The Small Sizes and High Implied Densities of “Little Red Dots” with Balmer Breaks Could Explain Their Broad Emission Lines without an Active Galactic Nucleus

Abstract Early JWST studies found an apparent population of massive, compact galaxies at redshifts z ≳ 7. Recently three of these galaxies were shown to have prominent Balmer breaks, demonstrating that their light at λ rest ∼ 3500 Å is dominated by a stellar population that is relatively old (∼200 Myr). All three also have broad H β emission with σ > 1000 km s −1 , a common feature of such “little red dots.” From Sérsic profile fits to the Near Infrared Camera images in F200W we find that the stellar light of galaxies is extremely compact: the galaxies have half-light radii of r e ∼ 100 pc, in the regime of ultracompact dwarfs in the nearby Universe. Their…