Extremely ultraviolet-bright starbursts at the end of cosmic reionization

Vol. 711
4. Extragalactic astronomy

Extremely ultraviolet-bright starbursts at the end of cosmic reionization

J0217–0208 at z = 6.204 (MUV =−23.4) was observed with JWST/NIRCam and NIRSpec. Its spectral energy distribution indicates a young starburst (∼5 Myr) with a stellar mass of a billion solar masses and a high specific star formation rate (∼100 Gyr−1). Its very compact size (r_eff ~ 260 pc) corresponds to stellar mass and star formation rate surface densities ≈100 times higher than those of typical galaxies at comparable redshifts. There is strong nebular emission (high electron density, n_e ≃1000/cm3) and a super-solar oxygen-to-nitrogen ratio (log(N/O)≃−0.30). J0217–0208 shows broad components in several rest-optical emission lines, which indicates powerful ionized outflows. They appear to be heavily obscured (E(B−V)out ≃0.6) despite the nearly dust-free stellar continuum (E(B−V)⋆ = 0.01±0.01). ALMA observations show a massive (Mdust ≃2 × 108 M⊙), extended (∼1.4 kpc), and cold (Tdust ≃25 K) dust reservoir. These findings point to dusty, feedback-driven outflows carrying and pushing dust well beyond the stellar core and likely boosting the observed UV luminosity.