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The detection of two $z\sim 13$ galaxy candidates has opened a new window ongalaxy formation at an era only $330$ Myr after the Big Bang. Here, weinvestigate the physical nature of these sources: are we witnessing starforming galaxies or quasars at such early epochs? If powered by star formation,the observed ultraviolet (UV) luminosities and number densities can be jointlyexplained if: (i) these galaxies are extreme star-formers with star formationrates $5-24\times$ higher than those expected from extrapolations of averagelower-redshift relations; (ii) the star formation efficiency increases withhalo mass and is countered by increasing dust attenuation from $z \sim 10-5$;(iii) they form stars with an extremely top-heavy initial mass function. Thequasar hypothesis is also plausible, with the UV luminosity produced by blackholes of $\sim 10^8 \, \mathrm{M_{\odot}}$ accreting at or slightly above theEddington rate ($f_{\rm Edd}\sim 1.0$). This black hole mass at $z\sim13$ wouldrequire very challenging, but not implausible, growth parameters. Ifspectroscopically confirmed, these two sources will represent a remarkablelaboratory to study the Universe at previously inaccessible redshifts.

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