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

TL;DR

JWST-detected Little Red Dots are tested for hosting SMBHs with ultra-deep Chandra observations behind Abell 2744. Individually, LRDs show no X-ray detections; stacking reveals a tentative 2.6σ signal in the broad-Hα subset, corresponding to MBH ~3.2×10^6 Msun, well below JWST virial masses. The full LRD population thus appears not to harbor over-massive BHs under Eddington-limited accretion, though uncertainties in stellar mass estimates (AGN contamination) could alter this conclusion. The findings highlight potential biases in high-z BH mass inferences and have implications for BH seeding scenarios, suggesting a coexistence of seeding channels and varying accretion histories in the early universe.

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$α$ emission. For the LRDs exhibiting broad-line H$α$ emission, there is a hint of a stacked signal ($\sim2.6σ$), 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$α$ line and virial relations may not be applicable for high-redshift LRDs.

Figures (4)

• Figure 1: Merged Chandra ACIS-I image of the lensing cluster, Abell 2744, in the $2-7$ keV band. The total exposure time of the merged data is $\approx2.07$ Ms. We focus on a sample of 23 LRDs (see Table \ref{['tab:sample']}) in the redshift range of $z=3-8.5$ that were identified by JWST NIRCAM data labbe23b. The location of the galaxies is highlighted with small white circles. The Chandra X-ray data provides data for all galaxies.
• Figure 2: Stacked $2-7$ keV band Chandra X-ray images of LRD galaxies using different samples. The solid circular region represents a $1.9\arcsec$ region and the dashed annulus shows a $3\arcsec-6\arcsec$ annulus. The regions shown here are only for illustration purposes, the actual source and background regions used in our analysis were calculated using the local PSF for each observation (Section \ref{['sec:stackfast']} and Appendix A). The stacked samples are as follows. Top left: The 21 individual LRDs in our sample; Top right: 9 LRDs from the greene23 sample with broad-line H$\alpha$ emission and an inferred SMBH mass. We obtain a tentative $2.6\sigma$ detection for this subset.; Bottom left: 11 massive galaxies with stellar mass $\geq2.5\times10^{9} \ \rm{M_{\odot}}$; Bottom right: 10 low-mass galaxies with stellar masses $<2.5\times10^{9} \ \rm{M_{\odot}}$.
• Figure 3: The relationship between the SMBH mass and the stellar masses of galaxies. The yellow data points represent the upper limits obtained for the individual LRDs. The thick dark green data point corresponds to the upper limit measured for the stacked sample of 21 LRDs assuming Eddington-limited accretion. The uncertainty in stellar mass corresponds to the standard deviation. For reference, we also show the SMBH mass upper limits for the full stacked sample assuming accretion at $10\%$ and $1\%$ of the Eddington limit (green thin dashed). The tentative $2.6\sigma$ detection for the sample of nine LRDs with broad H$\alpha$ line is shown with the solid dark green data point. The arrow pointing left from this data point indicates that the stellar masses used for this plot from labbe23b assume galaxy-only fits to calculate the masses. However, galaxy+AGN fits lead to lower AGN masses barro2024 by up to 2 orders of magnitude, therefore this point might shift leftwards, as illustrated by the green dashed arrow. We over-plot the scaling relations observed for local early-type galaxies using the relation observed by kormendy13 that was scaled following reines15, the best-fit relation for local AGN reines15, and for AGN up to $z<2.5$suh20. The black data point (square) shows the median SMBH mass inferred from the broad line H$\alpha$ emission line measured by JWSTgreene23. We also plot the $z=10.07$ galaxy, UHZ 1, which hosts an over-massive SMBH bogdan24.
• Figure 4: The distribution of 90% energy encircling radius ($R_{\rm 90}$) for the 21 AGN candidates presented in labbe23b in the Chandra observations of Abell 2744. This radius varies as a function of distance from Chandra ACIS-I pointing center. The median $R_{\rm 90}$ for all source-observation pairs for these sources is $1.9\arcsec$ (shown with black dashed line).