MOLECULAR ABSORPTION-LINES TOWARD THE NUCLEUS OF CENTAURUS-A

We present measurements of absorption lines of 12CO, 13CO, HCO+, HCN, CN, and CS toward the nuclear continuum source of Centaurus A obtained with the Swedish-ESO Submillimeter Telescope (SEST). The spectra show a number of features with line widths of a few km s-1. They can be attributed to individual molecular clouds with properties similar to dense molecular clouds in our Galaxy. CO absorption-line ratios, as well as the hyperfine line ratios for CN and HCN, indicate that the deepest absorption at the systemic velocity of Centaurus A originates in quiescent molecular clouds that are cold, dense, and clumped and happen to lie along the line of sight to the central compact continuum source. Combining CO, H I, and 9.7 μm silicate absorption data, we obtain a 12CO fractional abundance of (5-11) × 10-5 which is in excellent agreement with the typical values derived for the dense interstellar medium in our Galaxy (8 × 10-5). Fractional abundances also of the other observed molecular species are comparable to those found in Galactic molecular clouds and in other galaxies. For the central absorption feature the 12CO and 13CO J = 2-1 to J = 1-0 line ratio, as well as a limit on this ratio for CN, indicate that the absorption occurs in very cold and/or subthermally excited gas. For this absorbing component LTE and non-LTE model calculations indicate a kinetic temperature between 4 K and 10 K, 12CO column densities of about (3-5) × 1017 cm-2, and hydrogen volume densities near the critical density of 2 × 104 cm3 required to populate the J = 2 level. The physical properties of the absorbing gas are comparable to those of the emitting molecular gas in the disk of Centaurus A. The fact that the absorbing molecular gas is cold and dense is in agreement with the finding that at the mean distance of the absorbing gas of 1 kpc radiative excitation from the nuclear source is not important. For the weak, broad, redshifted absorption features, neither a close proximity to the nucleus nor the importance of the nuclear continuum radiation field for their molecular excitation can be excluded.