Dense gas in the dust lane of Centaurus A

Abstract

The interstellar medium of Centaurus A (NGC 5128) has been studied extensively in recent years, using mostly molecular lines tracing low to medium density gas (500 to several $10^3$ cm${}^{-3}$). The amount and distribution of the dense molecular gas was largely unknown. Here we present new millimeter data of the HCN(1-0), CS(2-1), and CS(3-2) rotational transitions towards the nearby radio galaxy Centaurus A observed with the SEST on La Silla, Chile. We obtained spectra of the HCN(1-0) emission which traces dense 10${}^4$ cm${}^{-3}$ molecular gas at the center and along the prominent dust lane at offset positions $\pm 60^{\prime \prime }$ and $\pm 100^{\prime \prime }$. We also obtained a few spectra of CS(2-1) and (3-2) tracing densities of $\sim 10^5$ cm${}^{-3}$. The emission in these lines is weak and reaches a few mK at the available angular resolutions of 54${}^{\prime \prime }$ - 36${}^{\prime \prime }$. At the central position, the integrated intensity ratio I(HCN)/I(CO) peaks at 0.064, and decreases to $\sim$0.02 to 0.04 in the dust lane.

7 pages, 5 figures, accepted by A.&A. 04. May 2000