# Spectroscopic confirmation of an ultra-faint galaxy at the epoch of reionization

## Abstract

Within one billion years of the Big Bang, intergalactic hydrogen was ionized by sources emitting ultraviolet and higher energy photons. This was the final phenomenon to globally affect all the baryons (visible matter) in the Universe. It is referred to as cosmic reionization and is an integral component of cosmology. It is broadly expected that intrinsically faint galaxies were the primary ionizing sources due to their abundance in this epoch1,2. However, at the highest redshifts (z > 7.5; lookback time 13.1 Gyr), all galaxies with spectroscopic confirmations to date are intrinsically bright and, therefore, not necessarily representative of the general population3. Here, we report the unequivocal spectroscopic detection of a low luminosity galaxy at z > 7.5. We detected the Lyman-α emission line at 10,504 Å in two separate observations with MOSFIRE4 on the Keck I Telescope and independently with the Hubble Space Telescope’s slitless grism spectrograph, implying a source redshift of z = 7.640 ± 0.001. The galaxy is gravitationally magnified by the massive galaxy cluster MACS J1423.8+2404 (z = 0.545), with an estimated intrinsic luminosity of MAB = −19.6 ± 0.2 mag and a stellar mass of $M ☆ = 3.0 − 0.8 + 1.5 × 10 8$ solar masses. Both are an order of magnitude lower than the four other Lyman-α emitters currently known at z > 7.5, making it probably the most distant representative source of reionization found to date.

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## Acknowledgements

A.H. and this work were supported by NASA (National Aeronautics and Space Administration) Headquarters under the NASA Earth and Space Science Fellowship Program, Grant ASTRO14F-0007. The data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA. The observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors thank L. Rizzi and M. Kassis for help with the Multi-Object Spectrometer for Infra-Red Exploration (MOSFIRE) observations and data reduction. The authors recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This work is also based on observations made with the NASA/European Space Agency Hubble Space Telescope (HST), obtained at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contracts NAS5-26555 and NNX08AD79G, and the European Southern Observatory Very Large Telescopes. Support for the Grism Lens-Amplified Survey from Space (GLASS) (HST-G0-13459) was provided by NASA through a grant from the STScI. We are very grateful to the staff of the Space Telescope Science Institute for their assistance in planning, scheduling and executing the observations, and in setting up the GLASS public release website. Support for this work was also provided by NASA through an award issued by the Jet Propulsion Laboratory, California Institute of Technology and through HST-AR-13235, HST-GO-13177, HST-GO-10200, HST-GO-10863 and HST-GO-11099 from STScI. Observations were also carried out using the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was also provided by NASA through a Spitzer award issued by the Jet Propulsion Laboratory, California Institute of Technology.

## Author information

Authors

### Contributions

A.H. handled the MOSFIRE reduction and analysis, led the lens modelling analysis and was the principal author of the paper. M.B. and M.T. designed and planned the MOSFIRE observations, contributed to the GLASS survey and contributed to writing the paper. M.B. also carried out the 27 May 2015 MOSFIRE observations. T.T. designed the GLASS survey and contributed to the design of the MOSFIRE observations and to writing the paper. K.B.S. handled the GLASS reduction and analysis and contributed to writing the paper. K.H.H. performed the HST and Spitzer photometry, led the stellar population modelling and contributed to writing the paper. B.C.L. contributed to the MOSFIRE analysis and to writing the paper. J.H. contributed to the lens modelling analysis. S.R.B. carried out the 19 March 2016 MOSFIRE observations. L.E.A., C.A.M., T.M. and L.P. contributed to the GLASS survey and to writing the paper. T.S. contributed to the lens modelling analysis.

### Corresponding author

Correspondence to Austin Hoag.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

## Supplementary information

### Supplementary Information

Supplementary Figure 1 and Supplementary References. (PDF 215 kb)

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Reprints and Permissions

Hoag, A., Bradacˇ, M., Trenti, M. et al. Spectroscopic confirmation of an ultra-faint galaxy at the epoch of reionization. Nat Astron 1, 0091 (2017). https://doi.org/10.1038/s41550-017-0091

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