The Spiral Arms of M 51: Physical Properties of the Gas

Abstract

We have mapped key molecular line probes (¹²CO, ¹³CO, C¹⁸O, HCN, HCO+) in two distinct regions in the spiral arms of the Whirlpool galaxy M51 using the Owens Valley Radio Observatory (OVRO) mm-interferometer and the IRAM 30m single dish telescope. Line Velocity Gradient (LV G ) analysis performed at a linear resolution of 210pc (4.5″; both regions) and 135 pc (2.9″; western region only) suggests physical conditions in the Giant Molecular Cloud complexes (GMCs) of M 51 very similar to those in the Milky Way: cold (T_kin ∼ 15 K) clouds with moderate H₂ density (n(H₂) < 10^2.7 cm^-3). We find indications for a galacto-centric trend with higher kinetic temperature for smaller radii. The data show little evidence for cloud heating by the massive star formation at our resolution of 135 pc. CO(2-1) data at our highest angular resolution of 1.3” reveal the presence of multiple unresolved GMCs, with a tendency to higher velocity dispersion upstream, i.e. the inner side, of the spiral arm.