2005AJ....129.1849M

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 (≳5 σ); for the remainder of the sample we provide upper limits on the nuclear CO content. Adopting a standard Galactic I_CO-to-H₂ conversion factor implies molecular gas masses of (3.3-9.8)x10⁶ M_☉ 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≲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.