VLBI observations of the CORALZ sample: young radio sources at low redshift

TL;DR

This study uses VLBI observations to analyze the morphology, size, and expansion speeds of young, low-redshift radio-loud active galactic nuclei in the CORALZ sample, providing insights into their early evolution.

Contribution

First VLBI-based morphological and size analysis of the CORALZ sample, with constraints on expansion speeds and evidence supporting synchrotron self-absorption as the cause of spectral turnovers.

Findings

Low expansion velocity in one source compared to high-redshift GPS sources.

Spectral turnovers are consistent with synchrotron self-absorption after size correction.

Correlation between spectral peak frequency and size is confirmed after flux-density correction.

Abstract

Young radio-loud active galactic nuclei form an important tool to investigate the evolution of extragalactic radio sources. To study the early phases of expanding radio sources, we have constructed CORALZ, a sample of 25 compact ($\theta<2"$) radio sources associated with nearby ($z<0.16$) galaxies. In this paper we determine the morphologies, linear sizes, and put first constraints on the lobe expansion speeds of the sources in the sample. We observed the radio sources from the CORALZ sample with MERLIN at 1.4 GHz or 1.6 GHz, the EVN at 1.6 GHz, and global VLBI at 1.6 GHz and/or 5.0 GHz. Radio maps, morphological classifications, and linear sizes are presented for all sources in the CORALZ sample. We have determined a first upper limit to the expansion velocity of one of the sources, which is remarkably low compared to the brighter GPS sources at higher redshifts, indicating a relation between radio luminosity and expansion speed, in agreement with analytical models. In addition we present further strong evidence that the spectral turnovers in GPS and CSS sources are caused by synchrotron self-absorption (SSA): the CORALZ sources are significantly offset from the well-known correlation between spectral peak frequency and angular size, but this correlation is recovered after correcting for the flux-density dependence, as predicted by SSA theory.