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Stacking X-ray Observations of "Little Red Dots": Implications for their AGN Properties
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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.
Forward citations
Cited by 10 Pith papers
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BlackTHUNDER Reveals a Massive Filament around a Compact AGN at $z\simeq5.23$
GN-77652, a compact broad-line AGN at z=5.229, lies beside a 12 kpc multi-source filament at the same redshift whose coalescence will erase the compact AGN appearance within ~150–440 Myr.
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ATLAS. II. Extremely High Incidence of Balmer Line Absorption with Predominant Blueshifts in LRDs: Statistical Insights through Comparison with Type 1 AGNs
Balmer-line absorption occurs in ~35% (14/40) of JWST little-red-dot AGNs, roughly 850x the rate in SDSS type-1 AGNs, with mostly slow blueshifted absorber velocities.
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An (in)complete NIRSpec census of Balmer absorption in Type 1 AGN -- radiation-driven outflows in little red dots, quasars and variable stars
About 44% of little red dots have hydrogen-alpha absorption from outflowing gas, implying radiatively driven outflows rather than static atmospheres.
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Reduced Incidence of Little Red Dots at z < 3 from Number Density and Halo Mass Evolution
LRDs transition from underdense low-halo-mass environments at z>4 to typical galaxy conditions by z~3.5, with halo growth leading to larger sizes and SED changes that explain their disappearance at lower redshifts.
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Discovery of Multiply Ionized Iron Emission Powered by an Active Galactic Nucleus in a z~7 Little Red Dot
The z=6.68 Little Red Dot THRILS 46403 shows a 4.5 sigma [FeVII] line, the first robust coronal-line detection among z>5 LRDs, plus narrow Balmer absorption, supporting an AGN with direct sightlines to gas at or beyon...
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Do Little Red Dots Vary?
Super-Eddington accretion models can explain why little red dots show almost no variability, whereas standard sub-Eddington AGN variability models predict changes that should already have been seen.
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MEGA: Spectrophotometric SED Fitting of Little Red Dots Detected in JWST MIRI
Standard AGN and star-forming templates cannot fully reproduce the SEDs of eight MIRI-detected little red dots; dense-gas models plus extra hot dust partly work but fail on UV and narrow emission lines.
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Little red dots sit in lower-density environments than typical galaxies, and Bayesian model comparison favors non-AGN interpretations for most of them.
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Little Red reionization factories
LRDs may drive cosmic reionization: tidal fields are said to funnel intergalactic hydrogen into colliding streams at LRD sites, igniting starbursts that ionize the gas.
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