VLBI images at 327 MHz of compact steep spectrum and GHz-peaked spectrum sources from the 3C and PW samples

TL;DR

This study uses low-frequency VLBI imaging at 327 MHz to analyze the structure, energetics, and ages of CSS and GPS radio sources, revealing their youth and the dominance of lobes in energy storage.

Contribution

First low-frequency VLBI imaging of CSS and GPS sources, providing insights into their structure, energetics, and age estimates, highlighting the importance of lobes in source evolution.

Findings

80% of sources show double/triple structures

Lobes contain most of the energy budget

Dynamical ages range from 2,000 to 50,000 years

Abstract

We present results on global very long baseline interferometry (VLBI) observations at 327 MHz of eighteen compact steep-spectrum (CSS) and GHz-peaked spectrum (GPS) radio sources from the 3C and the Peacock & Wall catalogues. About 80 per cent of the sources have a 'double/triple' structure. The radio emission at 327 MHz is dominated by steep-spectrum extended structures, while compact regions become predominant at higher frequencies. As a consequence, we could unambiguously detect the core region only in three sources, likely due to self-absorption affecting its emission at this low frequency. Despite their low surface brightness, lobes store the majority of the source energy budget, whose correct estimate is a key ingredient in tackling the radio source evolution. Low-frequency VLBI observations able to disentangle the lobe emission from that of other regions are therefore the best way to infer the energetics of these objects. Dynamical ages estimated from energy budget arguments provide values between 2x10^3 and 5x10^4 yr, in agreement with the radiative ages estimated from the fit of the integrated synchrotron spectrum, further supporting the youth of these objects. A discrepancy between radiative and dynamical ages is observed in a few sources where the integrated spectrum is dominated by hotspots. In this case the radiative age likely represents the time spent by the particles in these regions, rather than the source age.