# Challenging the AGN scenario for JWST/NIRSpec broad H$α$ emitters/Little Red Dots in light of non-detection of NIRCam photometric variability and X-ray

**Authors:** Mitsuru Kokubo, Yuichi Harikane  
**arXiv:** [2407.04777](https://arxiv.org/abs/2407.04777)  
**Date:** 2024-07-05  
**Summary page:** https://www.summarizepaper.com/en/paper/2407.04777v1/

## Key points

- The James Webb Space Telescope (JWST) has identified a significant population of high-redshift galaxies with broad H$\alpha$ emission lines.
- These galaxies, known as the 'Little Red Dots,' have compact structures and extremely red rest-frame optical colors.
- The prevailing hypothesis suggests that these emitters could be attributed to type 1-1.9 Active Galactic Nuclei (AGNs), indicating a higher density of low-luminosity AGNs than previously estimated.
- A study on five JWST broad H$\alpha$ emitters found no significant flux variation within the rest-frame wavelength range of approximately 1,500-9,000 Å, setting stringent upper limits on variability amplitudes.
- Alternative explanations for the observed broad H$\alpha$ emission include peculiar Compton-thick broad-line AGNs, scattering-dominated AGN emission, non-AGN sources like star formation-driven outflows or Raman scattering of stellar UV continua by neutral hydrogen atoms.

## Summary

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The James Webb Space Telescope () has recently identified a significant population of high-redshift galaxies with broad H$\alpha$ emission lines. These galaxies, particularly those with Full Width at Half Maximum values exceeding 1,000 km s$^{-1}$, have been dubbed the 'Little Red Dots' due to their compact structure and extremely red rest-frame optical colors. The prevailing hypothesis suggests that these broad H$\alpha$ emitters could be attributed to type 1-1.9 Active Galactic Nuclei (), indicating a higher density of low-luminosity AGNs than previously estimated. To further investigate this scenario, a study was conducted on five of these JWST broad H$\alpha$ emitters using multi-epoch, multi-band JWST/NIRCam imaging data. The temporal sampling interval of the NIRCam data aligns well with typical variability timescales observed in AGNs with black hole masses around $10^7 M_{\odot}$. However, no significant flux variation was detected within the rest-frame wavelength range of approximately 1,500-9,000 Å, setting stringent upper limits on the variability amplitudes. This finding is complemented by the confirmation of X-ray faintness through ultra-deep Chandra data analysis. These combined results challenge the AGN scenario and suggest alternative explanations for the observed broad H$\alpha$ emission. One possibility is the existence of peculiar Compton-thick broad-line AGNs with unique characteristics such as an intrinsically non-variable AGN disk continuum or a host galaxy-dominated continuum. Alternatively, scattering-dominated AGN emission or non-AGN sources could also account for the observed phenomena. Non-AGN scenarios propose that the broad-line emission may originate from fast and dense/low-metallicity star formation-driven outflows or inelastic Raman scattering of stellar UV continua by neutral hydrogen atoms. This comprehensive investigation sheds light on the complex nature of high-redshift galaxies exhibiting broad H$\alpha$ emission lines and challenges traditional interpretations based solely on AGN scenarios. Further research and observations will be crucial in unraveling the true origins and mechanisms behind these enigmatic 'Little Red Dots' and their counterparts in the early universe.