What are the megahertz peaked-spectrum sources?

TL;DR

This study uses EVN observations to analyze megahertz peaked-spectrum sources, revealing most are compact, young AGN with sizes under 1.1 kpc, supporting their use as high-redshift AGN indicators.

Contribution

The paper demonstrates that low-frequency colour-colour diagrams effectively select compact AGN, with EVN confirming their small sizes and young nature, and distinguishes them from blazars.

Findings

Nine out of eleven sources detected with EVN.

All detected sources are compact AGN smaller than 1.1 kpc.

Sources are likely young and not blazars.

Abstract

Megahertz peaked-spectrum (MPS) sources have spectra that peak at frequencies below 1 GHz in the observer's frame and are believed to be radio-loud active galactic nuclei (AGN). We recently presented a new method to search for high-redshift AGN by identifying unusually compact MPS sources. In this paper, we present European VLBI Network (EVN) observations of 11 MPS sources which we use to determine their sizes and investigate the nature of the sources with ~10 mas resolution. Of the 11 sources, we detect nine with the EVN. Combining the EVN observations with spectral and redshift information, we show that the detected sources are all AGN with linear sizes smaller than 1.1 kpc and are likely young. This shows that low-frequency colour-colour diagrams are an easy and efficient way of selecting small AGN and explains our high detection fraction (82%) in comparison to comparable surveys. Finally we argue that the detected sources are all likely compact symmetric objects and that none of the sources are blazars.