Introduction

The radio source PKS 1322-427 (Centaurus A, NGC 5128) is associated with a peculiar elliptical galaxy and is the closest radio galaxy to us, at a distance of 3.5 0.2 Mpc [Hui et al. 1993]. Centaurus A was one of the first radio sources to be identified with an extragalactic object [Bolton, Stanley, & Slee 1949] and is one of the brightest extragalactic radio sources in the sky with an integrated flux density of 681 34 Jy at a wavelength of 6 cm [Junkes et al. 1993]. As the closest radio galaxy, Centaurus A offers the best spatial resolution return for high angular resolution (1 61 kpc, 1' 1 kpc, 1'' 17 pc, and 1 mas 0.017 pc) and the most detailed view of an extragalactic radio jet on sub-pc-scales.

The entire radio source occupies approximately 3.5 8.5 on the sky, including the faint extended emission which is orientated along a position angle of approximately 0 [Junkes et al. 1993]. Straddling the optical galaxy along a position angle of approximately 50 and separated by approximately 15 kpc are two lobes of radio emission which have been imaged in detail with the VLA [Clarke, Burns, & Norman 1992]. An unresolved and inverted spectrum radio core which is coincident with the centre of the optical galaxy is connected to the north-east lobe by a radio jet [Burns, Feigelson, & Schreier 1983]. At the dynamic range of the existing VLA images, no jet has been found connecting the radio core to the south-west radio lobe.

VLBI observations of Centaurus A are aimed at imaging the inverted spectrum radio core. Wade et al. [1971] undertook the first high resolution radio observations of Centaurus A and reported a source less than 0.''5 in extent, although noting that the inverted spectrum of the core implied a much smaller extent. Preston et al. [1983], from 2.3 GHz VLBI observations, showed the compact core to consist of a component 50 mas in length at a position angle of 30 20 , identifying the component as a jet which possibly aligned with the kpc-scale radio structure. Preston et al. [1983] detected no structure with spatial extent less than 0.02 pc whereas at higher frequencies a component of 0.01 pc size was known to exist. Preston et al. [1983] therefore proposed a model for the nuclear radio source in Centaurus A which consisted of two components, the 50 mas jet which is seen with low frequency observations and a more compact component likely to be the core, seen only at higher frequencies since it is severely self-absorbed at lower frequencies.

VLBI observations of Centaurus A were undertaken during the first SHEVE observing session in 1982. Meier et al. [1989] described the 2.3 and 8.4 GHz observations aimed at detecting both the compact self-absorbed core component and the 50 mas jet component. Meier et al. [1989] extended the suggested model of Preston et al. [1983] and constructed from their two-frequency observations a model for the source which consisted of a weak self-absorbed core and an extended jet component some 100 mas away which can be seen at both frequencies. The core-jet position angle was estimated to be 51 3 , in good agreement with the jet position angle from VLA images of the kpc-scale radio structure.

In 1991, VLBI imaging observations of Centaurus A were successful for the first time at 8.4 GHz. Within only a few months significant changes were seen in the sub-pc-scale structure of Centaurus A [Meier et al. 1993; Tingay et al. 1994], in stark contrast to the results at 2.3 GHz. The observations presented in this chapter were motivated by the results of these early 8.4 GHz observations.

The contents of this chapter describe the results of the first concerted effort to monitor the radio core of Centaurus A at frequencies of 4.8 and 8.4 GHz with VLBI arrays which allow high quality images to be formed. This program of observations had two primary aims:

1] To determine the detailed sub-pc-scale structure of the nuclear radio source, especially its frequency dependent structure.

2] To follow evolution in the nuclear source on the sub-pc-scale over an extended period of time.

In 5.2, near simultaneous observations at 4.8 and 8.4 GHz which examine the sub-pc-scale structure are presented. In 5.3, the results of approximately 4.3 years of monitoring observations at 8.4 GHz and an examination of the evolution in the sub-pc-scale structure are presented. In 5.4, observations combining the VLBA and SHEVE arrays which yield the highest resolution images of Centaurus A yet and show evidence for a sub-pc-scale counterjet are presented. Finally, 5.6 presents the conclusions.