Are the Newly-Discovered z 13 Drop-out Sources Starburst Galaxies or Quasars?

The detection of two z ~ 13 galaxy candidates by Harikane et al. (2021) has opened a new window on galaxy formation at an era only 330 Myr after the Big Bang. Here, we investigate the physical nature of these sources: are we witnessing star forming galaxies or quasars at such early epochs? If powered by star formation, the observed ultraviolet (UV) luminosities and number densities can be jointly explained if: (i) these galaxies are extreme star-formers with star formation rates 5-25× higher than those expected from extrapolations of average lower-redshift relations; (ii) the star formation efficiency increases with halo mass and is countered by increasing dust attenuation from z ~ 10-5; (iii) they form stars with an extremely top-heavy initial mass function. The quasar hypothesis is also plausible, with the UV luminosity produced by black holes of ~10⁸ M_⊙ accreting at or slightly above the Eddington rate (f_Edd ~ 1.0). This black hole mass at z ~ 13 would require challenging, but not implausible, growth parameters. If spectroscopically confirmed, these two sources will represent a remarkable laboratory to study the Universe at previously inaccessible redshifts.