ATLAS. IV. A JWST+MUSE Demographic Study of Ly$α$ Profiles in Little Red Dots

Yuta Kageura, Masami Ouchi, Hiroto Yanagisawa, Makoto Ando, Yuichi Harikane, Tomokazu Kiyota, Minami Nakane, Yoshiaki Ono, Yui Takeda

First listed 2026-08-17 | Last updated 2026-08-14

Abstract

We present an initial demographic study of Ly$α$ profiles in little red dots (LRDs) at $z=3$--9 using $R\sim1000$--4000 spectroscopy. Our sample consists of 8 LRDs observed in the VLT/MUSE Deep and Wide surveys and 23 LRDs observed with JWST/NIRSpec grating spectroscopy from JADES, CANUCS, and GO programs including SPURS. We identify Ly$α$ emission in 5 MUSE LRDs and 9 JWST LRDs. Only two of them exhibit broad Ly$α$ emission (FWHM $>1000\ \mathrm{km\ s^{-1}}$), both reported previously. The other Ly$α$-emitting LRDs show narrow Ly$α$ emission (FWHM $<1000\ \mathrm{km\ s^{-1}}$), with FWHMs mostly in the range 300--600 $\mathrm{km\ s^{-1}}$, comparable to or slightly larger than those of high-redshift star-forming galaxies (100--500 $\mathrm{km\ s^{-1}}$). We measure the fraction of broad Ly$α$ emitters above a broad Ly$α$ luminosity threshold of $L_{\mathrm{Ly}α,\mathrm{broad}}=10^{42}\ \mathrm{erg\ s^{-1}}$, obtaining $0.10^{+0.12}_{-0.07}$ for LRDs, about five times higher than the $2σ$ upper limit of $<0.02$ for high-redshift star-forming galaxies. Although we reproduce the broad Ly$α$ component reported in a previous stacking analysis of eight LRDs, albeit with a large uncertainty, we find no evidence for broad Ly$α$ emission in either the larger JWST stack, after excluding the two individually detected broad Ly$α$ emitters, or the higher-resolution MUSE stack. These results suggest that broad Ly$α$ emission is not ubiquitous among LRDs. Instead, LRDs with broad Ly$α$ emission appear to represent a rare population that may correspond to a particular evolutionary stage, potentially associated with unusually strong outflows or other distinctive physical conditions.

Short digest

Kageura et al. assemble resolved Lyα spectroscopy for 31 little red dots at z=3–9, combining eight VLT/MUSE sources with 23 JWST/NIRSpec grating targets from JADES, CANUCS, and GO programs. Lyα is detected in 14 objects, but only the previously reported CANUCS-LRD-z8.6 and Abell2744-QSO1 show broad emission above 1000 km s−1; the rest are predominantly narrow, with FWHM of roughly 300–600 km s−1. At L_Lyα,broad > 10^42 erg s−1, the inferred broad-line fraction is 0.10^{+0.12}_{-0.07}, above the <0.02 upper limit for high-redshift star-forming galaxies, yet stacks excluding those two detections show no significant broad component. Broad Lyα is therefore not a generic LRD signature, but may mark a rare phase with unusually porous, low-covering-factor, or outflowing nuclear gas that lets AGN-associated Lyα escape.

Key figures to inspect

• Figure 4. This is the central object-level evidence: the nine JWST/NIRSpec Lyα detections show that most resolved LRD profiles are narrow, while GN-1020514 requires a distinct broad component. It grounds the demographic conclusion in the actual grating spectra and profile fits.
• Figure 6. The three stack fits directly test whether broad Lyα is hidden below individual-detection thresholds. The non-detections of statistically significant broad components in the larger 21-object JWST stack and the MUSE stack are the key counterpoint to the earlier eight-object stacking claim.
• Figure 7. This figure supplies the paper's main quantitative demographic result: broad-Lyα emitter fractions as a function of luminosity threshold for LRDs and star-forming galaxies. It makes clear that the apparent LRD excess is driven by only two secure broad emitters and remains uncertain.
• Figure 8. The FWHM comparison places the narrow LRD Lyα lines, generally 300–600 km s−1, alongside JWST AGNs and MUSE star-forming galaxies. It visually supports the conclusion that the majority of Lyα-emitting LRDs resemble star-forming galaxies more than broad-line AGNs in Lyα width.
• Figure 12. The schematic distills the proposed physical interpretation: most LRDs retain broad Lyα inside a nearly closed, high-column-density envelope, whereas rare sources reveal it through lower covering factor or gaps in a porous medium. It connects the observed rarity of broad Lyα to the dense-gas picture for LRD nuclei.