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Radio Detection of a Local Little Red Dot

L. F. Rodriguez, I. F. Mirabel

TL;DR

This work tackles why high-redshift Little Red Dots (LRDs) are radio silent by examining two local LRD analogs with archival VLA data. It finds that J1047+0739 exhibits persistent, optically-thin synchrotron radio emission with little variability, while J1025+1402 shows a nearby potential compact jet; both emission scenarios include accreting IMBH/SMBH activity or clustered supernovae, though the steady flux favors a black hole origin or a population of SNe rather than a single event. The authors quantify the radio luminosity and compare it to Seyfert galaxies, arguing that moderate BH accretion could explain the observations, while a luminous SN plus multiple events remains plausible. They also predict that identical sources at cosmological distances would be extremely faint (tens of nJy at cm wavelengths for $z\sim5$), but could be detectable with long ngVLA integrations, highlighting the importance of long-term radio monitoring to unravel the radio properties of early-universe LRDs.

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.

Radio Detection of a Local Little Red Dot

TL;DR

This work tackles why high-redshift Little Red Dots (LRDs) are radio silent by examining two local LRD analogs with archival VLA data. It finds that J1047+0739 exhibits persistent, optically-thin synchrotron radio emission with little variability, while J1025+1402 shows a nearby potential compact jet; both emission scenarios include accreting IMBH/SMBH activity or clustered supernovae, though the steady flux favors a black hole origin or a population of SNe rather than a single event. The authors quantify the radio luminosity and compare it to Seyfert galaxies, arguing that moderate BH accretion could explain the observations, while a luminous SN plus multiple events remains plausible. They also predict that identical sources at cosmological distances would be extremely faint (tens of nJy at cm wavelengths for ), but could be detectable with long ngVLA integrations, highlighting the importance of long-term radio monitoring to unravel the radio properties of early-universe LRDs.

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 1178 Jy and its in-band spectral index was -0.850.24, 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 1309 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.

Paper Structure

This paper contains 10 sections, 4 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Observations and Data Reduction
  3. LLRD J1047+0739
  4. LLRD J1025+1402
  5. Discussion
  6. The Nature of the Radio Emission of LLRD J1047+0739
  7. Can a Supernova Account for the Radio Emission Observed in J1047+0739?
  8. Can an IMBH/SMBH Account for the Radio Emission Observed in J1047+0739?
  9. Expected Radio Flux Density at Cosmological Distances
  10. Summary and Conclusions

Figures (3)

  • Figure 1: (Top) VLA contour image of the J1047+0739 region from project 10B-156. Contours start at $\pm$3-$\sigma$ and increase by factors of $\sqrt{2}$, where $\sigma$ = 9.0 $\mu$Jy beam$^{-1}$, the rms in this region of the image. The synthesized beam ($4\hbox{.}^{"}7 \times 3\hbox{.}^{"}5; -69^\circ$) is shown in the bottom left corner. (Bottom) VLA contour image of the J1047+0739 region from project 18A-413. Contours start at $\pm$3-$\sigma$ and increase by factors of $\sqrt{2}$, where $\sigma$ = 8.0 $\mu$Jy beam$^{-1}$, the rms in this region of the image. The synthesized beam ($0\hbox{.}^{"}30 \times 0\hbox{.}^{"}27; -56^\circ$) is shown in the bottom left corner. In both images the cross marks the optical position of the source from the Gaia Early Data Release 3 (Gaia Collaboration 2020).
  • Figure 2: Flux density as a function of frequency for J1047+0739 from the data of project 18A-413. The least squares fit is indicated with a dashed line.
  • Figure 3: VLA contour image of the J1025+1402 region from project 10B-156. Contours start at $\pm$3-$\sigma$ and increase by factors of $\sqrt{2}$, where $\sigma$ = 10.0 $\mu$Jy beam$^{-1}$, the rms in this region of the image. The synthesized beam ($4\hbox{.}^{"}8 \times 3\hbox{.}^{"}4; -73^\circ$) is shown in the bottom left corner. The cross marks the optical position of the source from the Gaia Early Data Release 3 (Gaia Collaboration 2020).