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

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$\alpha$ 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$\alpha$ 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 $2.9\sigma$ and $3.2\sigma$ significance, and with $4.1\sigma$ significance when combining the two bands. Nevertheless, we find that the soft (hard) band $3\sigma$ upper limit is $\sim1$dex ($\sim 0.3$dex) lower than the expected level from the $L_\text{X}-L_{\text{H}\alpha}$ 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 $L_\text{X}/L_{\text{H}\alpha}$ ratios observed in LRDs solely by absorption. It is also unlikely that fast outflows have major contributions to the broad H$\alpha$ 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.