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

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 Hβ 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 masses are uncertain, as they depend on the contribution of possible light from an active galactic nucleus (AGN) to the flux at λ_rest > 5000 Å. If the AGN contribution is low beyond the Balmer break region, the masses are M_* ∼ 10¹⁰-10¹¹ M_☉, and the central densities are higher than those of any other known galaxy population by 1 order of magnitude. Interestingly, the implied velocity dispersions of ∼1500 km s⁻¹ are in very good agreement with the measured Hβ line widths. We suggest that some of the broad lines in “little red dots” are not due to AGNs, but simply reflect the kinematics of the galaxies, and speculate that the galaxies are observed in a short-lived phase where the central densities are much higher than at later times. We stress, however, that the canonical interpretation of AGNs causing the broad Hβ lines also remains viable.