2025/05/14 by Andrea Sacchi, Sacchi, Andrea, Akos Bogdan +1 · 1 voice · 5 citations
#astro-ph.GA #astro-ph.CO #astro-ph.HE
paper · pdf · doi:10.48550/arxiv.2505.09669
One of the most puzzling discoveries by JWST is the population of high-redshift, red, and compact galaxies dubbed little red dots (LRDs). Based on broad-line diagnostics, these galaxies have been argued to host accreting 107-108 M_\odot supermassive black holes (SMBHs), a claim with crucial consequences for our understanding of how the first black holes form and grow over cosmic time. A key feature of LRDs is their extreme X-ray weakness: analyses of individual and stacked sources have yielded non-detections or only tentative, inconclusive X-ray signals, except for a handful of individual cases. Although high obscuration is the most straightforward way to explain the X-ray weakness of LRDs, JWST rest-frame optical/UV spectra initially argued against the presence of Compton-thick gas clouds. Instead, several authors have proposed that LRDs are intrinsically X-ray weak due to super-Eddington accretion rates. In this work, we observationally test these tailored models by stacking X-ray data for 55 LRDs in the Chandra Deep Field South, accumulating a total exposure time of nearly 400 Ms. Despite reaching unprecedented X-ray depths, our stack still yields a non-detection. The corresponding upper limits are deep enough to rule out current super-Eddington accretion models, and are compatible only with extremely high levels of obscuration (N\rm H\gtrsim1025 cm-2). To explain the X-ray weakness of LRDs, we therefore speculate that the SMBHs in these systems are neither as massive nor as luminous as currently believed.