Radio emission from little red dots may reveal their true nature

Muhammad A. Latif, Ammara Aftab, Daniel J. Whalen, Mar Mezcua

First listed 2025-02-06 | Last updated 2025-02-06

Abstract

The unprecedented sensitivity of the \textit{James Webb Space Telescope} (\textit{JWST}) has revolutionized our understanding of the early universe. Among the most intriguing \textit{JWST} discoveries are red, very compact objects showing broad line emission features nicknamed as little red dots (LRDs). The discovery of LRDs has triggered great interest about their origin as either extremely starbursting galaxies or highly-obscured active galactic nuclei (AGN). Their exact nature still remains unknown. 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 employ the fundamental plane of black hole (BH) accretion to estimate radio emission from AGN and the stellar radio fluxes from their host galaxies. We assume a range of BH mass, X-ray luminosity ($\rm L_{X}$) and star formation rate (SFR) to bracket the likely properties of LRDs. Our findings suggest that BH radio fluxes from LRDs are 10-100 times higher than the stellar fluxes from their host galaxies, depending on BH mass, $\rm L_X$ and SFR. The detection of a $\sim$ 500 nJy signal above 2 GHz at $z \geq$ 5 or a $\sim$ 2000 nJy flux at $z =$ 3-4 would be a smoking gun for the presence of AGN provided that SFRs in the host galaxies are $\rm < 30~ M_{\odot} ~yr^{-1}$. We find that LRDs are most likely radio quiet AGN otherwise would have been already detected in the current radio surveys. Our findings suggest that LRDs can be detected with the upcoming radio observatories such as ngVLA and SKA with integration times of 10-100 hrs, respectively.

Short digest

Using the black-hole fundamental plane alongside star-formation radio scaling, the authors predict continuum fluxes for little red dots (LRDs) across BH mass, X-ray luminosity, SFR, and redshift (≈3–7). They find AGN radio fluxes are typically 10–100× the host stellar contribution at 3–8 GHz, and that a ≥500 nJy detection above 2 GHz at z ≥ 5 or ≥2000 nJy at z = 3–4 would be a clear AGN signature if SFR < 30 M⊙/yr. The analysis implies most LRDs are radio-quiet AGN, consistent with current survey non-detections. Forecasts show ngVLA and SKA could confirm typical LRD AGN in ~10–100 hr integrations, providing a dust-robust discriminator between starbursts and accretion.

Key figures to inspect

• Figure 1: Check where the AGN/SFR flux ratio at z = 7 crosses unity across the BH-mass–LX grid; this identifies the combinations where the AGN outshines star-formation for SFR = 1, 10, 30 M⊙/yr.
• Figure 2: Read off absolute AGN fluxes (min/max FP) versus redshift alongside host-galaxy SFR fluxes and the ngVLA/SKA 1/10/100 hr sensitivity curves; this shows the 1–3000 nJy AGN range, its decline with z, and where the ≥500 nJy (z ≥ 5) and ≥2000 nJy (z = 3–4) “smoking gun” thresholds lie.
• Figure 3: Compare radio-quiet and radio-loud AGN/SFR ratios at z = 7; note how introducing radio-loudness boosts ratios by orders of magnitude and makes AGN dominance robust even at higher SFR.
• Figure 4: Contrast radio-quiet vs radio-loud absolute fluxes across redshift against ngVLA/SKA limits at 3 and 8 GHz; use this to estimate which exposure times enable detections for typical LRD parameter choices.