THE JAMES CLERK MAXWELL TELESCOPE NEARBY GALAXIES LEGACY SURVEY. I. STAR-FORMING MOLECULAR GAS IN VIRGO CLUSTER SPIRAL GALAXIES

We present large-area maps of the CO <i>J</i> = 3-2 emission obtained at the James Clerk Maxwell Telescope for four spiral galaxies in the Virgo Cluster. We combine these data with published CO <i>J</i> = 1-0, 24 μm, and Hα images to measure the CO line ratios, molecular gas masses, and instantaneous gas depletion times. For three galaxies in our sample (NGC 4254, NGC 4321, and NGC 4569), we obtain molecular gas masses of 7 x 10<sup>8</sup> - 3 x 10<sup>9</sup> <i>M</i> ⊙ and disk-averaged instantaneous gas depletion times of 1.1-1.7 Gyr. We argue that the CO <i>J</i> = 3-2 line is a better tracer of the dense star-forming molecular gas than the CO <i>J</i> = 1-0 line, as it…