Radio Detection of a Local Little Red Dot

TL;DR

This study investigates whether local analogs of high-redshift luminous red galaxies emit radio waves, finding a compact source with properties suggesting either supernova activity or accreting black holes, with evidence favoring the latter.

Contribution

First analysis of radio emission from local analogs of high-redshift LRDs, providing insights into their possible black hole activity.

Findings

Detected a compact radio source with synchrotron emission characteristics.

Flux stability over eight years suggests accreting black holes rather than supernovae.

Supports the hypothesis that high-redshift LRDs may host active black holes.

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 $\mu$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 $\mu$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.