Cl and CO in the center of M 51

We present J = 2-1, J = 3-2, J = 4-3 ¹²CO maps as well as J = 2-1, J = 3-2 ¹³CO 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) ≈ 10 ³) with kinetic temperature T_kin ≈ 100 K, combined with either a less dense (≈ 10² cm^-3) component of the same temperature, or a more dense (n(H₂) ≈ 3 × 10 ³ cm^-3) and much cooler (T_kin = 10-30 K) 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 ± 20 M pc^-2, and a well-established CO-to-H₂ conversion ratio X four to five times lower than the standard Galactic value.