Evolution of the pc-scale structure of PKS 1934-638 revisited: first science with the ASKAP and New Zealand telescopes

TL;DR

This study revisits the structure and evolution of the radio galaxy PKS 1934-638 using new high-resolution VLBI observations, revealing that frequency-dependent effects, rather than expansion, dominate its apparent structural changes over 40 years.

Contribution

First VLBI observations with ASKAP and New Zealand telescopes at 1.4 GHz, highlighting the importance of opacity effects in interpreting source evolution.

Findings

Frequency-dependent structure effects are significant in PKS 1934-638.

Evidence for source expansion over 40 years is weak.

Opacity effects influence the perceived age and evolution of the radio source.

Abstract

We have studied the archetypal Gigahertz Peaked Spectrum radio galaxy, PKS 1934-638, using the Australian Long Baseline Array, augmented with two new telescopes that greatly improve the angular resolution of the array. These VLBI observations represent the first scientific results from a new antenna in NZ and the first antenna of the Australian SKA Pathfinder (ASKAP). A compact double radio source, PKS 1934-638, has been monitored over a period of 40 years, and the observation described here provides the latest datum, eight years after the previous observation, to aid in the study of the long-term evolution of the source structure. We take advantage of these new long baselines to probe PKS 1934-638 at the relatively low frequency of 1.4 GHz, in order to examine the effects of optical depth on the structure of the radio source. Optical depth effects, resulting in the observation of frequency dependent structure, may have previously been interpreted in terms of an expansion of the source as a function of time. Expansion and frequency dependent effects are important to disentangle in order to estimate the age of PKS 1934-638. We show that frequency dependent structure effects are likely to be important in PKS 1934-638 and present a simple two-dimensional synchrotron source model in which opacity effects due to synchrotron self-absorption are taken into account. Evidence for expansion of the radio source over 40 years is therefore weak, with consequences for the estimated age of the radio source.