The THRILS Factor: Investigating the properties of Little Red Dots (LRDs) at 3<z<6 with JWST/NIRSpec

Ananya Ganapathy, Rebecca L. Larson, Erini Lambrides, Guillermo Barro, Taylor A. Hutchison, Pablo Arrabal Haro, Anthony J. Taylor, Dale Kocevski, Casey Papovich, Steven L. Finkelstein, Anton M. Koekemoer, Jonathan R. Trump, Pablo G. Pérez-González, Weida Hu, Volker Bromm, Dan Coe, Kelcey Davis, Michaela Hirschmann, Nikko J. Cleri, Ray A. Lucas, Stephan R. McCandliss, L. Y. Aaron Yung, Jorge A. Zavala

First listed 2026-09-11 | Last updated 2026-09-09

Abstract

JWST has uncovered a class of objects called LRDs, whose nature is still widely debated. In this work, we present a comprehensive spectroscopic analysis of nine LRDs in the Extended Groth Strip (EGS) field studied as part of THRILS and C3PO, both JWST Cycle 3 programs. These targets, photometrically selected based on their compact red appearance, are observed with deep spectroscopic exposures ($\geq8$ hours), enabling robust detections of broad Balmer lines, He I emission, and other spectral features characteristic of AGN activity. Using the [SII] $λ\lambda6716,6731$ doublet, we find electron densities ($n_e$) between $2.33 < \log (n_e) < 2.97 \ \mathrm{cm^{-3}}$, comparable to those in narrow line regions (NLR) of local AGN and high-$z$ galaxies. The spectroscopic depth further enables detailed characterization of broad Balmer line profiles. We fit both Gaussian and convolved exponential models to each source and find that five LRDs are statistically better described by the latter model. We measure optical depths $τ_{\rm sc} = 0.56-0.91$, scattering fractions $f_{\rm SC} = 0.40-0.82$ which correspond to column densities log(N$_e$) $\sim$ 23.93-24.14 cm$^{-2}$, and covering fractions $c_f = 0.43-0.59$. These results indicate a clumpier broad line region (BLR) geometry that deviates from conventional LRD models, which predict covering fractions close to unity. Furthermore, these Compton-thick gas columns may explain the X-ray weakness of LRDs. We also find that THRILS LRDs are narrow-line dominated compared to literature AGN-dominated LRDs, and show that exponential profile fitting corrects for systematic overestimation of black hole masses from Gaussian-based measurements.

Short digest

Ganapathy et al. present deep JWST/NIRSpec spectroscopy of nine compact, V-shaped little red dots at 3<z<6 in the EGS, selected through the THRILS and C3PO programs. Broad Balmer and He I emission establish widespread AGN-like activity, while [S II] ratios imply electron densities comparable to local AGN narrow-line regions and other high-redshift galaxies. Five objects favor convolved exponential rather than Gaussian broad-line profiles, yielding inferred scattering columns of log(Ne/cm−2)=23.93–24.14 and sub-unity covering fractions that point to a clumpy BLR rather than the nearly closed obscuring geometry often assumed for LRDs. The resulting Compton-thick columns offer a route to their X-ray weakness, and the profile modeling also lowers black-hole mass estimates relative to Gaussian-only fits; the THRILS sample is notably narrow-line dominated compared with previously reported AGN-dominated LRDs.

Key figures to inspect

• Figure 1. This figure establishes the photometric basis of the nine-object sample, showing how the compact, V-shaped THRILS LRDs occupy the adopted color-selection space relative to the full parent catalog and CEERS LRDs.
• Figure 5. The multi-object Balmer-line fitting summary is the clearest direct evidence for the paper’s central profile result: five LRDs require convolved exponential broad components, while the remaining sources are adequately described by Gaussian profiles.
• Figure 7. This diagnostic translates the measured [S II] doublet ratios into electron densities and shows that the THRILS LRDs resemble non-LRD comparison objects in density, supporting an NLR-like interpretation rather than an exceptional low-density gas phase.
• Figure 8. This figure shows the practical consequence of profile choice for black-hole inference: convolved-profile measurements revise the Gaussian-based virial mass estimates, placing the THRILS LRDs in context with local and high-redshift black-hole–stellar-mass relations.
• Figure 9. The full population comparison connects the sample to the paper’s evolutionary interpretation, contrasting the predominantly narrow-line-dominated THRILS LRDs with literature LRDs and broad-line AGN while relating UV luminosity to [O III] output.