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

Minghao Yue, Anna–Christina Eilers+4 more|arXiv (Cornell University)|Apr 20, 2024

Advanced X-ray Imaging Techniques4 citations

TL;DR

This study stacks Chandra X-ray observations of 19 spectroscopically selected 'Little Red Dots' (LRDs) at z ≳ 4 to test their active galactic nucleus (AGN) nature. Despite broad Hα emission suggesting type-I AGNs, the stacked X-ray data show non-detections with soft-band (0.5–2 keV) upper limits ~1 dex below the expected L_X–L_Hα relation for typical type-I AGNs, implying LRDs likely have intrinsically weak X-ray emission or broad Hα lines are driven by outflows rather than AGN activity.

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 $(0.5-2 ext{ keV})$ and hard $(2-8 ext{ keV})$ 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 $L_ ext{X}-L_{ ext{H}α}$ 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 $L_ ext{X}/L_{ ext{H}α}$ 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.

Motivation & Objective

• To test the hypothesis that 'Little Red Dots' (LRDs) host type-I active galactic nuclei (AGNs) using X-ray stacking analysis.
• To constrain the X-ray luminosities of LRDs, given their ambiguous nature despite broad Hα emission.
• To assess whether the observed low L_X/L_Hα ratios in LRDs are due to absorption, intrinsic weakness, or non-AGN processes like outflows.
• To evaluate the reliability of standard AGN scaling relations (e.g., L_X–L_Hα) when applied to LRDs.

Proposed method

• Selected 19 spectroscopically confirmed LRDs with FWHM(Hα) > 1000 km s⁻¹ from the literature.
• Stacked archival Chandra ACIS-I observations in soft (0.5–2 keV) and hard (2–8 keV) X-ray bands using the cstack and stackfast tools.
• Applied spectral fitting with XSPEC to derive flux upper limits and luminosity constraints.
• Compared the derived X-ray luminosities to the empirical L_X–L_Hα relation from low-redshift type-I AGNs.
• Assessed the impact of dust extinction and scattering on observed Hα luminosities to estimate intrinsic L_X/L_Hα ratios.
• Evaluated competing hypotheses: intrinsic X-ray weakness vs. outflow-driven broad emission lines.

Experimental results

Research questions

• RQ1 Do LRDs exhibit detectable X-ray emission consistent with typical type-I AGNs?
• RQ2 Is the observed low L_X/L_Hα ratio in LRDs due to X-ray absorption or intrinsic X-ray weakness?
• RQ3 Could strong outflows be responsible for the broad Hα emission lines in LRDs instead of AGN activity?
• RQ4 To what extent can standard AGN scaling relations be reliably applied to LRDs?

Key findings

• The stacked soft X-ray (0.5–2 keV) flux upper limit is ~1 dex lower than the expected level from the L_X–L_Hα relation for typical type-I AGNs.
• The stacked hard X-ray (2–8 keV) flux upper limit is consistent with the L_X–L_Hα relation, indicating a possible soft X-ray deficit.
• The observed low L_X/L_Hα ratio in LRDs is unlikely to be explained solely by X-ray absorption, as the hard band limit is consistent with expectations.
• The results suggest LRDs may have intrinsically weak X-ray emission or that broad Hα lines are primarily driven by galactic-scale outflows.
• Dust attenuation and scattering effects may further reduce the observed L_X/L_Hα ratio, implying even lower intrinsic ratios than observed.
• Empirical relations derived for standard type-I AGNs should be applied with caution to LRDs due to significant systematic uncertainties.