2000A&A...359..483W

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 10³cm^–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 10⁴cm^–3 molecular gas at the center and along the prominent dust lane at offset positions ±60'' and ±100''. We also obtained a few spectra of CS(2-1) and (3-2) tracing densities of ∼10⁵cm^–3. The emission in these lines is weak and reaches a few mK at the available angular resolutions of 54''-36''. At the central position, the integrated intensity ratio I(HCN)/I(CO) peaks at 0.064, and decreases to ∼0.02 to 0.04 in the dust lane. Using the new high density tracer data, we estimate the amount, distribution and physical conditions of the dense molecular gas in the dust lane of Centaurus A. We find that Cen A and the Milky Way are comparable in their HCN(1-0) line luminosity. However, towards the nucleus the fraction of dense molecular gas measured via the line luminosity ratio L(HCN)/L(CO) as well as the star formation efficiency L_FIR/L_CO is comparable to ultra-luminous infrared galaxies (ULIRGs). Within the off-nuclear dust lane and for Cen A as a whole these quantities are between those of ULIRGs and normal and infrared luminous galaxies. This suggests that most of the FIR luminosity of Centaurus A originates in regions of very dense molecular gas and high star formation efficiency.