Lyα in Compact Broad-Line Emitters, Including Little Red Dots, Is More Closely Linked to the Host Galaxy than to the Broad-Line Nucleus
Abstract
JWST has found many faint broad-line AGN at z>4, but it is unclear whether the Lyα we see from them comes from the accreting black hole or from its host galaxy. The answer matters for how these sources are interpreted and for how many ionizing photons they release. We use deep JWST/NIRSpec spectroscopy from the DIVER program to measure Lyα in 12 compact broad-line emitters (CBEs) at z=4.2--5.5 in GOODS-N. CBEs include little red dots (LRDs), little blue dots, and other broad-line AGN. Six are detected in Lyα. Every source shows a broad-line region, yet Lyα does not follow it: the sources with the strongest Hα relative to the UV include weak detections and nondetections, and every detected Lyα line (FWHM ≈300--1000~km~s−1) is narrower than broad Hα (1100--3300~km~s−1). Instead, Lyα appears to track the host galaxy. We model the AGN-subtracted host with a dust-attenuated SED fit. Including the upper limits, the observed Lyα follows the Lyα production in this host model more closely than it follows Hα/UV, in both equivalent width and luminosity. Every detection exceeds its host-model value by a factor of 1.5--18, leaving room for a nuclear contribution. The black hole is visible in these sources, but the Lyα we observe is more closely tied to its host. The median effective Lyα escape fraction of the detections is only 0.09, which suggests that ionizing escape from CBEs is typically modest and varies widely.
Related
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BibTeX
@misc{zhu2026lyα,
title = {{Ly$α$ in Compact Broad-Line Emitters, Including Little Red Dots, Is More Closely Linked to the Host Galaxy than to the Broad-Line Nucleus}},
author = {Yongda Zhu and Zheng Ma and Xiaojing Lin and Junyu Zhang and Jianwei Lyu and Xiaohui Fan and Eiichi Egami and Andrew J. Bunker and Sijia Cai and Laura C. Keating and Joseph Shields and Zhiyuan Ji and Xiangyu Jin and Weizhe Liu and Suprabhas Narisetty and George H. Rieke and Marcia J. Rieke and Pierluigi Rinaldi and Fengwu Sun},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.03801},
eprint = {2611.03801},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.03801},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Lyα in Compact Broad-Line Emitters, Including Little Red Dots, Is More Closely Linked to the Host Galaxy than to the Broad-Line Nucleus
AU - Yongda Zhu
AU - Zheng Ma
AU - Xiaojing Lin
AU - Junyu Zhang
AU - Jianwei Lyu
AU - Xiaohui Fan
AU - Eiichi Egami
AU - Andrew J. Bunker
AU - Sijia Cai
AU - Laura C. Keating
AU - Joseph Shields
AU - Zhiyuan Ji
AU - Xiangyu Jin
AU - Weizhe Liu
AU - Suprabhas Narisetty
AU - George H. Rieke
AU - Marcia J. Rieke
AU - Pierluigi Rinaldi
AU - Fengwu Sun
PY - 2026
DA - 2026/11/02
PB - arXiv
DO - 10.48550/arxiv.2611.03801
UR - https://arxiv.org/abs/2611.03801
AB - JWST has found many faint broad-line AGN at z>4, but it is unclear whether the Lyα we see from them comes from the accreting black hole or from its host galaxy. The answer matters for how these sources are interpreted and for how many ionizing photons they release. We use deep JWST/NIRSpec spectroscopy from the DIVER program to measure Lyα in 12 compact broad-line emitters (CBEs) at z=4.2--5.5 in GOODS-N. CBEs include little red dots (LRDs), little blue dots, and other broad-line AGN. Six are detected in Lyα. Every source shows a broad-line region, yet Lyα does not follow it: the sources with the strongest Hα relative to the UV include weak detections and nondetections, and every detected Lyα line (FWHM ≈300--1000~km~s^−1) is narrower than broad Hα (1100--3300~km~s^−1). Instead, Lyα appears to track the host galaxy. We model the AGN-subtracted host with a dust-attenuated SED fit. Including the upper limits, the observed Lyα follows the Lyα production in this host model more closely than it follows Hα/UV, in both equivalent width and luminosity. Every detection exceeds its host-model value by a factor of 1.5--18, leaving room for a nuclear contribution. The black hole is visible in these sources, but the Lyα we observe is more closely tied to its host. The median effective Lyα escape fraction of the detections is only 0.09, which suggests that ionizing escape from CBEs is typically modest and varies widely.
ER -