Extragalactic peaked-spectrum radio sources at low frequencies are young radio galaxies^⋆

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: slob@strw.leidenuniv.nl
² ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, Dwingeloo 7991 PD, The Netherlands
³ Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁴ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
⁵ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁶ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK

Received: 1 August 2022
Accepted: 28 October 2022

Abstract

We present a sample of 373 peaked-spectrum (PS) sources with spectral peaks around 150 MHz, selected using a subset of the two LOw Frequency ARray (LOFAR) all-sky surveys, the LOFAR Two Meter Sky Survey and the LOFAR LBA Sky Survey. These LOFAR surveys are the most sensitive low-frequency widefield surveys to date, allowing us to select low-luminosity peaked-spectrum sources. Our sample increases the number of known PS sources in our survey area by a factor 50. The 5 GHz luminosity distribution of our PS sample shows we sample the lowest luminosity PS sources to date by nearly an order of magnitude. Since high-frequency gigahertz-peaked spectrum sources and compact steep-spectrum sources are hypothesised to be the precursors to large radio galaxies, we investigate whether this is also the case for our sample of low-frequency PS sources. Using optical line emission criteria, we find that our PS sources are predominately high-excitation radio galaxies instead of low-excitation radio galaxies, corresponding to a quickly evolving population. We compute the radio source counts of our PS sample, and find they are scaled down by a factor of ≈40 compared to a general sample of radio-loud active galactic nuclei (AGN). This implies that the lifetimes of PS sources are 40 times shorter than large-scale radio galaxies if their luminosity functions are identical. To investigate this, we compute the first radio luminosity function for a homogeneously selected PS sample. We find that for 144 MHz luminosities ≳10²⁵ W Hz⁻¹, the PS luminosity function has the same shape as an unresolved radio-loud AGN population, but shifted down by a factor of ≈-pagination10. We interpret this as strong evidence that these high-luminosity PS sources evolve into large-scale radio-loud AGN. For local low-luminosity PS sources, there is a surplus of PS sources, which we hypothesise to be the addition of frustrated PS sources that do not evolve into large-scale AGN.