The IRAM M33 CO(2-1) Survey - A complete census of the molecular gas out\n to 7 kpc

Abstract

In order to study the ISM and the interplay between the atomic and molecular\ncomponents in a low-metallicity environment, we present a complete high angular\nand spectral resolution map and data cube of the 12CO(2-1) emission from the\nLocal Group galaxy M33. Its metallicity is roughly half-solar, such that we can\ncompare its ISM with that of the Milky Way with the main changes being the\nmetallicity and the gas mass fraction. The data have a 12" angular resolution\n(50pc) with a spectral resolution of 2.6 km/s and a mean noise level of 20 mK\nper channel in antenna temperature. A radial cut along the major axis was also\nobserved in the 12CO(1-0) line. The CO data cube and integrated intensity map\nare optimal when using HI data to define the baseline window and the velocities\nover which the CO emission is integrated. Great care was taken when building\nthese maps, testing different windowing and baseline options and investigating\nthe effect of error beam pickup. The total CO(2-1) luminosity is 2.8e7 K km/s\npc2, following the spiral arms in the inner disk. There is no clear variation\nin the CO(2-1/1-0) intensity ratio with radius and the average value is roughly\n0.8. The total molecular gas mass is estimated, using a N(H2)/Ico(1-0)=4e20\ncm-2/(K km/s) conversion factor, to be 3.1e8 Msol. The CO spectra in the cube\nwere shifted to zero velocity by subtracting the velocity of the HI peak from\nthe CO spectra. Hence, the velocity dispersion between the atomic and molecular\ncomponents is extremely low, independently justifying the use of the HI line in\nbuilding our maps. Stacking the spectra in concentric rings shows that the CO\nlinewidth and possibly the CO-HI velocity dispersion decrease in the outer\ndisk. Using the CO(2-1) emission to trace the molecular gas, the probability\ndistribution function of the H2 column density shows an excess at high column\ndensity above a log normal distribution.\n