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

Abstract

We present the results of CO(J=3-2) on-the-fly mappings of two nearby non-barred spiral galaxies, NGC628 and NGC7793, 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 NGC628, NGC7793, and M83, and compared it with global luminosities of a JCMT (James Clerk Maxwell Telescope) Nearby Galaxy Legacy Survey sample. We found a striking linear L′ CO(3-2)-LIR correlation over the four orders of magnitude, and the correlation is consistent even with that for ultraluminous IR galaxies and submillimeter-selected galaxies. In addition, we examined the spatially resolved relationship between CO(J = 3-2) intensities (ICO(3-2)) and extinctioncorrected star formation rates (SFRs) for NGC628, NGC7793, and M83, and compared it with that for Giant Molecular Clouds in M33 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.