2023MNRAS.520.2445Z

The recent James Webb Space Telescope tentative discovery of a population of super-early (redshift z > 10), relatively massive (stellar mass M_* = 10⁸-10⁹M_⊙) and evolved (metallicity Z ≈ 0.1 Z_⊙) galaxies, which nevertheless show blue (β ≃ -2.6) spectra, and very small dust attenuation (A_V ≲ 0.02), challenges our interpretation of these systems. To solve the puzzle, we propose two solutions in which dust is either (a) ejected by radiation pressure, or (b) segregated with respect to UV-emitting regions. We clarify the conditions for which the two scenarios apply, and show that they can be discriminated by ALMA observations, such as the recent non-detection of the 88μm dust continuum in GHZ2 (z ≃ 12) favouring dust ejection.