Mm-wave HCO+, HCN and CO absorption toward NGC1052

Abstract

We used the Plateau de Bure Interferometer to observe $\lambda$3mm J=1-0 absorption lines of \hcop, HCN and CO toward the core of the nearby elliptical, megamaser-host galaxy NGC1052. The lines are relatively weak, with peak optical depths 0.03 for \hcop and HCN and 0.1 for CO. Nonetheless the inferred column density of molecular gas 2N(\HH) $\simeq 5\times10^{21}~\pcc$ is consistent with the degree of reddening inferred toward the nucleus from observations of the Balmer series of hydrogen. Mm-wave absorption line profiles are somewhat broader than those of H I and OH, perhaps because lower free-free opacity at mm-wavelengths exposes higher-velocity material nearer the nucleus. Overall, the OH/\hcop ratio in NGC1052 is as expected from the strong relationship established in local diffuse clouds but the optical depth ratio varies strongly over the line profiles. Similar variations are also seen toward Cen A, which has very different line ratios among H I, OH and \hcop for very nearly the same amount of OH absorption.