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 spectralline observations toward the nuclear regions of 15 edge-on, low surfacebrightness (LSB) spiral galaxies. Our sample comprises extreme late-type LSBspirals with disk-dominated morphologies and rotational velocities V_rot<~120km/s. We report detections of four galaxies in at least one transition(>5sigma); for the remainder of the sample we provide upper limits on thenuclear CO content. Adopting a standard Galactic I_CO-to-H_2 conversion factorimplies 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 withsamples of late-type spirals from the literature, we find that the CO-detectedLSB spirals adhere to the same M_H2-FIR correlation as more luminous and highersurface brightness galaxies. The amount of CO in the central regions oflate-type spirals appears to depend more strongly on mass than on centraloptical surface brightness, and CO detectability declines significantly formoderate-to-low surface brightness spirals with V_rot<~90 km/s; no LSB spiralshave so far been detected in CO below this threshold. Metallicity effects aloneare unlikely to account for this trend, and we speculate that we are seeing theeffects of a decrease in the mean fraction of a galaxy disk able to supportgiant molecular cloud formation with decreasing galaxy mass.Comment: accepted to A