Stacking X-ray Observations of "Little Red Dots": Implications for their AGN Properties

TL;DR

LRDs are candidate AGN at z ≳ 4. This study stacks archival Chandra observations of 34 spectroscopically confirmed LRDs to test the AGN hypothesis, analyzing soft and hard X-ray bands. It finds tentative detections but X-ray luminosities that are lower than expected from the L_X-L_Hα relation for typical Type-I AGNs, implying intrinsically weak X-ray emission or distinct properties for LRDs. The results suggest AGN activity in LRDs but warn that standard AGN relations may not apply, with absorption or fast outflows unable to fully explain the low L_X/L_Hα, underscoring the need for caution when interpreting black hole properties in this population.

Abstract

Recent James Webb Space Telescope (JWST) observations have revealed a population of compact extragalactic objects at $z\gtrsim4$ with red near-infrared colors, which have been dubbed as ``Little Red Dots" (LRDs). The spectroscopically-selected LRDs exhibit broad H$α$ emission lines, which likely indicates that type-I active galactic nuclei (AGN) are harbored in the galaxies' dust-reddened cores. However, other mechanisms, like strong outflowing winds, could also produce broad H$α$ emission lines, and thus, the nature of LRDs is still under debate. We test the AGN hypothesis for LRDs by stacking the archival {\em Chandra} observations of 34 spectroscopically-selected LRDs. We obtain tentative detections in the soft $(0.5-2\text{ keV})$ and hard $(2-8\text{ keV})$ X-ray bands with $2.9σ$ and $3.2σ$ significance, and with $4.1σ$ significance when combining the two bands. Nevertheless, we find that the soft (hard) band $3σ$ upper limit is $\sim1$dex ($\sim 0.3$dex) lower than the expected level from the $L_\text{X}-L_{\text{H}α}$ relation for typical type-I AGNs. Our results indicate that AGN activity is indeed likely present in LRDs, though these objects have significantly different properties compared to previously identified type-I AGNs, i.e., LRDs may have intrinsically weak X-ray emissions. We find it difficult to explain the low $L_\text{X}/L_{\text{H}α}$ ratios observed in LRDs solely by absorption. It is also unlikely that fast outflows have major contributions to the broad H$α$ lines. Our findings indicate that empirical relations (e.g., for black hole mass measurements) established for typical type-I AGNs should be used with caution when analyzing the properties of LRDs.