The THRILS Factor: Investigating the properties of Little Red Dots (LRDs) at 3<z<6 with JWST/NIRSpec
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 (≥8 hours), enabling robust detections of broad Balmer lines, He I emission, and other spectral features characteristic of AGN activity. Using the [SII] λλ6716,6731 doublet, we find electron densities (ne) between 2.33 < log (ne) < 2.97 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 τsc = 0.56−0.91, scattering fractions fSC = 0.40−0.82 which correspond to column densities log(Ne) ∼ 23.93-24.14 cm−2, and covering fractions cf = 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.
Related
- An (in)complete NIRSpec census of Balmer absorption in Type 1 AGN – radiation-driven outflows in little red dots, quasars and variable stars
- From “The Cliff” to “Virgil”: Mapping the Spectral Diversity of Little Red Dots with JWST/NIRSpec
- Little red dots host black hole stars: a unified family of gas-reddened AGN revealed by JWST /NIRSpec spectroscopy
- IRAC-Excess Galaxies in the NIRSpec Wide Survey: physical properties of bright EELGs at z∼7 and implications for Reionization
- The lifetimes and properties of Little Red Dots in the AMBRA simulation
BibTeX
@misc{ganapathy2026thrils,
title = {{The THRILS Factor: Investigating the properties of Little Red Dots (LRDs) at 3<z<6 with JWST/NIRSpec}},
author = {Ananya Ganapathy and Rebecca L. Larson and Erini Lambrides and Guillermo Barro and Taylor A. Hutchison and Pablo Arrabal Haro and Anthony J. Taylor and Dale Kocevski and Casey Papovich and Steven L. Finkelstein and Anton M. Koekemoer and Jonathan R. Trump and Pablo G. Pérez-González and Weida Hu and Volker Bromm and Dan Coe and Kelcey Davis and Michaela Hirschmann and Nikko J. Cleri and Ray A. Lucas and Stephan R. McCandliss and L. Y. Aaron Yung and Jorge A. Zavala},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.10803},
eprint = {2609.10803},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.10803},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The THRILS Factor: Investigating the properties of Little Red Dots (LRDs) at 3<z<6 with JWST/NIRSpec
AU - Ananya Ganapathy
AU - Rebecca L. Larson
AU - Erini Lambrides
AU - Guillermo Barro
AU - Taylor A. Hutchison
AU - Pablo Arrabal Haro
AU - Anthony J. Taylor
AU - Dale Kocevski
AU - Casey Papovich
AU - Steven L. Finkelstein
AU - Anton M. Koekemoer
AU - Jonathan R. Trump
AU - Pablo G. Pérez-González
AU - Weida Hu
AU - Volker Bromm
AU - Dan Coe
AU - Kelcey Davis
AU - Michaela Hirschmann
AU - Nikko J. Cleri
AU - Ray A. Lucas
AU - Stephan R. McCandliss
AU - L. Y. Aaron Yung
AU - Jorge A. Zavala
PY - 2026
DA - 2026/09/09
PB - arXiv
DO - 10.48550/arxiv.2609.10803
UR - https://arxiv.org/abs/2609.10803
AB - 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 (≥8 hours), enabling robust detections of broad Balmer lines, He I emission, and other spectral features characteristic of AGN activity. Using the [SII] λλ6716,6731 doublet, we find electron densities (n_e) between 2.33 < log (n_e) < 2.97 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_sc = 0.56−0.91, scattering fractions f_SC = 0.40−0.82 which correspond to column densities log(N_e) ∼ 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.
ER -