Detections of CO in late-type, low surface brightness spiral galaxies

Using the IRAM 30 m telescope, we have obtained (CO)-C-12 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 less than or similar to 120 km s(-1). We report detections of four galaxies in at least one transition (greater than or similar to 5 sigma); 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) x 10(6) M-circle dot 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 CO-detected LSB spirals adhere to the same M-H2-far-infrared 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 less than or similar to 90 km s(-1); 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.