SPURS: An Ultra-deep View Inside the Compact, Nitrogen-Enriched Nuclei of Little Red Dots
Abstract
We present the first ultra-deep rest-UV spectroscopy of four UV-bright Little Red Dots (LRDs), obtained from the SPURS Cycle 4 Large Program. The spectra reveal broad CIV (FWHM ≈2700−2800 km s−1) in two LRDs, alongside narrow-line densities elevated above star-forming galaxies (ne∼104−105 cm−3, reaching 106 cm−3 in the most extreme source) and nitrogen-enhancements in all four LRDs. We detect broad HeII emission (FWHM ≈930 km s−1) in one LRD, and two others with fast P-Cygni absorption (≳2200 km s−1). Strong interstellar absorption lines and Lyα damping wings reveal the UV continuum is deeply embedded in neutral gas (NHI≳1022 cm−2) in all four LRDs. Detections of fluorescent FeII and OI emission and fine-structure absorption indicate this gas lies close to the UV-emitting region. In archival z>4 samples, we find nitrogen and strong CIII] emission are significantly more common in LRDs than in the galaxy population. The transmission of broad CIV, tracing the broad-line region or cocoon, depends on rest-optical color within our sample, consistent with an orientation-dependent picture in which bluer, less obscured sightlines offer a more direct, polar view of the central engine and its outflows. We find several potential signatures of very massive stars, whose winds may contribute to nitrogen enhancement. We investigate other abundance patterns expected from supermassive stars but our results are inconclusive. Our results place the UV-emitting region within ≲8 pc of the LRD nucleus, consistent with an actively assembling nuclear star cluster. Dynamical interactions in this extremely dense environment, including tidal disruption of stars, may explain the high incidence of nitrogen enhancements in LRDs.
Related
- Broad-band host measurements of little red dots and similar compact nuclei require source-conditioned polychromatic PSFs
- Formation of Heavy Seed Black Holes and Little Red Dots-like Compact Clusters in Metal-enriched Star-forming Regions
- Inside the cocoon: A comprehensive explanation of the spectra of little red dots
- Can LISA See Inside the Little Red Dots?
- The UV Side of Little Red Dots: Red, Compact, and Iron-Enhanced Rest-UV Emission with a Strong Downturn around Lyα
BibTeX
@misc{tang2026spurs,
title = {{SPURS: An Ultra-deep View Inside the Compact, Nitrogen-Enriched Nuclei of Little Red Dots}},
author = {Mengtao Tang and Daniel P. Stark and Charlotte A. Mason and Zuyi Chen and Tucker Jones and Sarah Searle Grannis and Peter Senchyna and Lily Whitler and Keerthi Vasan G. C. and Viola Gelli},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.11094},
eprint = {2609.11094},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.11094},
}
RIS
TY - UNPB
TI - SPURS: An Ultra-deep View Inside the Compact, Nitrogen-Enriched Nuclei of Little Red Dots
AU - Mengtao Tang
AU - Daniel P. Stark
AU - Charlotte A. Mason
AU - Zuyi Chen
AU - Tucker Jones
AU - Sarah Searle Grannis
AU - Peter Senchyna
AU - Lily Whitler
AU - Keerthi Vasan G. C.
AU - Viola Gelli
PY - 2026
DA - 2026/09/10
PB - arXiv
DO - 10.48550/arxiv.2609.11094
UR - https://arxiv.org/abs/2609.11094
AB - We present the first ultra-deep rest-UV spectroscopy of four UV-bright Little Red Dots (LRDs), obtained from the SPURS Cycle 4 Large Program. The spectra reveal broad CIV (FWHM ≈2700−2800 km s^−1) in two LRDs, alongside narrow-line densities elevated above star-forming galaxies (n_e∼10^4−10^5 cm^−3, reaching 10^6 cm^−3 in the most extreme source) and nitrogen-enhancements in all four LRDs. We detect broad HeII emission (FWHM ≈930 km s^−1) in one LRD, and two others with fast P-Cygni absorption (≳2200 km s^−1). Strong interstellar absorption lines and Lyα damping wings reveal the UV continuum is deeply embedded in neutral gas (N_HI≳10^22 cm^−2) in all four LRDs. Detections of fluorescent FeII and OI emission and fine-structure absorption indicate this gas lies close to the UV-emitting region. In archival z>4 samples, we find nitrogen and strong CIII] emission are significantly more common in LRDs than in the galaxy population. The transmission of broad CIV, tracing the broad-line region or cocoon, depends on rest-optical color within our sample, consistent with an orientation-dependent picture in which bluer, less obscured sightlines offer a more direct, polar view of the central engine and its outflows. We find several potential signatures of very massive stars, whose winds may contribute to nitrogen enhancement. We investigate other abundance patterns expected from supermassive stars but our results are inconclusive. Our results place the UV-emitting region within ≲8 pc of the LRD nucleus, consistent with an actively assembling nuclear star cluster. Dynamical interactions in this extremely dense environment, including tidal disruption of stars, may explain the high incidence of nitrogen enhancements in LRDs.
ER -