Chandra Rules Out Super-Eddington Accretion Models For Little Red Dots

TL;DR

This study uses deep X-ray stacking of 55 high-redshift little red dots to test super-Eddington accretion models, finding they are unlikely and suggesting extreme obscuration or lower black hole activity.

Contribution

It provides the first deep X-ray stacking analysis of LRDs, challenging super-Eddington accretion as an explanation for their X-ray weakness.

Findings

Deep X-ray stacking yields non-detections, ruling out super-Eddington models.

Results are consistent only with extremely high obscuration levels.

Suggests SMBHs in LRDs are less massive or luminous than previously thought.

Abstract

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 $10^7-10^8$ 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 ($N_{\rm H}\gtrsim10^{25}$ 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.