1996A&A...309..705M

The central 80"x80" of the nearby edge-on starburst galaxy NGC 4945 has been mapped in the J=3-2 line of ¹²CO with a resolution of 15". Spectra of the J=3-2 transition of ¹³CO and tentative detections of the J=4-3 line of HCN and the J_Ka_Kc=3₁₂-2₁₁ 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=3.7Mpc. 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≤600pc is ∼3x10⁹M_☉. Applying a "standard" N(H₂)/I(CO) conversion factor of 2x10²⁰cm^–2/(K.km/s), the central molecular gas mass is ∼9x10⁸M_☉. Like in the case of the central regions of the Milky Way and NGC253, integrated ¹²CO 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 0.15-0.35x10²⁰cm^–2/(K.km/s) for the central region of NGC 4945. An embedded active nucleus may contribute to the very high "star forming efficiency" L_IR/M_gas∼140L_☉/M_☉. The contribution of the CO lines to the "continuum" flux density in a 50GHz 1.3mm 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(H₂)=10^3.5-10⁴cm^–3) and is clumped with a beam filling factor of roughly 20%.