Are the newly-discovered z 13 drop-out sources starburst galaxies or quasars?

The detection of two z ~ 13 galaxy candidates 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 5-24 × 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 $\sim 10^8 \, \rm M_\odot$ accreting at or slightly above the Eddington rate (f_Edd ~ 1.0). This black hole mass at z ~ 13 would require very 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.