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X-ray View of Little Red Dots: Do They Host Supermassive Black Holes?
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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.
Forward citations
Cited by 10 Pith papers
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An (in)complete NIRSpec census of Balmer absorption in Type 1 AGN -- radiation-driven outflows in little red dots, quasars and variable stars
About 44% of little red dots have hydrogen-alpha absorption from outflowing gas, implying radiatively driven outflows rather than static atmospheres.
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LRDs transition from underdense low-halo-mass environments at z>4 to typical galaxy conditions by z~3.5, with halo growth leading to larger sizes and SED changes that explain their disappearance at lower redshifts.
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Little Red Dots from Small-Scale Primordial Black Hole Clustering
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