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

TL;DR

This study uses 12 orbits of HST/WFC3/IR G141 slitless grism spectroscopy to confirm GN-z11 as a galaxy at $z=11.09^{+0.08}_{-0.12}$, based on a pronounced continuum break near $1.47\,\mu$m interpreted as the Ly$\alpha$ break. The team demonstrates rigorous contamination modeling for slitless spectra and rules out lower-redshift contaminants, including extreme emission-line, dusty, or quiescent scenarios. GN-z11 emerges as an exceptionally luminous and relatively massive system for its epoch, with $M_{UV}\approx-22.1$, $\log M/M_\odot\approx9.0$, SFR $\approx24\,M_\odot\,\mathrm{yr}^{-1}$, and age $\lesssim40$ Myr, suggesting rapid early star formation. The discovery challenges expectations for the bright end of the UV luminosity function at $z>10$ and highlights the potential for JWST and WFIRST to greatly expand the census of the earliest galaxies.

Abstract

We present Hubble WFC3/IR slitless grism spectra 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 $λ=1.47\pm0.01~μ$m. The new grism data, 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$α$ break redshifted to ${z_\mathrm{grism}=11.09^{+0.08}_{-0.12}}$, just $\sim$400 Myr after the Big Bang. This observation extends the current spectroscopic frontier by 150 Myr to well before the Planck (instantaneous) cosmic reionization peak at z~8.8, demonstrating that galaxy build-up was well underway early in the reionization epoch 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 $μ$m of this galaxy are consistent with a stellar mass of ${\sim10^{9}~M_\odot}$. 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.

Figures (11)

• Figure 1: CANDELS $H$-band image around the location of our target source GN-z11. The arrows and dashed lines indicate the direction along which sources are dispersed in the slitless grism spectra for our two individual epochs (magenta and blue) and for the pre-existing AGHAST data (green). The latter are significantly contaminated by bright neighbors along the dispersion direction of GN-z11 (see Fig \ref{['fig:ContamModel']}).
• Figure 2: Our model of the contaminating flux from neighboring sources in the slitless grism spectra around the trace of our source (i.e., the line along which we expect its flux; indicated by red lines). From top to bottom, the panels show the final model contamination in our spectra in epochs 1 and 2 and in the pre-existing AGHAST spectra. Note that the contamination model includes emission lines for the neighboring sources as calibrated from our two-epoch data. The high contaminating flux in the AGHAST data makes these spectra inadequate for studying GN-z11. Our orientations were chosen based on extensive simulations to minimize such contamination from neighbors. However, some zeroth order flux in epoch 2 could not be avoided while at the same time making the observations schedulable with HST in cycle 22.
• Figure 3: 2D grism data of GN-z11. The five panels show from top to bottom (1) the original 2D spectrum from a stack of all our G141 grism data, (2) the modeled contaminating flux from neighboring sources, (3) the cleaned 2D spectrum of GN-z11, (4) the model of a $z=11.09$ source with the same morphology and H-band magnitude as GN-z11, and (5) the residual spectrum after subtracting the $z=11.09$ continuum model. The observed grism flux is completely consistent with the model flux, as can be seen from the clean residual. The observed spectrum also falls off at $\sim1.65~\mu$m, exactly as expected based on the drop in the G141 grism sensitivity providing further strong support that the observed flux is indeed the continuum of GN-z11. The spatial direction extends over 3.6 arcsec, and the two red lines indicate the trace of GN-z11.
• Figure 4: The new 12-orbit deep grism spectra in combination with the photometry of GN-z11 exclude lower redshift solutions. The main contaminants for high-redshift galaxy selections are sources with extreme emission lines or with very strong 4000 Å breaks. The top left panel shows the photometry together with three example SEDs for the possible nature of GN-z11 (dark red: a $z=11.09$ star-forming galaxy, blue: an extreme line emitter at $z=2.1$, green: a dusty+quiescent galaxy at $z=2.5$). The last one is only shown for illustration purposes as it can be clearly excluded based on the longer wavelength photometry ($\Delta\chi_p^2>8000$ relative to the best fit SED model). The remaining panels compare the observed 1D spectrum with the expected grism fluxes for the same three cases. The best-fit to the grism data is provided by the high-redshift LBG template which interprets the observed break as a Ly$\alpha$ break. This solution has a reduced $\chi^2=1.2$. The other two cases can be excluded based on the difference in $\chi^2$ in the grism spectra as well as from the photometry ($\chi_p^2$).
• Figure 5: Grism Spectrum of GN-z11. The top panel shows the (negative) 2D spectrum from the stack of our cycle 22 data (12 orbits) with the trace outlined by the dark red lines. For clarity the 2D spectrum was smoothed by a Gaussian indicated by the ellipse in the lower right corner. The bottom panel is the un-smoothed 1D flux density using an optimal extraction rebinned to one resolution element of the G141 grism (93 Å). The black dots show the same further binned to 560 Å, while the blue line shows the contamination level that was subtracted from the original object spectrum. We identify a continuum break in the spectrum at $\lambda=1.47\pm0.01~\mu$m. The continuum flux at $\lambda>1.47~\mu m$ is detected at $\sim1-1.5\sigma$ per resolution element and at 3.8$\sigma$ per 560 Å bin. After excluding lower redshift solutions (see text and Fig \ref{['fig:ContamSpectra']}), the best-fit grism redshift is $z_\mathrm{grism}=11.09^{+0.08}_{-0.12}$. The red line reflects the Ly$\alpha$ break at this redshift, normalized to the measured H-band flux of GN-z11. The agreement is excellent. The fact that we only detect significant flux along the trace of our target source, which is also consistent with the measured H-band magnitude, is strong evidence that we have indeed detected the continuum of GN-z11 rather than any residual contamination.