2016MNRAS.463.3556M

Using Hubble data, including new grism spectra, Oesch et al. recently identified GN-z11, an M_UV = -21.1 galaxy at z = 11.1 (just 400 Myr after the big bang). With an estimated stellar mass of ∼10⁹ M_☉, this galaxy is surprisingly bright and massive, raising questions as to how such an extreme object could form so early in the Universe. Using MERAXES, a semi-analytic galaxy-formation model developed as part of the Dark-ages Reionization And Galaxy-formation Observables from Numerical Simulations (DRAGONS) programme, we investigate the potential formation mechanisms and eventual fate of GN-z11. The volume of our simulation is comparable to that of the discovery observations and possesses two analogue galaxies of similar luminosity to this remarkably bright system. Existing in the two most massive subhaloes at z = 11.1 (M_vir = 1.4 x 10¹¹ M_☉ and 6.7 x 10¹⁰ M_☉), our model analogues show excellent agreement with all available observationally derived properties of GN-z11. Although they are relatively rare outliers from the full galaxy population at high-z, they are no longer the most massive or brightest systems by z = 5. Furthermore, we find that both objects possess relatively smooth, but extremely rapid mass growth histories with consistently high star formation rates and UV luminosities at z > 11, indicating that their brightness is not a transient, merger-driven feature. Our model results suggest that future wide-field surveys with the James Webb Space Telescope may be able to detect the progenitors of GN-z11 analogues out to z ∼ 13-14, pushing the frontiers of galaxy-formation observations to the early phases of cosmic reionization and providing a valuable glimpse of the first galaxies to reionize the Universe on large scales.