X-ray View of Little Red Dots: Do They Host Supermassive Black Holes?

TL;DR

This study uses deep X-ray observations to investigate whether Little Red Dots, high-redshift galaxies discovered by JWST, host supermassive black holes, finding no strong evidence for over-massive SMBHs and highlighting discrepancies with JWST-based estimates.

Contribution

The paper provides the first X-ray constraints on SMBH presence in LRDs, challenging previous JWST-based mass estimates and suggesting different scaling relations may be needed at high redshift.

Findings

LRDs remain undetected in X-ray, setting SMBH mass upper limits.

Stacked analysis hints at SMBHs with masses around 3.2 million solar masses.

Results imply LRDs do not host over-massive SMBHs or accrete at high Eddington ratios.

Abstract

The discovery of Little Red Dots (LRDs) -- a population of compact, high-redshift, dust-reddened galaxies -- is one of the most surprising results from JWST. However, the nature of LRDs is still debated: does the near-infrared emission originate from accreting supermassive black holes (SMBHs), or intense star formation? In this work, we utilize ultra-deep Chandra observations and study LRDs residing behind the lensing galaxy cluster, Abell~2744. We probe the X-ray emission from individual galaxies but find that they remain undetected and provide SMBH mass upper limits of $\lesssim(1.5-16)\times10^{6}~\rm{M_{\odot}}$ assuming Eddington limited accretion. To increase the signal-to-noise ratios, we conduct a stacking analysis of the full sample with a total lensed exposure time of $\approx87$~Ms. We also bin the galaxies based on their stellar mass, lensing magnification, and detected broad-line H$\alpha$ emission. For the LRDs exhibiting broad-line H$\alpha$ emission, there is a hint of a stacked signal ($\sim2.6\sigma$), corresponding to a SMBH mass of $\sim3.2\times10^{6}~\rm{M_{\odot}}$. Assuming unobscured, Eddington-limited accretion, this BH mass is at least 1.5 orders of magnitude lower than that inferred from virial mass estimates using JWST spectra. Given galaxy-dominated stellar mass estimates, our results imply that LRDs do not host over-massive SMBHs and/or accrete at a few percent of their Eddington limit. However, alternative stellar mass estimates may still support that LRDs host over-massive BHs. The significant discrepancy between the JWST and Chandra data hints that the scaling relations used to infer the SMBH mass from the H$\alpha$ line and virial relations may not be applicable for high-redshift LRDs.