CO(J=3-2) On-the-fly Mapping of the Nearby Spiral Galaxies NGC 628 and
  NGC 7793: Spatially-resolved CO(J=3-2) Star-formation Law
  We present the results of CO(J=3-2) on-the-fly mappings of two nearby
non-barred spiral galaxies NGC 628 and NGC 7793 with the Atacama Submillimeter
Telescope Experiment at an effective angular resolution of 25". We successfully
obtained global distributions of CO(J=3-2) emission over the entire disks at a
sub-kpc resolution for both galaxies. We examined the spatially-resolved
(sub-kpc) relationship between CO(J=3-2) luminosities (L'CO(3-2)) and infrared
(IR) luminosities (LIR) for NGC 628, NGC 7793, and M 83, and compared it with
global luminosities of JCMT Nearby Galaxy Legacy Survey sample. We found a
striking linear L'CO(3-2)-LIR correlation over the 4 orders of magnitude, and
the correlation is consistent even with that for ultraluminous infrared
galaxies and submillimeter selected galaxies. In addition, we examined the
spatially-resolved relationship between CO(J=3-2) intensities (ICO(3-2)) and
extinction-corrected star formation rates (SFRs) for NGC 628, NGC 7793, and M
83, and compared it with that for GMCs in M 33 and 14 nearby galaxy centers. We
found a linear ICO(3-2)-SFR correlation with 1 dex scatter. We conclude that
the CO(J=3-2) star formation law (i.e., linear L'CO(3-2)-LIR and ICO(3-2)-SFR
correlations) is universally applicable to various types and spatial scales of
galaxies, from spatially-resolved nearby galaxy disks to distant IR-luminous
galaxies, within 1 dex scatter.
