Star Clusters in M51: Connection between Molecular Gas, Stars and Dust

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. Line Velocity Gradient (LVG) analysis performed at a linear resolution of 135–210 pc (2.9–4.5 arcsec) suggests physical conditions in the Giant Molecular Cloud complexes (GMCs) of M51 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 galactocentric 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. Our new deep ∼ 2 arcsec radio continuum images at 3.6 and 6 cm reveal the presence of some highly dust-obscured young star-forming regions within the molecular spiral arms.