HCO+ and HCN J=3-2 absorption toward the center of Centaurus A

TL;DR

This study investigates dense molecular gas in Centaurus A using HCO+ and HCN absorption lines, revealing relatively low gas densities around a few 10^4 cm^-3, which impacts understanding of the galaxy's central molecular environment.

Contribution

First detection and analysis of HCO+ and HCN (3-2) absorption lines toward Centaurus A's core, providing new constraints on the molecular gas density in this region.

Findings

HCO+ (3-2) absorption detected near systemic velocity

Gas density estimated at a few 10^4 cm^-3

No evidence for higher density molecular gas along the line of sight

Abstract

We have investigated the presence of dense gas toward the radio source Cen A by looking at the absorption of the HCO+ and HCN (3-2) lines in front of the bright continuum source with the Submillimeter Array. We detect narrow HCO+ (3-2) absorption, and tentatively HCN (3-2), close to the systemic velocity. For both molecules, the J=3-2 absorption is much weaker than for the J=1-0 line. From simple excitation analysis, we conclude that the gas density is on the order of a few 10^4 cm^-3 for a column density N(HCO+)/dV of 3x10^12 cm^-2 km^-1 s and a kinetic temperature of 10 K. In particular, we find no evidence for molecular gas density higher than a few 10^4 cm^-3 on the line of sight to the continuum source. We discuss the implications of our finding on the nature of the molecular gas responsible for the absorption toward Cen A.