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

Abstract

JWST has found many faint broad-line AGN at $z>4$, but it is unclear whether the Ly$α$ 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 Ly$α$ 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 Ly$α$. Every source shows a broad-line region, yet Ly$α$ does not follow it: the sources with the strongest H$α$ relative to the UV include weak detections and nondetections, and every detected Ly$α$ line (FWHM $\approx300$--1000~km~s$^{-1}$) is narrower than broad H$α$ (1100--3300~km~s$^{-1}$). Instead, Ly$α$ appears to track the host galaxy. We model the AGN-subtracted host with a dust-attenuated SED fit. Including the upper limits, the observed Ly$α$ follows the Ly$α$ production in this host model more closely than it follows H$α$/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 Ly$α$ we observe is more closely tied to its host. The median effective Ly$α$ escape fraction of the detections is only 0.09, which suggests that ionizing escape from CBEs is typically modest and varies widely.