ASTROPHYSICS Molecular excitation in Centaurus A : the 13 CO J = 1 – 0 map and CO line ratios ?

We present a fully sampled map of the 13 CO J=1–0 emission in the dust lane of Centaurus A as well as 13 CO 2–1 and for the first time C 18 O 1–0 spectra at selected positions. The morphology of the 13 CO map is similar to the 12 CO 1–0 and 2–1 maps. Maps of the 12 CO 2–1/1–0 and 12 CO/ 13 CO 1–0 ratios show small variations of the ratios throughout the disk with a rise of the 12 CO 2–1/1–0 ratio towards the southeast. The values found are within the range as found in other galaxies. Radiative transfer calculations indicate a kinetic temperature of 10–15 K and H 2 densities on the order of 3 × 10 4 cm −3. In the southeastern part of the dust lane, the gas appears to be warmer (20–30 K) which may be due to a greater star formation activity. The total molecular mass is about 1.2 × 10 8 M. The observed 12 CO 2–1 intensity is consistent with the molecular gas being distributed in a thin, warped disk as modelled by Quillen et al. (1992). Comparison of the measurements with the kinematic model shows good agreement.