The molecular gas distribution in M51

Abstract

CO emission has been mapped along four strips in M51 using the 14 m Five College Radio Astronomy Observatory telescope in order to sample the radial distribution of molecular gas. The derived abundance of H2 shows that three-fourths of the interstellar medium nucleons are in H2 rather than H I in the disk at R < 10 kpc. For galactic radii 1-10 kpc, the radial distributions of CO emission, optical light from the disk, far-infrared flux, and nonthermal radio emission can be described by similar exponentials in R between 1 and 10 kpc with scale lengths ≡4 kpc. This contrasts sharply with the distribution of H I emission, which exhibits a hole in the center and a much flatter disk. The kinematics of CO are consistent with a total mass distribution (stars + gas) with surface density proportional to the disk light (i.e., constant M/L outside the nucleus).