2006A&A...445..907I

We present J=2-1, J=3-2, J=4-3 ¹²CO maps as well as J=2-1, J=3-2 3co and 492 GHz [CI] measurements of the central region in M 51. The distribution of CO is strongly concentrated towards the spiral arms. The center itself is poor in, though not devoid of, CO emission. The observed line intensities require modelling with a multi-component molecular gas. A dense component must be present (n(H₂)≃10³) with kinetic temperature T_kin≃100K, combined with either a less dense (≃10²cm^–3) component of the same temperature, or a more dense (n(H₂)≃3x10³cm^–3) and much cooler (T_kin=10-30K) component. Atomic carbon amounts are between 5 and 10 times those of CO. Much of the molecular gas mass is associated with the hot PDR phase. The center of M 51 has a face-on gas mass density of about 40±20M_☉/pc², and a well-established CO-to-H₂ conversion ratio X four to five times lower than the standard Galactic value.