HI line absorption against the centre of Cen A over the velocity range of 500-600 km s-1 was first recognized in single-dish observations by Roberts (1970) and Whiteoak & Gardner (1971; 1976b). It was studied at high spatial (2.6" × 11") and moderate velocity (6 km s-1) resolutions by Van der Hulst, Golisch & Haschick (1983). Subsequently, centimetre wavelength absorption by various molecular species (OH, H2CO, C3H2, NH3) was observed by Gardner & Whiteoak 1976a, b; 1979; Seaquist & Bell 1986; 1988; 1990). The HI observations by van der Hulst et al. (1983) show the inner jets (20" and 30" northeast of the nucleus) as well as the nuclear source in absorption. The inner jets are also seen in H2CO absorption (Seaquist & Bell 1990), while H2CO absorption is likewise found in the direction of the dust band, 4' southeast from the nucleus (Gardner & Whiteoak 1976b).
The flat and strong nuclear continuum spectrum at millimetre wavelengths (Sect. 5.4) provides the opportunity to extend absorption studies to wavelengths where molecular line transitions are most abundant. High velocity resolution (V 2.6 km s-1) absorption spectra of the lower transitions of 12CO and 13CO were obtained by Israel et al. (1990; 1991) and Eckart et al. (1990b). Various other molecular species, such as HCO+, H13CO+, HCN, HNC, CS, C2H, CN, C3H2 and H2CO have likewise been detected in absorption (Eckart et al. 1990b; Israel et al. 1991; Wiklind & Combes 1997; Israel et al. unpublished). Centaurus A is unique in this sense: as of yet, no other active galactic nucleus is known to exhibit absorption by a molecular disk (Drinkwater, Combes & Wiklind 1996).
Below 10 GHz, emission from the inner and nuclear jet dominates the continuum, whereas above 30 GHz the continuum emission arises mostly or wholly from the nucleus. Thus, centimetre-wavelength single-dish measurements sample sightlines as far as 500 pc away from the nucleus while aperture synthesis observations, after elimination of the inner jet contribution, preferentially sample sightlines missing the nucleus itself by about 1.5 parsec, i.e. 200 times the diameter of the nuclear source. Only at (sub)millimetre wavelengths does the absorption sample a narrow line of sight (0.5 milliarcsec) directly to the nucleus.
A typical molecular line spectrum of the Centaurus A nucleus consists of a relatively strong emission line arising from the molecular disk (Sect. 4) as well as weaker and broader emission representing the circumnuclear disk (Sect. 4.2) superposed on the nuclear continuum (cf. Fig. 1 in Israel, 1992). As absorption occurs over a significant fraction of the total emission line spectrum, the intrinsic shape of the latter is hard to determine but of crucial importance in determining the pure absorption spectrum.
The absorption spectra show numerous peaks at various velocities (Fig. 12), relatively strong between 540 km s-1 VHel 555 km s-1, i.e. within 10 km s-1 from the systemic velocity. It is not clear which of these lines, if any, marks the systemic velocity. There are at least five (blended) components in this velocity range but only two of these have discernible HI counterparts in VLA spectra (van der Hulst et al. 1983; J.M. van der Hulst, private communication). As these two HI features are also seen against the inner jets at 30" (500 pc) above the circumnuclear disk, they originate most likely in the more distant molecular disk associated with the dust band structure (cf. van der Hulst et al. 1983). This is consistent with the apparent low excitation of the corresponding molecular absorption lines, which suggest the material to be located at considerable distance to the nucleus (Gardner & Whiteoak 1979; Seaquist & Bell 1990; Eckart et al. 1990b). The velocity range is consistent with the velocity dispersion in the molecular disk (Quillen et al. 1992).
Figure 12. Nuclear absorption spectrum in HCO+. Left: low velocity resolution (4.7 km s-1) spectrum showing continuum, molecular line emission from circumnuclear disk and dark band, and molecular line absorption covering large velocity range. Right: high velocity resolution (0.3 km s-1) spectrum showing several narrow absorption lines and "forest" of redshifted lines. Horizontal scale is VLRS, vertical scale is antenna temperature (corresponding to 0.75 × main-beam brightness temperature). From Israel et al. 1991, and unpublished data.
Of particular interest is a second absorption line system, extending from VHel = 560 km s-1. It is seen only against the nuclear source, and not against the inner jets (disregarding the marginal H2CO feature seen by Seaquist & Bell 1990). This high-velocity, redshifted absorption system takes the shape of continous molecular absorption with a few individual peaks of relatively low optical depth (Israel et al. 1991; Wiklind & Combes 1997). The redshifted absorption system is also prominent in HI (Gardner & Whiteoak 1976b; van der Hulst et al. 1983). With respect to e.g. HCO+, the HI absorption is relatively strong at 560 km s-1 VHel 600 km s-1, and weak at higher velocities. This redshifted system is generally interpreted as due to infalling clouds (Gardner & Whiteoak 1976b; van der Hulst et al. 1983; Seaquist & Bell 1990; Israel et al. 1991). It implies accretion rates sufficiently high to explain the overall radio luminosity of Centaurus A (van Gorkom et al. 1989). As the absorption optical depth ratios of the various species appear to be a function of (infall) velocity, this opens the interesting possibility of modelling the processing of material falling into the nucleus of Centaurus A. As the optical depths and consequently signal-to-noise ratios are low, this is not an easy task.
At velocities 500 km s-1 VHel 540 km s-1 there is a hint of a very low optical depth blueshifted HCO+ absorption wing (cf. Israel 1992; Wiklind & Combes 1997). Its nature is not clear, and it is not seen in HI (J.M. van der Hulst, private communication). High resolution OH maser absorption spectra against the nuclear source in the four transitions at 18 cm wavelength show exactly conjugate behaviour at all velocities in the satellite lines: the sum of the two transitions is zero (van Langevelde et al. 1995). This effect is caused by the competition of the two transitions for the same pumping photons and allows a direct determination of the OH column density N(OH) 6 × 10 15cm-2.