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

Paper 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 \textit{JWST}. However, the nature of LRDs is still debated: some studies suggest that these galaxies host accreting supermassive black holes (SMBHs), while others conclude that the near-infrared emission primarily originates from intense star formation. In this work, we utilize ultra-deep \textit{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 <~(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 ~87~Ms. We also bin the galaxies based on their stellar mass, lensing magnification, and detected broad-line H\alpha emission. All but one stacked sample remains undetected with SMBH mass upper limits of <~2.5\times10^{6}~\rm{M_{\odot}}. We obtain a tentative, ~2.6\sigma detection for LRDs exhibiting broad-line H\alpha emission. Taking this detection at face value, the inferred mean SMBH mass is ~3.2\times10^{6}~\rm{M_{\odot}} assuming Eddington-limited accretion, about 1.5 orders of magnitude lower than that inferred from \textit{JWST} data. Our results imply that LRDs do not host over-massive SMBHs and/or accrete at a few percent of their Eddington limit. The significant discrepancy between the \textit{JWST} and \textit{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.