Molecular distribution and kinematics in nearby galaxies .1. NGC 253

The central 80 '' x 60 '' of the edge-on starburst galaxy NGC 253 has been mapped in the J = 2 - 1 line of (CO)-C-12 with the LRAM 30 m telescope (beam width: 12 ''). After a Lucy deconvolution, the resolution is enhanced to similar to 4 ''. CO emission is strongly concentrated toward a region elongated along the major axis with a FWHP size of 34 '' x 11 '' (410 pc x 130 pc at D=2.5 Mpc). Toward the center of NGC 253, the (CO)-C-12 (2 - 1) line contributes between 20% and 35% to the total measured 1.3 mm ''continuum'' in a 50 GHz wide bandpass. Position-velocity maps reveal two condensations symmetrically placed with respect to the nucleus. Distinct velocity features suggest the presence of a compact molecular spiral, although a molecular bar or ring cannot be excluded. The dynamical mass in the inner 480 pc is 2.6 10(9) M.. Applying a ''standard'' N(H-2)/I(CO) conversion factor of 2 10(20) cm(-2)/K km s(-1), the molecular gas mass of the central condensations would be similar to 3 10(8) M.. Other tracers of molecular mass, namely optically thin (CO)-O-18 rotational lines and the mm-wave dust continuum yield gas masses of 4 10(7) ... 6 10(7) M.. The most plausible explanation of this discrepancy is that the ''standard'' H-2 mass/CO luminosity ratio, which has been derived far Galactic disk clouds, cannot be applied to the bulge region of galaxies such as NGC 253. Possible reasons are discussed in terms of excitation conditions and the influence of the stellar gravitational potential. It is not the gas mass that makes NGC 253 an outstanding galaxy but its infrared luminosity and the ''star forming efficiency'' L(IR)/M(gas) of 90 L./M..