Detections of CO in Late-Type, Low Surface Brightness Spiral Galaxies

Using the IRAM 30-m telescope, we have obtained 12CO J=1-0 and 2-1 spectral line observations toward the nuclear regions of 15 edge-on, low surface brightness (LSB) spiral galaxies. Our sample comprises extreme late-type LSB spirals with disk-dominated morphologies and rotational velocities V_rot<~120 km/s. We report detections of four galaxies in at least one transition (>5sigma); for the remainder of the sample we provide upper limits on the nuclear CO content. Adopting a standard Galactic I_CO-to-H_2 conversion factor implies molecular gas masses of (3.3-9.8)x10**6 M_sun in the nuclear regions (inner 1.1-1.8 kpc) of the detected galaxies. Combining our new data with samples of late-type spirals from the literature, we find that the CO-detected LSB spirals adhere to the same M_H2-FIR correlation as more luminous and higher surface brightness galaxies. The amount of CO in the central regions of late-type spirals appears to depend more strongly on mass than on central optical surface brightness, and CO detectability declines significantly for moderate-to-low surface brightness spirals with V_rot<~90 km/s; no LSB spirals have so far been detected in CO below this threshold. Metallicity effects alone are unlikely to account for this trend, and we speculate that we are seeing the effects of a decrease in the mean fraction of a galaxy disk able to support giant molecular cloud formation with decreasing galaxy mass.