High‐Resolution Molecular Gas Maps of M33

Abstract

New observations of CO (J=1-->0) line emission from M33, using the 25 element BEARS focal plane array at the Nobeyama Radio Observatory 45 m telescope, in conjunction with existing maps from the BIMA interferometer and the FCRAO 14 m telescope, give the highest resolution (13") and most sensitive (σ rms ~60 mK) maps to date of the distribution of molecular gas in the central 5.5 kpc of the galaxy.A new catalog of giant molecular clouds (GMCs) has a completeness limit of 1.3×10 5 M solar. The fraction of molecular gas found in GMCs is a strong function of radius in the galaxy, declining from 60% in the center to 20% at galactocentric radius R gal ~4 kpc. Beyond that radius, GMCs are nearly absent, although molecular gas exists. Most (90%) of the emission from low-mass clouds is found within 100 pc projected separation of a GMC. In an annulus 2.1 kpc gal <4.1 kpc, GMC masses follow a power-law distribution with index -2.1. Inside that radius, the mass distribution is truncated, and clouds more massive than 8×10 5 M solar are absent. The cloud mass distribution shows no significant difference in the grand-design spiral arms versus the interarm region. The CO surface brightness ratio for the arm to interarm regions is 1.5, typical of other flocculent galaxies.