining 20 sources, we collected sensitive single-dish spectra in order to evaluate the large-scale flux recovery. We demonstrate that the measured ratios of flux density recovered are a function of the signal-to-noise of the interferometric data. We examine the degree of central peakedness of the molecular surface density distributions and show that the distributions exhibit their brightest CO emission within the central 6'' in only 20/44 or 45% of the sample. We show that all three Local Group spiral galaxies have CO morphologies that are represented in SONG, though the Milky Way CO luminosity is somewhat below the SONG average, and M31 and M33 are well below average. This survey provides a unique public database of integrated intensity maps, channel maps, spectra, and velocity fields of molecular emission in nearby galaxies. It also lays the groundwork for extragalactic surveys by more powerful future millimeter-wavelength interferometers like CARMA and ALMA.