CI and CO in the center of M 51

¹ Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: israel@strw.leidenuniv.nl
² Joint Astronomy Centre, 660 N. A'ohoku Pl., Hilo, Hawaii, 96720, USA

Received: 20 March 2005
Accepted: 13 September 2005

Abstract

We present , , maps as well as , 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 () with kinetic temperature 100 K, combined with either a less dense () component of the same temperature, or a more dense () 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  pc^-2, and a well-established CO-to-H₂ conversion ratio X four to five times lower than the standard Galactic value.