COLD DUST EMISSION FROM THE SPIRAL ARMS OF M51

Using the IRAM 30-m telescope equipped with the MPIfR 7-channel bolome ter array, we have mapped the lambda 1.2 mm continuum emission of the inner part of M 51. This emission traces the central molecular disk an d the inner spiral arms and correlates remarkably well with the (CO)-C -12 (2-1) line emission. The lambda 1.2 mm continuum to CO line intens ity ratio, averaged over the 70 GHz-wide bolometer band, is 7; a simil ar value was derived for NGC 891. The lambda 1.2 mm emission arises mo stly from cold dust (T-d less than or equal to 20 K) associated with m olecular clouds. Using the local Galactic value of the dust absorption cross section, corrected for the larger metallicity observed near M 5 1's center, we derive the gas distribution from this emission. The der ived molecular masses are similar or equal to 4 times smaller than tho se obtained from the CO luminosity, using Strong et al.'s Galactic CO to Hz conversion factor, but close to the masses estimated from (CO)-C -13 and from visual extinction.