Eight per cent leakage of Lyman continuum photons from a compact, star-forming dwarf galaxy


One of the key questions in observational cosmology is the identification of the sources responsible for ionization of the Universe after the cosmic ‘Dark Ages’, when the baryonic matter was neutral. The currently identified distant galaxies are insufficient to fully reionize the Universe by redshift z ≈ 6 (refs 1, 2, 3), but low-mass, star-forming galaxies are thought to be responsible for the bulk of the ionizing radiation4,5,6. As direct observations at high redshift are difficult for a variety of reasons, one solution is to identify local proxies of this galaxy population. Starburst galaxies at low redshifts, however, generally are opaque to Lyman continuum photons7,8,9. Small escape fractions of about 1 to 3 per cent, insufficient to ionize much surrounding gas, have been detected only in three low-redshift galaxies10,11. Here we report far-ultraviolet observations of the nearby low-mass star-forming galaxy J0925+1403. The galaxy is leaking ionizing radiation with an escape fraction of about 8 per cent. The total number of photons emitted during the starburst phase is sufficient to ionize intergalactic medium material that is about 40 times as massive as the stellar mass of the galaxy.

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Figure 1: The O32R23 diagram for star-forming galaxies.
Figure 2: Near-ultraviolet image of J0925+1403 from the HST.
Figure 3: The double-peaked Lyα emission line in the COS spectrum of J0925+1403.
Figure 4: The COS spectrum of J0925+1403.


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This Letter is based on observations made with the NASA/ESA HST, obtained from the data archive at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5–26555. Support for this work was provided by NASA through grant number HST-GO-13744.001-A from the STScI. I.O. acknowledges a grant GACR 14–20666P of the Czech Science Foundation. The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions. GALEX is a NASA mission managed by the Jet Propulsion Laboratory. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.

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All authors participated in the design of the HST observational program. Y.I.I. and N.G.G. selected the galaxy sample. T.X.T. and Y.I.I. led the observations. G.W. reduced the HST data. I.O. did part of the HST data analysis. Y.I.I. and D.S. did the SED modelling and interpretation. A.V. and Y.I.I. did the Lyα interpretation. The bulk of the text was written by Y.I.I. All authors commented on the manuscript at all stages.

Corresponding author

Correspondence to Y. I. Izotov.

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Extended data figures and tables

Extended Data Figure 1 The diagnostic diagram for narrow emission lines.

The foundations of this diagram are given in ref. 29. The galaxy J0925+1403 is shown by a large filled star, and the Luminous Compact Galaxies13 by small dark-grey circles. Also plotted are the 100,000 emission-line galaxies from SDSS DR7 (cloud of light-grey dots). The solid line30 separates star-forming galaxies (SFG) from active galactic nuclei (AGN).

Extended Data Figure 2 SED fitting of the optical spectrum of J0925+1403.

The rest-frame extinction-corrected spectrum is shown by a grey line. The stellar, ionized gas, and total modelled SEDs are shown by black dotted, dashed and solid lines, respectively.

Extended Data Figure 3 A comparison of the observed ultraviolet and optical spectrum with the modelled SED.

The observed spectrum is shown by a grey line. The total GALEX and SDSS photometric fluxes are represented by filled squares and filled circles, respectively, while the SDSS photometric fluxes within a round spectroscopic aperture of 3″ diameter are shown by open circles. Modelled SEDs, which are reddened by the Milky Way with RV,MW = 3.1 and internal extinction with different values of RV,int, are shown by black lines. Dotted, dashed and solid lines correspond to RV,int = 3.1, 2.7, and 2.4, respectively.

Extended Data Table 1 Emission-line fluxes and equivalent widths in the optical spectrum
Extended Data Table 2 Physical conditions and chemical composition
Extended Data Table 3 Global characteristics of J0925+1403

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Izotov, Y., Orlitová, I., Schaerer, D. et al. Eight per cent leakage of Lyman continuum photons from a compact, star-forming dwarf galaxy. Nature 529, 178–180 (2016). https://doi.org/10.1038/nature16456

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