NEXUS: Spectral Variability of Little Red Dots and Blue Active Galactic Nuclei at $2 \lesssim z \lesssim 6$

Zachary Stone, Yue Shen, Ming-Yang Zhuang, Junyao Li, Zhiwei Pan, Jenny E. Greene, Feige Wang

First listed 2026-08-04 | Last updated 2026-08-03

Abstract

We present spectral measurements for 17 Little Red Dots (LRDs) and 14 blue broad-line active galactic nuclei (AGNs) at $2\lesssim z \lesssim 6$ using multi-epoch JWST NIRSpec MSA spectra from the NEXUS program, sampling rest-frame timescales of $\sim 1-3$ months. Overall, the LRD population shows significantly enhanced Balmer decrement compared with both blue JWST AGNs at similar redshifts and 56 low-redshift broad-line AGNs matched in H$\rmα$ luminosity. The rest-optical continua of LRDs show little ensemble variability (rms $\lesssim 3\%$), and the total H$\rmα$ emission also shows weaker ensemble variability compared with low-redshift AGNs matched in H$\rmα$ luminosity and rest-frame timescales. Based on the flux uncertainties, we constrain the intrinsic H$\rmα$ rms variability to be $\lesssim 4\%$ for the LRD population over these timescales. Combining our results with recent broad-line variability measurements of LRDs over yearly to decade timescales reveals a low-level white-noise pattern across all timescales, in stark contrast to the variability amplitude ($\sim 6\%$ over monthly timescales) and red-noise pattern observed in normal AGNs. These results add to the growing observational studies that suggest population-wise, LRDs have weak variability both in optical continuum and broad-line emission. Furthermore, the distinct white-noise broad-line variability pattern suggests different production mechanisms of broad-line emission in LRDs as opposed to normal AGNs, and/or different properties of the driving ionizing flux from the central engine.

Short digest

Using multi-epoch JWST/NIRSpec MSA spectra from NEXUS, Stone et al. measure rest-frame optical continuum and broad-line changes over roughly 1–3 months for 17 little red dots (LRDs) and 14 blue broad-line AGNs at z≈2–6. LRDs have enhanced Balmer decrements relative to both the blue JWST AGNs and Hα-luminosity-matched low-redshift AGNs, while their rest-optical continua show ensemble rms variability below 3% and their intrinsic Hα rms is constrained to below 4%. Combined with published longer-baseline broad-line measurements, the LRD results trace low-level, white-noise-like variability from monthly through decade timescales, unlike the stronger red-noise variability of ordinary AGNs. The distinct line-variability behavior suggests that LRD broad-line emission, or the ionizing continuum driving it, differs systematically from the normal AGN picture.

Key figures to inspect

• Figure 1. This establishes the NEXUS LRD and blue-AGN samples in luminosity-redshift space, identifies the Hα-variability subsample, and shows how the authors construct the low-redshift SDSS-RM comparison.
• Figure 6. The multi-line Balmer and Paschen ratios provide the central spectral evidence that LRDs have systematically enhanced Balmer decrements relative to blue AGNs and the comparison populations.
• Figure 8. This is the conclusion-driving broad-line diagnostic: Hα pairwise variability is plotted against rest-frame lag alongside SDSS-RM AGNs and longer-timescale LRD constraints, making the low-amplitude white-noise pattern directly visible.
• Figure 9. The continuum structure functions show that weak variability is not confined to Hα, linking the spectroscopic result to the similarly suppressed rest-optical continuum fluctuations of the LRD population.