Variability and parsec-scale radio structure of candidate compact symmetric objects

TL;DR

This study uses multi-epoch VLA and VLBA observations to analyze the structure, variability, and evolution of candidate compact symmetric objects, revealing diverse properties and rapid evolution in some sources, challenging their classification as classical radio galaxies.

Contribution

It provides new high-resolution VLBA imaging and variability analysis of faint CSO candidates, demonstrating that many are young, evolving sources with diverse structures and behaviors.

Findings

42% of sources show resolved double structures

Most sources exhibit decade-long variability, with some blazar-like behavior

Estimated ages of some sources are a few tens to hundreds of years

Abstract

We report results on multi-epoch Very Large Array (VLA) and pc-scale Very Long Baseline Array (VLBA) observations of candidate compact symmetric objects (CSOs) from the faint sample of high frequency peakers. New VLBA observations could resolve the radio structure in about 42 per cent of the observed sources, showing double components that may be either mini-lobes or core-jet structures. Almost all the sources monitored by the VLA show some variability on time scale of a decade, and only 1 source does not show any significant variation. In 17 sources the flux density changes randomly as it is expected in blazars, and in 4 sources the spectrum becomes flat in the last observing epoch, confirming that samples selected in the GHz regime are highly contaminated by beamed objects. In 16 objects, the pc-scale and variability properties are consistent with a young radio source in adiabatic expansion, with a steady decrease of the flux density in the optically-thin part of the spectrum, and a flux density increase in the optically-thick part. For these sources we estimate dynamical ages between a few tens to a few hundreds years. The corresponding expansion velocity is generally between 0.1c and 0.7c, similar to values found in CSOs with different approaches. The fast evolution that we observe in some CSO candidates suggests that not all the objects would become classical Fanaroff-Riley radio sources.