Molecular absorption lines toward the nucleus of Centaurus A

Abstract

Measurements of absorption in the lines of (C-12)O, (C-13)O, HCO(+), HCN, CN, and CS toward the nuclear continuum source in Centaurus A are reported. The spectra show a number of features which can be attributed to individual molecular clouds with properties similar to dense molecular clouds in the Galaxy. The deepest absorption at the systemic velocity of Cen 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. A (C-12)O fractional abundance is obtained which is in excellent agreement with typical values derived for the dense ISM in the Galaxy. For the central absorption feature, the (C-12)O and (C-13)O J = 2-1 to J = 1-0 line ratio and the limit on this ratio for CN indicate that the absorption occurs in very cold and/or subthermlly excited gas. 45 refs.