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

Abstract

Context. 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 (θ < 2 ) radio sources associated with nearby (z < 0.16) galaxies. Aims. In this paper we determine the morphologies, linear sizes, and place first constraints on the lobe expansion speeds of the sources in the sample. Methods. 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. Results. 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.

The Dynamic Evolution of Young Extragalactic Radio Sources

“…The observed range of spectral indices above the peak for samples of CSS and GPS sources is α = 0.5-0.9 (O'Dea et al 1990), which partially overlaps with the CSO range. The same range was derived for the spectral index above spectra peak for GPS and FR galaxies, suggesting that particle acceleration and energy loss mechanism preserve the same average spectral index over most of the lifetime of the source (de Vries et al 2009). Variability of the spectral index has also been associated with the HFP sources and to identify truly young sources (Orienti et al 2007).…”

“…The current (P rad -D) diagram contains similar but also different groups of data points than earlier versions, particularly for the region of early development of CSO radio sources. A sample of low-power GPS sources (de Vries et al 2009) has been included in the bottom left corner of the diagram as likely precursors of the low-power MSO sources (KunertBajraszewska et al 2010). In addition, there are more data points for the group of low-power FR Is (<10 24 W Hz −1 ; Laing et al 1983;Fanti et al 1987Fanti et al , 2001).…”

The dynamical ages of two low-luminosity young radio sources

“…It is therefore important to review the correlation between expansion speed and luminosity found by de Vries et al (2009). The expansion speed we determine here for CORALZ J083139+460800, exceeds the previously derived upper limit, but is still the lowest expansion speed measured in any GPS radio source so far.…”

Radio source evolution

“…At the time, selection effects of the available surveys were hiding the low-luminosity population of radio sources. More recent surveys of low-luminosity sources (e.g., de Vries et al 2009, Kunert-Bajraszewska et al 2010) have revealed a number of sources distributed throughout the diagram. A classification adapted to the new scenario revealed by low-luminosity observations has been proposed by An & Baan (2012), including predicted tracks depending on the initial power of the source and the activity period of the active galactic nucleus (AGN).…”