The evolution of extragalactic peaked-spectrum sources down to 54 megahertz

¹ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
² NSF NOIRLab, Gemini Observatory, 670 N A’ohoku Place Hilo, HI 96720, USA
³ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁴ CSIRO Space and Astronomy, ATNF, PO Box 1130 Bentley, WA 6102, Australia
⁵ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy

^⋆ Corresponding author; szhai@strw.leidenuniv.nl

Received: 13 November 2024
Accepted: 28 January 2025

Abstract

Peaked-spectrum sources, known for their distinct peaked radio spectra, are a type of radio-loud active galactic nuclei. One subtype, megahertz-peaked-spectrum (MPS) sources, which exhibit a spectral peak at a frequency of a hundred megahertz, have emerged as a potential tool for identifying high-redshift candidates. However, the potential evolutionary link between the fraction of these sources and their redshifts remains unclear and requires further investigation. The recent, high-sensitivity Low Frequency Array (LOFAR) surveys enable statistical studies of these objects down to ultra-low frequencies (< 150 MHz). In this study we first used the multi-radio data to investigate the evolution of spectral index with redshift for 1187 quasars from the 16th SDSS quasar catalog. For each quasar, we analyzed available data from the LOFAR Low Band Antenna at 54 MHz, the High Band Antenna at 144 MHz, and the Very Large Array Faint Images of the Radio Sky at Twenty centimeters at 1.4 GHz. We measured the spectral index (α₅₄¹⁴⁴ and α₁₄₄¹⁴⁰⁰) and find no significant change in their median values with redshift. Extended sources have steeper spectral indices than compact sources, which is consistent with previous findings. Based on the spectral index information, we identified MPS sources using the criteria α₅₄¹⁴⁴ > = 0.1 and α₁₄₄¹⁴⁰⁰ < 0, and analyzed their properties. We find that the fraction of MPS sources is constant with the redshift (0.1 − 4.8), bolometric luminosity (10⁴⁴ − 10⁴⁸ erg/s), and supermassive black hole mass (10⁷ − 10^10.5 M_⊙), which suggests that MPS sources have relatively stable physical conditions or formation mechanisms across various evolutionary stages and environments.