Galaxies are surrounded by large reservoirs of gas, mostly hydrogen, that are fed by inflows from the intergalactic medium and by outflows from galactic winds. Absorption-line measurements along the lines of sight to bright and rare background quasars indicate that this circumgalactic medium extends far beyond the starlight seen in galaxies, but very little is known about its spatial distribution. The Lyman-α transition of atomic hydrogen at a wavelength of 121.6 nanometres is an important tracer of warm (about 104 kelvin) gas in and around galaxies, especially at cosmological redshifts greater than about 1.6 at which the spectral line becomes observable from the ground. Tracing cosmic hydrogen through its Lyman-α emission has been a long-standing goal of observational astrophysics1,2,3, but the extremely low surface brightness of the spatially extended emission is a formidable obstacle. A new window into circumgalactic environments was recently opened by the discovery of ubiquitous extended Lyman-α emission from hydrogen around high-redshift galaxies4,5. Such measurements were previously limited to especially favourable systems6,7,8 or to the use of massive statistical averaging9,10 because of the faintness of this emission. Here we report observations of low-surface-brightness Lyman-α emission surrounding faint galaxies at redshifts between 3 and 6. We find that the projected sky coverage approaches 100 per cent. The corresponding rate of incidence (the mean number of Lyman-α emitters penetrated by any arbitrary line of sight) is well above unity and similar to the incidence rate of high-column-density absorbers frequently detected in the spectra of distant quasars11,12,13,14. This similarity suggests that most circumgalactic atomic hydrogen at these redshifts has now been detected in emission.
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The observations of the HUDF discussed in this paper were made using European Southern Observatory (ESO) Telescopes at the La Silla Paranal Observatory under programme IDs 094.A-0289, 095.A-0010, 096.A-0045 and 096.A-0045. The corresponding data are available on the ESO archive at http://archive.eso.org/cms.html. The data of the HDFS were obtained during MUSE commissioning observations and are available at http://muse-vlt.eu/science/hdfs-v1-0/.
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We thank the ESO staff for the support that made these observations possible. L.W., J.K., R.S. and T.U. acknowledge support by the Competitive Fund of the Leibniz Association through grants SAW-2013-AIP-4 and SAW-2015-AIP-2. R.B., H.I., F.L. and M.A. are supported by the ERC advanced grant 339659-MUSICOS. J.B. acknowledges support by FCT grants UID/FIS/04434/2013 and IF/01654/2014/CP1215/CT0003 and by FEDER through COMPETE2020 (POCI-01-0145-FEDER-007672). J.R. acknowledges support from the ERC starting grant 336736-CALENDS. P.M.W. received support through BMBF Verbundforschung, grant 05A17BAA. T.C., N.B. and B.G. acknowledge support by ANR FOGHAR (ANR-13-BS05-0010-02). T.C. and N.B. were also supported by OCEVU Labex (ANR-11-LABX-0060), and by the A*MIDEX project (ANR-11-IDEX-0001-02) funded by the “Investissements d’avenir” French government programme. A.M.I. acknowledges support from MINECO through project AYA2015-68217-P. S.C. acknowledges support from Swiss National Science Foundation grant PP00P2_163824.
Nature thanks G. Micheva and the other anonymous reviewer(s) for their contribution to the peer review of this work.