Astron. Astrophys. 324, 51-64 (1997)

1. Introduction

The nearby giant elliptical galaxy Cen A (NGC 5128) contains a warped disk of dust and dense gas. Optically the disk is seen as obscuration against a background of the old stellar population belonging to the elliptical galaxy. The presence of H emission in the disk (cf. Nicholson et al. 1992) suggest that formation of massive stars are currently taking place in the disk.

The disk is also a source of molecular line emission. Several studies have shown that it contains about of H₂. The distribution of the molecular gas has been traced by the CO emission (Eckart et al. 1990a, Quillen et al. 1992, Rydbeck et al. 1993, Wild et al. 1997). The emission extends to a galactocentric distance of approximately 1 kpc. The molecular gas distribution and its kinematics is consistent with a thin disk which is severly warped (Quillen et al. 1992). The main components are a ring or spiral arm at a galactocentric distance of 800 pc (adopting a distance of 3 Mpc to Cen A, which means that 1" corresponds to 14.5 pc.) and a circumnuclear ring at a radius of 100 pc (Israel et al. 1990, Rydbeck et al. 1993). The inner ring is seen as high velocity wings in spectra towards the center when the angular resolution is better than 25-30". It has also been imaged with the aid of deconvolution of single dish CO(2-1) data (Rydbeck et al. 1993). The inner ring is inclined relative to the outer disk but aligned perpendicular to the inner radio jet. The molecular gas properties of the disk appears to be similar to those found in normal spiral galaxies (cf. Israel et al. 1990, Eckart et al. 1990, Wild et al. 1997).

The radio core is hidden behind a large column of dense obscuring gas. The combination of a strong radio continuum source and a large column of dense gas makes the line of sight towards the center of Cen A a rich source of molecular absorption lines.

The properties of the gas seen in absorption is still largely unknown. Several studies have come up with conflicting results, both concerning the location of the absorption components relative to the nucleus as well as the temperature and density of the gas. The HI absorption towards the nucleus shows three absorption components; one strong at the systemic velocity around 552 km s^-1 and two redshifted ones at 596 and 609 km s^-1, respectively. Towards the inner jet, only the main absorption at 552 km s^-1 is seen (van der Hulst et al. 1983). This has been interpreted as evidence that the main 552 km s^-1 line is situated far out in the disk, while the redshifted lines are situated very close to the nucleus, possibly falling in to the center. However, Seaquist & Bell (1990) report a detection of redshifted H₂ CO 2cm absorption against the inner jet at a velocity of 576 km s^-1. This is not necessarily a proof against the redshifted component being situated close to the nucleus. The inner jet is seen at a projected distance of 20" from the core, which corresponds to 300 pc. The inner molecular disk can be extended on these scales (cf. Israel et al. 1991, Rydbeck et al. 1993, Hawarden et al. 1993). The molecular absorption lines seen in the millimeter range only occurs towards the radio core of Cen A, since the inner jet has a steep radio spectrum with a completely negligible continuum flux at mm wavelengths.

Molecular absorption lines seen in our Galaxy (cf. Lucas & Liszt 1996) and towards high redshift galaxies (Wiklind & Combes 1996a, b, 1995, Combes & Wiklind 1996) almost exclusively arise in very cold gas (in terms of excitation temperature). Although the abundance ratio of HCN/HNC imply that the kinetic temperature can be in the range 10-20 K, the excitation temperature is comparable to the cosmic microwave background. This suggests diffuse gas with n(H₂) cm^-3. Are the molecular absorption lines seen in Cen A likewise coming from diffuse gas? Unfortunately very few multiline transitions of the same molecule have been observed. One exception is H₂ CO, for which Seaquist & Bell (1990) derive an upper limit to the excitation temperature of 3.9 K. Several OH absorption features have been detected by van Langevelde et al. (1995), their interpretation is complicated by the presence of maser lines. Some come from diffuse gas, and some features point to dense clumps (n(H₂) 10⁴ cm^-3). The three lowest rotational lines of CO have been seen in absorption, both the lines around the systemic velocity and the redshifted components. The mere detection of the CO(3-2) line (cf. Israel et al. 1991) implies that the excitation temperature is relatively high (10-20 K). For CO, however, the analysis is complicated by confusion with emission, especially for the two lowest transitions.

In this paper we present new high quality observations of the (1-0), HCN(1-0), HNC(1-0) and CS(2-1) absorption lines, as well as the previously unobserved (1-0) transition, in order to shed some light on the physical properties and location of the molecular absorption towards the radio core of Cen A.

In Sect. 2 we present the observations, in Sect. 3 we identify the different absorption components and question their possible time variations and in Sect. 4 we derive column densities and abundance ratios. In Sect. 5 we discuss the implied properties of the absorbing gas in Cen A, and its assumed distance from the center.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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