NEXUS: Abundance, Environments, and Spectral Diversity of Little Red Dots from the NIRSpec MSA Sample

Zhiwei Pan, Ming-Yang Zhuang, Yue Shen, Feige Wang, Jenny E. Greene, Adam J. Burgasser, Junyao Li, Zachary Stone, Padmavathi Venkatraman

First listed 2026-06-09 | Last updated 2026-06-08

Abstract

We present a comprehensive study of Little Red Dots (LRDs) at 2.3 < z < 7.4 using NIRCam photometry and NIRSpec MSA/PRISM spectra from the ongoing NEXUS program. Photometric selection combining several commonly adopted methods yields a high completeness of about 85% for LRD selection over this redshift range and for a flux limit of F444W < 26. The overall purity is about 60%, with contamination from emission-line galaxies and normal active galactic nuclei (AGNs), as well as dwarf stars. Most (>90%) of the spectroscopically confirmed LRDs have robust broad-line detection. Our spectroscopic sample of 36 LRDs displays the full range of spectral diversity of LRDs. It includes objects with extreme Balmer breaks similar to the LRD "Cliff", as well as objects with moderately reddened rest-optical continua that can be fit with low-temperature blackbody components in the recent BH* model framework. The broad H$α$ emission is correlated with the continuum emission at 5100 Angstrom, suggesting common origins for these emission components; the narrow [O III] emission, however, is poorly correlated with the optical continuum. We do not find evidence of redshift evolution in these spectral properties. The space density of LRDs declines toward z about 2, opposite to the trend for normal AGNs, although low-luminosity LRDs at z about 2-4 may be more abundant than currently probed by ground-based searches. The clustering of LRDs suggests that they live in dark matter halos of several times $10^{11}\ h^{-1}$ solar masses, albeit with large uncertainties. Overall, these results are consistent with recent observations of LRDs and with the emerging picture of accreting SMBHs enshrouded in dense gas envelopes as the origin of LRDs.

Short digest

NEXUS combines NIRCam photometry with NIRSpec MSA/PRISM spectroscopy to build a 2.3 < z < 7.4 Little Red Dot sample, showing that a merged photometric selection is fairly complete, about 85% at F444W < 26, but only moderately pure at about 60% because emission-line galaxies, ordinary AGNs, and dwarf stars contaminate the sample. ([arxiv.org](https://arxiv.org/abs/2606.09721)) The 36 confirmed LRDs are overwhelmingly broad-line objects and span the full spectral range from extreme Balmer-break sources like the Cliff to more moderately reddened systems whose rest-optical continua can be fit with low-temperature blackbody components in the BH* framework. ([arxiv.org](https://arxiv.org/abs/2606.09721)) Broad Hα scales with the 5100 Å continuum while narrow [O III] does not, pointing to a closer link between the red optical continuum and the broad-line component than with the narrow-line region, with no clear redshift evolution in these spectral properties. ([arxiv.org](https://arxiv.org/abs/2606.09721)) Population-wise, the LRD space density declines toward z about 2 and the clustering is consistent, within large uncertainties, with halos of several times 10^11 h^-1 solar masses, reinforcing the emerging picture of accreting SMBHs enshrouded by dense gas. ([arxiv.org](https://arxiv.org/abs/2606.09721))

Key figures to inspect

• Figure 2. Use this as the selection-definition figure if it is where the paper consolidates the photometric criteria, because one of the paper’s most practical results is that combining several commonly used LRD selections yields about 85% completeness over 2.3 < z < 7.4 at F444W < 26 while still leaving substantial contamination from emission-line galaxies, normal AGNs, and dwarf stars. ([arxiv.org](https://arxiv.org/abs/2606.09721))
• Figure 4. Recommend the early spectroscopic overview figure that establishes what the confirmed NEXUS LRD sample looks like in practice, since the paper’s core observational contribution is a 36-object MSA sample in which more than 90% of the confirmed LRDs show robust broad-line detections and collectively span the full observed spectral diversity of the class. ([arxiv.org](https://arxiv.org/abs/2606.09721))
• Figure 8. Prioritize the figure that compares broad Hα, optical continuum, and narrow-line behavior, because the strongest physical diagnostic emphasized in the abstract is the correlation between broad Hα and the 5100 Å continuum together with the poor correlation between narrow [O III] and the optical continuum. That contrast is one of the clearest arguments that the red optical component is tied to the broad-line-producing region rather than the narrow-line region. ([arxiv.org](https://arxiv.org/abs/2606.09721))
• Figure 12. Include the later population-level comparison figure that captures abundance or environment, because this paper is not only about spectral classification but also about where LRDs sit cosmologically: their space density declines toward z about 2, opposite to the trend for normal AGNs, and their clustering points to host halos of several times 10^11 h^-1 solar masses, albeit with large uncertainties. ([arxiv.org](https://arxiv.org/abs/2606.09721))
• Figure 16. Choose this late synthesis figure if it is the composite-spectrum or BH* interpretation figure, since the paper’s bottom-line physical picture is that the observed LRD diversity, from Cliff-like extreme Balmer breaks to more moderately reddened continua, is broadly consistent with accreting SMBHs embedded in dense gas envelopes. A late diagnostic or composite figure is especially valuable here because it turns the sample diversity into the paper’s conclusion rather than leaving it as a gallery of examples. ([arxiv.org](https://arxiv.org/abs/2606.09721))