Mm-wave HCO⁺, HCN and CO absorption toward NGC 1052

¹ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, 22903-2475, USA e-mail: hliszt@nrao.edu
² Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France

Received: 12 July 2004
Accepted: 26 August 2004

Abstract

We used the Plateau de Bure Interferometer to observe λ3 mm  absorption lines of HCO⁺, HCN and CO toward the core of the nearby elliptical, megamaser-host galaxy NGC 1052. The lines are relatively weak, with peak optical depths 0.03 for HCO⁺ and HCN and 0.1 for CO. Nonetheless the inferred column density of molecular gas 2N(H₂)  5  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/HCO⁺ ratio in NGC 1052 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 HCO⁺ for very nearly the same amount of OH absorption.