The dependence of CO emission on luminosity and the rate of star formation in SC galaxies

A sample of eight Sc galaxies covering a large range in total luminosity has been observed in the 2.6 mm CO line using the 14 m FCRAO telescope (HPBW = 50''). An approximately linear correlation is revealed between the CO and blue luminosities measured for the same regions in both the nuclei and the inner disks. Assuming the CO emission is proportional to the abundance of molecular gas, disks. this correlation implies that low-luminosity regions have little H/sub 2/ while high-luminosity regions have large amounts. Within this sample, the molecular masses out to a fixed radius, R< or =2.5 kpc, range from less than 6 x 10/sup 7/ M/sub sun/ for M33 to approximately 2 x 10/sup 9/ M/sub sun/ for NGC 6946. However, the ratio of H/sub 2/ mass to blue luminosity is relatively constant, with M/sub H//sub 2//L/sub B/ = 0.17 +- 0.08 M/sub sun//L/sub sun/ over two orders of magnitude in L/sub B/ even including the Seyfert galaxy NGC 1068. If the blue luminosity is an indicator of the recent star formation rate, these results suggest the star formation rate per nucleon in Sc galaxies is constant. In contrast, the H/sub 2/ I ratio in the centralmore » R< or =2.5 kpc varies from less than 0.4 in M33 to approximately 32 in M51.« less