Radio Detection of a Local Little Red Dot

L. F. Rodriguez, I. F. Mirabel

First listed 2025-12-03 | Last updated 2025-12-29

Abstract

Context. One of the most important discoveries by the James Webb Space Telescope (JWST) is the unexpected existence in the Early Universe (z > 4) of very large quantities of "Little Red Dots" (LRDs), compact luminous red galaxies of intriguing physical properties. One of those intriguing properties is the absence of radio detections in high redshift LRDs. Aims. We wish to know if LRDs have radio emission that may be produced by accreting Intermediate/Supermassive Black Holes (IMBHs/SMBHs) or by frequent supernovae (SNe) from a cluster of massive stars. Methods. Assuming LRDs at high redshifts have not been detected at radio wavelengths due to their large distances and/or present limitations of observational capabilities, we analyse here archive Very Large Array radio observations of J1047+0739 and J1025+1402, two analog candidates of LRDs in the Local Universe (LLRDs) at redshifts z = 0.1 - 0.2. Results. The LLRD source J1047+0739 at z = 0.1682 is detected at 6.0 GHz in 2018 with the VLA-A (Very Large Array) as a compact source with radius less than 0.2 arcsec ($<$700 pc at d = 750 Mpc). Its flux density was 117$\pm$8 $μ$Jy and its in-band spectral index was -0.85$\pm$0.24, which is typical of optically-thin synchrotron emission. It was also detected at 5.0 GHz in 2010 with the VLA-C, showing a flux density of 130$\pm$9 $μ$Jy. Conclusions. The observed flux densities can be provided by either a radio luminous supernova or an accreting IMBH/SMBH. However, the lack of important variation in flux density over eight years favors the IMBH/SMBH hypothesis. Radio time monitoring of this and other LLRDs could help clarify the mystery of the radio silence of its cosmological counterparts.

Short digest

Archival VLA imaging of two local LRD analogs (LLRDs) targets the radio-quiet puzzle seen at z>4 by testing whether nearby counterparts emit at cm wavelengths. J1047+0739 (z=0.1682) is detected as a compact (<0.2″) source with S6GHz=117±8 µJy in 2018 (A-config) and an in-band α=−0.85±0.24; an earlier 5 GHz detection in 2010 (C-config) of 43±3 µJy implies a flux rise by 2.3±0.2 over 7.5 yr. The optically thin spectrum and year-scale variability are argued to be consistent with a radio‑luminous SN scenario, though an accreting IMBH/SMBH cannot be excluded. For J1025+1402, a 4σ (57±14 µJy) unresolved source 2″ east hints at a jet-like counterpart, motivating radio monitoring of LLRDs to explain the high‑z LRD radio silence.

Key figures to inspect

• Figure 1 (2010 C-config, 5 GHz): Verify the compact detection of J1047+0739 at the Gaia EDR3 position, the ~43 µJy flux relative to the 14 µJy rms, and the larger beam that could dilute structure compared to 2018.
• Figure 1 (2018 A-config, 6 GHz): Check the unresolved morphology (<0.2″), brighter ~117 µJy flux, and positional agreement with Gaia; contrast beam sizes to assess whether structural resolution affects measured flux.
• Figure 2 (in-band spectrum, 2018): Inspect the four 1‑GHz sub-band points and the log–log fit yielding α≈−0.85±0.24, confirming optically thin synchrotron without curvature.
• Figure 3 (J1025+1402 field, 5 GHz): Examine the 4σ point source ~2″ east of the optical position (57±14 µJy), its unresolved size limit (<1700 pc) and spatial offset relevant to a possible compact jet association.