, R. Zylka
, P.G. Mezger
, C.G.T. Haslam
,
E. Kreysa
IRAM, 300 rue de la piscine, F-38406 S
Martin d'Hères,
France
Max Planck Institut für Radioastronomie, Auf dem Hügel
69, D-53121 Bonn 1, Germany 
1.2 mm continuum emission of the inner
part of M51 (Fig. 6).
This emission traces the central molecular disk and the inner
spiral arms and correlates remarkably well with the 
CO (2--1) line
emission. The 
1.2 mm continuum to CO line intensity
ratio, averaged over the 70 GHz-wide bolometer band, is 7; a similar
value was derived for NGC891.
The 
1.2 mm emission arises mostly from cold dust (
K)
associated with molecular clouds. Using the local Galactic value of the dust
absorption cross section, corrected for the larger metallicity observed
near M51's center, we derive the gas distribution from this emission.
The derived molecular masses are 
times smaller than those obtained
from the CO luminosity, using Strong et al.'s Galactic CO to H
conversion factor (
10
), but close to the masses estimated from
CO and from visual extinction.
It may be argued, considering the crude approximations inherent to each method,
that all estimates are encouragingly close. We wish to stress, however, that
the present work points towards an H
mass in the inner part of M51 a
factor of 4 lower than usually assumed from the 
CO integrated line
intensity. A similar conclusion was reached for NGC 891 (Guélin et al. 1995,
A&A 279, L37). It probably also applies to many galaxies and could have major
consequences on their evolution.
Figure 6: The 
1.2 mm continuum emission (
--- top) and
the velocity integrated CO line intensity
(
--- bottom)
in the inner part of M51, smoothed to a 15
angular resolution. The
position of selected H
regions are indicated on the continuum map
(numbering according to Carranza et al. A&A 1, 479). Contour levels range from
10 to 46 mJy (beam)
in steps of 3 mJy.(beam)
for the
continuum, and from
5 K.km
s to 32.5 K.km
s in
steps of 2.5 K.km
s for CO. The continuum emission was set to zero
outside the diamond-shaped grey area. Both maps were
observed with the IRAM 30-m telescope; the CO data are taken from
Garcia-Burillo et al. (1993, A&A 274, 123).