Abstract
The central 80'' x 80'' of thp nearby edge-on starburst galaxy NGC 4945 has been mapped in the J = 3 - 2 line of (CO)-C-12 with a resolution of 15''. Spectra of the J = 3 - 2 transition of (CO)-C-13 and tentative detections of the J = 4 - 3 line of HCN and the JK(a)K(c) = 3(12) - 2(11) line of H2CO are also presented. The CO J = 3 - 2 emission is concentrated toward the kinematical center of the galaxy with a deconvolved full width at half power (FWHP) size of (11.5+/-3)'' corresponding to (200+/-50) pc at a distance D = 3.7 Mpc. This is less than the extent of the nuclear molecular complex as derived from the distributions of the lower rotational CO transitions. Position-velocity maps reveal three condensations located at the center and at offsets of 5'' toward either side of the nucleus. The dynamical mass in the inner R less than or equal to 600 pc is similar to 3 10(9) M(circle dot).
Applying a ''standard'' N(H-2)/I(CO) conversion factor of 2 10(20) cm(-2)/(K km s(-1)), the central molecular gas mass is similar to 9 10(8) M(circle dot). Like in the case of the central regions of the Milky Way and NGC 253, integrated (CO)-C-12 line intensities combined with a standard conversion factor yield a gas mass which is 6 to 14 times higher than that obtained from multi-transition CO models and from the dust continuum emission. If the latter results are correct, we find a conversion factor of only 0.15-0.35 10(20) cm(-2)/(K km s(-1)) for the central region of NGC 4945. An embedded active nucleus may contribute to the very high ''star forming efficiency'' L(IR)/M(gas) similar to 140 L(circle dot)/M(circle dot). The contribution of the CO lines to the ''continuum'' flux density in a 50 GHz 1.3 mm band is comparable to that of the dust. From the strength of the CO J = 3 - 2 emission and our tentative detection of high density tracing molecules, the bulk of the CO emitting gas should have a high density (n(H-2) = 10(3.5) - 10(4) cm(-3)) and is clumped with a beam filling factor of roughly 20%.