PKS 2152-699, at a red shift of 0.0282 [Tadhunter et al. 1988], is a powerful radio galaxy with a monochromatic luminosity of 
W Hz 
at 5 GHz. It is one of the brightest radio sources in the sky at 2.3 GHz [Wall 1994] and was identified optically by Westerlund and Smith [1966].
The ATCA image of the kpc-scale radio structure [Fosbury et al. 1990] shows the double-lobed structure of FR II type radio sources. PKS 2152-699 is also known to harbour a pc-scale radio core between the two lobes. The astrometric VLBI observations of Reynolds et al. [1994] show that the core possesses an unresolved component on 800 km baselines at 2.3 and 8.4 GHz (angular resolution of approximately 4 mas at 8.4 GHz).
The optical counterpart to the radio source has been the target of observations which have revealed, in addition to strong nuclear emission line activity, a region of extra-nuclear activity with high ionisation emission lines and a highly polarised blue continuum [Tadhunter et al. 1987; di Serego Alighieri et al. 1988].
Tadhunter et al. [1988] report on extensive observations of the emission line regions and the kpc-scale radio source. They find the extra-nuclear emission line region (ENELR) to lie 10'' ( 
5 kpc for H=75 km/s/Mpc) from the nuclear region, covering position angles between 35 
and 50 . The north-east and south-west radio lobes lie approximately 46'' and 19'' respectively from the nuclear region and are connected along a position angle of 23 
2 [Tadhunter et al. 1988; Norris et al. 1990]. The properties of the ENELR and its near alignment with the radio axis led Tadhunter et al. [1988] to suggest it as the site of interaction between the radio jet connecting the central core and extended lobes, and a cloud of gas away from the nuclear region of the galaxy. They suggest that the 12-25 misalignment between the radio structure and ENELR could be caused by the deflection of the radio jet in the interaction.
Similarities have been found between PKS 2152-699 and 3C277.3, a source for which evidence of jet/cloud interactions is persuasive [Miley et al. 1981; van Breugal et al. 1985]: the emission line spectra are similar, both possess a strong ionising continuum local to the emission line region, the radio axes and the optical axes are misaligned by similar amounts. Tadhunter et al. [1988] found that the photoionisation models they considered showed that the ENELR in PKS 2152-699 could be ionised by the source of continuum radiation found local to the ENELR but not by a source at the site of the active nucleus, unless the radiation is strongly anisotropic and concentrated in the direction of the cloud. They suggested that the local ionising source could possibly be produced in the interaction between the radio jet and the cloud. However, primarily on the basis of the blue continuum and optical polarisation of the ENELR, di Serego Alighieri et al. [1988] and Fosbury et al. [1990] strongly favour a scenario in which the cloud is ionised by a highly anisotropic photon beam which originates at the galaxy nucleus.
The aims of this chapter are as follows:
1] To present the first VLBI imaging observations of PKS 2152-699, strengthening the evidence for a jet/cloud interaction in this source.
2] To present a model which, based upon observational data, plausibly explains the relationship between the optical and radio structures associated with this galaxy.
The results of the VLBI imaging observations of the compact central component at the core of the PKS 2152-699, which show that the nuclear radio jet is closely aligned with the ENELR are presented in 
8.2. The evidence for the previously suggested interaction is therein strengthened. A simple mechanism which plausibly explains the misalignment between radio axis and ENELR, consistent with the concept of an extra-nuclear interaction, is explored in 8.3.