A Remarkably Luminous Galaxy at z=11.1 Measured with Hubble Space Telescope Grism Spectroscopy

We present Hubble WFC3/IR slitless grismspectra of a remarkably bright $z\gtrsim10$ galaxy candidate, GN-z11, identified initially from CANDELS/GOODS-N imaging data. A significant spectroscopic continuum break is detected at $\lambda=1.47\pm0.01~\mu$m. The new grismdata, combined with the photometric data, rule out all plausible lower redshift solutions for this source. The only viable solution is that this continuum break is the Ly$\alpha$ break redshifted to ${z_\mathrm{ grism}=11.09^{+0.08}_{-0.12}}$, just $\sim$400 Myrafter the Big Bang. This observation extends the current spectroscopic frontier by 150 Myrto well before the Planck (instantaneous) cosmic reionizationpeak at z~8.8, demonstrating that galaxy build-up was well underway early in the reionizationepoch at z>10. GN-z11 is remarkably and unexpectedly luminous for a galaxy at such an early time: its UV luminosity is 3x larger than L* measured at z~6-8. The Spitzer IRAC detections up to 4.5 $\mu$m of this galaxy are consistent with a stellar massof ${\sim10^{9}~M_\odot}$. This spectroscopic redshift measurement suggests that the James Webb Space Telescope(JWST) will be able to similarly and easily confirm such sources at z>10 and characterize their physical properties through detailed spectroscopy. Furthermore, WFIRST, with its wide-field near-IR imaging, would find large numbers of similar galaxies and contribute greatly to JWST's spectroscopy, if it is launched early enough to overlap with JWST.