Letter to the Editor

Radio emission from little red dots may reveal their true nature

¹ Physics Department, College of Science, United Arab Emirates University, PO Box 15551, Al-Ain, UAE
² Institute of Cosmology and Gravitation, Portsmouth University, Dennis Sciama Building, Portsmouth PO1 3FX, UK
³ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Magrans, 08193 Barcelona, Spain
⁴ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels (Barcelona), Spain

^⋆ Corresponding author; latifne@gmail.com

Received: 27 November 2024
Accepted: 31 January 2025

Abstract

Context. The unprecedented sensitivity of the James Webb Space Telescope (JWST) has revolutionized our understanding of the early Universe. Among the most intriguing JWST discoveries are red, very compact objects that show broad line emission features, nicknamed little red dots (LRDs). The discovery of LRDs has triggered great interest in their origin as either extremely starbursting galaxies or highly obscured active galactic nuclei (AGNs). Their exact nature remains unknown.

Aims. The goal of this work is to estimate the radio emission from LRDs and predict which radio surveys would detect them. To achieve these objectives, we employed the fundamental plane of black hole (BH) accretion to estimate radio emission from AGNs and the stellar radio fluxes from their host galaxies. We assumed a range of BH masses, X-ray luminosities (L_X), and star formation rates (SFRs) to bracket the likely properties of LRDs.

Methods. Our findings suggest that BH radio fluxes from LRDs are 10–100 times higher than the stellar fluxes from their host galaxies, depending on the BH mass, L_X, and SFR. The detection of a ∼500 nJy signal above 2 GHz at z ≥ 5 or a ∼2000 nJy flux at z = 3–4 would be a smoking gun for the presence of AGNs provided that SFRs in the host galaxies are < 30 M_⊙ yr⁻¹.

Results. We find that LRDs are most likely radio-quiet AGNs; otherwise, they would have already been detected in the current radio surveys. Our findings suggest that LRDs can be detected with upcoming radio observatories such as ngVLA and SKA with integration times of 10–100 h, respectively.