Dense gas in the dust lane of Centaurus A

TL;DR

This study investigates dense molecular gas in Centaurus A using millimeter observations of HCN and CS lines, revealing the distribution and density of gas in the galaxy's dust lane and center.

Contribution

First detection and mapping of dense molecular gas in Centaurus A's dust lane using HCN and CS lines, expanding understanding of its interstellar medium.

Findings

Dense gas traced by HCN and CS is weak but present in the dust lane and center.

The HCN/CO ratio peaks at the galaxy's center, indicating higher dense gas concentration.

Densities of traced gas are around 10^4 to 10^5 cm^-3.

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 $\pm60^{\prime \prime}$ and $\pm100^{\prime \prime}$. We also obtained a few spectra of CS(2-1) and (3-2) tracing densities of $\sim10^5$ 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.