Radio Detection of a Local Little Red Dot

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. Aims. We wish to know if LRDs may host accreting Intermediate/Supermassive Black Holes (IMBHs/SMBHs) that may power LRDs, and compare them with the effect of clusters of massive stars. The spectrum of radio emission (synchrotron vs thermal) can be used to know which between these two types of energy sources is the dominant one. Methods. So far LRDs at high redshifts have not been detected at radio wavelengths and it is not known why. Assuming this could be due to their large distances and/or present limitations of observational capabilities, we analyze here archive Very Large Array radio observations of 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 of NRAO as a compact source with radii less than 0.2 arc sec (< 600 pc at d = 700 Mpc). Its flux density was 117$\pm$8 $μ$Jy and its spectral index was -0.85, which is typical of optically-thin synchrotron emission. It is also detected at 5.0 GHz in 2010 with the VLA-C, showing a flux density of 43$\pm$3 $μ$Jy. Conclusions. The observed flux densities can be provided by a radio luminous supernova. The increase in flux density over eight years can be explained as the result of two independent supernovae or as the radio re-brightening of a single one. Radio time monitoring of this and other LLRDs could help clarify the mystery of the radio silence of its cosmological counterparts.