CI and CO in the center of M 51

We present J = 2-1, J = 3-2, J = 4-3 (CO)-C-12 maps as well as J = 2-1, J = 3-2 (CO)-C-13 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(H-2) approximate to 10(3)) with kinetic temperature T-kin approximate to 100 K, combined with either a less dense (approximate to 10(2) cm(-3)) component of the same temperature, or a more dense (n(H-2)approximate to 3 x 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 circle dot pc(-2), and a well- established CO-to-H-2 conversion ratio X four to five times lower than the standard Galactic value.