Where the red rise stops: JWST/MIRI reveals a near-infrared turnover in the Little Red Dot Abell2744-QSO1

Abstract

Little Red Dots (LRDs) are compact sources with a blue rest-frame UV, a steep red optical continuum and, often, broad hydrogen lines. They have been interpreted as dense stellar systems, dust-reddened active galactic nuclei, or black holes embedded in dense, dust-poor gas, scenarios that are hard to tell apart in the rest-frame UV and optical. We present JWST/MIRI F770W and F1000W imaging of the triply lensed Abell2744-QSO1 at z=7.04, probing rest-frame 0.96 and 1.24um. QSO1 is unresolved, and its steep optical rise stops near rest-frame 1um. Up to 1.24um, a T~4450K blackbody, or a hotter one reddened by dust, describes the continuum, with no sign of the hot dust seen in most type-1 quasars. Our statistically preferred SED fits produce the red continuum with (3-4)x10^9 Msun of stars, over 100 times the dynamical limit on the host mass, and are therefore disfavoured. An accretion disc in a dense gas cocoon respects this limit but over-predicts the 1.24um flux by 5 sigma. Super-Eddington accretion models with a thermalising envelope reproduce the turnover, but only for a black hole ~2.5dex less massive than the dynamical estimate. For a dust-reddened accretion disc (A_V ~ 2 mag), dust at ~1200K can re-emit at most ~2 per cent of the absorbed luminosity, and most of it would have to emerge at ~70-600K, unless the dust covers only a small fraction of the sky. The dynamics favour an accretion-powered source, but our data cannot distinguish dust reddening from processing by dense gas. MIRI imaging at >18um could separate these scenarios.