Multiphase Gas Structure in JWST Little Red Dots
Abstract
JWST has uncovered a population of compact high-redshift galaxies, 'little red dots' (LRDs), exhibiting broad Balmer emission lines that often show narrow absorption features and strong Balmer breaks - spectral signatures attributed to dense gas surrounding rapidly growing black holes, but whose small-scale structure remains unknown. A striking feature of LRD spectra is that the Hβ absorption is systematically deeper than Hα, yet in any homogeneous gas this ordering is physically impossible. Here we model the Hα and Hβ line profiles using radiative transfer simulations for 11 LRDs with JWST sensitive high resolution spectroscopy, and independently constrain the n=2 hydrogen column density, NHI,n=2, from the Balmer-break continuum. We find that the NHI,n=2 from Hα in a homogeneous medium cannot reproduce the observed Hβ absorption in several objects, and that this NHI,n=2 typically falls two orders of magnitude below the lower bound set by the Balmer break. Both tensions are simultaneously resolved by a multiphase medium of dense clumps within diffuse hot ionized gas. This multiphase gas structure is a necessary ingredient for any viable LRD model, regardless of the nature of the central engine.
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BibTeX
@misc{chang2026multiphase,
title = {{Multiphase Gas Structure in JWST Little Red Dots}},
author = {Seok-Jun Chang and Max Gronke and Jorryt Matthee and Charlotte Mason},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.34552},
eprint = {2610.34552},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.34552},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Multiphase Gas Structure in JWST Little Red Dots
AU - Seok-Jun Chang
AU - Max Gronke
AU - Jorryt Matthee
AU - Charlotte Mason
PY - 2026
DA - 2026/10/25
PB - arXiv
DO - 10.48550/arxiv.2610.34552
UR - https://arxiv.org/abs/2610.34552
AB - JWST has uncovered a population of compact high-redshift galaxies, 'little red dots' (LRDs), exhibiting broad Balmer emission lines that often show narrow absorption features and strong Balmer breaks - spectral signatures attributed to dense gas surrounding rapidly growing black holes, but whose small-scale structure remains unknown. A striking feature of LRD spectra is that the Hβ absorption is systematically deeper than Hα, yet in any homogeneous gas this ordering is physically impossible. Here we model the Hα and Hβ line profiles using radiative transfer simulations for 11 LRDs with JWST sensitive high resolution spectroscopy, and independently constrain the n=2 hydrogen column density, N_HI,n=2, from the Balmer-break continuum. We find that the N_HI,n=2 from Hα in a homogeneous medium cannot reproduce the observed Hβ absorption in several objects, and that this N_HI,n=2 typically falls two orders of magnitude below the lower bound set by the Balmer break. Both tensions are simultaneously resolved by a multiphase medium of dense clumps within diffuse hot ionized gas. This multiphase gas structure is a necessary ingredient for any viable LRD model, regardless of the nature of the central engine.
ER -