X-ray View of Little Red Dots: Do They Host Supermassive Black Holes?
Tonima Tasnim Ananna, ‘Akos Bogd’an, Orsolya E. Kov’acs, Priyamvada Natarajan, Ryan C. Hickox
arXiv:2404.19010v1 Announce Type: new
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 $lesssim(1.5-16)times10^{6}~rm{M_{odot}}$ 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. All but one stacked sample remains undetected with SMBH mass upper limits of $lesssim2.5times10^{6}~rm{M_{odot}}$. We obtain a tentative, $approx2.6sigma$ detection for LRDs exhibiting broad-line H$alpha$ emission. Taking this detection at face value, the inferred mean SMBH mass is $approx3.2times10^{6}~rm{M_{odot}}$ 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.arXiv:2404.19010v1 Announce Type: new
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 $lesssim(1.5-16)times10^{6}~rm{M_{odot}}$ 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. All but one stacked sample remains undetected with SMBH mass upper limits of $lesssim2.5times10^{6}~rm{M_{odot}}$. We obtain a tentative, $approx2.6sigma$ detection for LRDs exhibiting broad-line H$alpha$ emission. Taking this detection at face value, the inferred mean SMBH mass is $approx3.2times10^{6}~rm{M_{odot}}$ 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.