Investigating the influence of radio-faint active galactic nuclei on the infrared-radio correlation of massive galaxies

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna Via Gobetti 93/3 I–40129 Bologna, Italy
² Jodrell Bank Centre for Astrophysics, University of Manchester Oxford Road Manchester M13 9PL, UK
³ Department of Physics, University of Pretoria Lynnwood Rd Hatfield Pretoria 0002, South Africa
⁴ CEA, Universitè Paris-Saclay, Université Paris Citè, CNRS, AIM 91191 Gif–sur–Yvette, France
⁵ University of the Witwatersrand Enoch Sontonga Ave Johannesburg, South Africa
⁶ University of Birmingham Birmingham B15 2TT, United Kingdom
⁷ INAF – Istituto di Radioastronomia Via Gobetti 101 I–40129 Bologna, Italy
⁸ Dipartimento di Fisica e Astronomia (DIFA), Università di Bologna Via Gobetti 93/2 I-40129 Bologna, Italy
⁹ Institute of Physics, Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL) 1290 Sauverny, Switzerland
¹⁰ Department of Physics, University of Zagreb Bijenička Cesta 32 Zagreb, Croatia
¹¹ Caltech/IPAC 1200 E. California Blvd. Pasadena CA 91125, USA
¹² University of Western Australia 35 Stirling Hwy Crawley WA 6009, Australia
¹³ Cosmic Dawn Center (DAWN), Denmark
¹⁴ DTU Space, Technical University of Denmark Elektrovej 327 2800 Kgs. Lyngby, Denmark
¹⁵ Swinburne University of Technology John Street Hawthorn Victoria 3122, Australia

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 8 July 2025
Accepted: 3 October 2025

Abstract

Context. It is well known that star-forming galaxies (SFGs) exhibit a tight correlation between their radio and infrared emissions, commonly referred to as the infrared-radio correlation (IRRC). Recent empirical studies have reported a dependence of the IRRC on the galaxy stellar mass, in which more massive galaxies tend to show lower infrared-to-radio ratios (q_IR) with respect to less massive galaxies. One possible, yet unexplored, explanation is a residual contamination of the radio emission from active galactic nuclei (AGNs), not captured through “radio-excess” diagnostics.

Aims. To investigate this hypothesis, we aim to statistically quantify the contribution of AGN emission to the radio luminosities of SFGs located within the scatter of the IRRC.

Methods. Our Very Large Baseline Array (VLBA) AGN-sCAN program has targeted 500 galaxies that follow the q_IR distribution of the IRRC, i.e., with no prior evidence for radio-excess AGN emission based on low-resolution (∼arcsec) VLA radio imaging. Our VLBA 1.4 GHz observations reach a 5σ sensitivity limit of 25 μJy/beam, corresponding to a radio-brightness temperature of T_b ∼ 10⁵ K. This classification serves as a robust AGN diagnostic, regardless of the host galaxy’s star formation rate.

Results. We detect four VLBA sources in the deepest regions, which are also the faintest VLBI-detected AGNs in SFGs to date. The effective AGN detection rate is 9%, when considering a control sample matched in mass and sensitivity, which is in good agreement with the extrapolation of previous radio AGN number counts. Despite the non-negligible AGN flux contamination (∼30%) in our individual VLBA detections, we find that the peak of the q_IR distribution is completely unaffected by this correction. Although we cannot rule out a high incidence of radio-silent AGNs at (sub)μJy levels among the VLBA non-detections, we derive a conservative upper limit of < 0.1 dex of their cumulative impact on the q_IR distribution. We conclude that residual AGN contamination from non-radio-excess AGNs is unlikely to be the primary driver of the M_★ – dependent IRRC.