Most of the ordinary matter in the Universe is found not in stars, but in the diffuse gas between galaxies: the intergalactic medium (IGM), which is mainly hydrogen. This gas is almost completely ionized, and has been so since the formation of the first stars and galaxies a few hundred million years after the Big Bang. But few details are known about the sources of the radiation that ionized the gas, or how this radiation escaped from its source galaxies. On page 178 of this issue, Izotov et al.1 report the detection of ionizing radiation from a star-forming dwarf galaxy in the local Universe, which may clarify the escape question.

The hydrogen gas that pervades the Universe has undergone phase changes over 13.8 billion years. The early Universe was too hot for protons and electrons to combine into neutral hydrogen, so the hydrogen was ionized. The Universe cooled as it expanded, and about 375,000 years after the Big Bang, the temperature decreased enough for neutral hydrogen to form.

The gas remained neutral for the next few hundred million years, until the epoch of reionization — the last major phase transition in the Universe (Fig. 1). This transition occurred when the first sources of photons that were energetic enough to ionize hydrogen appeared in sufficient numbers to reionize the IGM. These photons are known as Lyman continuum photons because they have wavelengths shorter than the Lyman limit of 912 ångströms, which corresponds to the energy required to ionize the hydrogen atom (13.6 electronvolts).

Figure 1: Escape of ionizing radiation from a galaxy.
figure 1

The hydrogen gas between galaxies was ionized by the first stars and galaxies when the Universe was about 400 million years old. This probably occurred gradually (main panel): radiation from stars and galaxies ionized increasingly large bubbles of gas, until the ionized regions completely overlapped. For this to happen, photons with enough energy to ionize hydrogen must escape from galaxies. Such escape is challenging, because galaxies are filled with neutral hydrogen gas that absorbs ionizing radiation (inset; arrows represent ionizing photons). Izotov et al.1 report the detection of ionizing photons from a compact starburst galaxy in the nearby Universe, a discovery that helps to explain the conditions that allow such radiation to escape from galaxies.

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We now know, from the scattering of cosmic microwave background photons by reionized electrons2 and from observations of the absorption by neutral hydrogen in the spectra of extremely distant quasars3, that reionization was a gradual process, the midpoint of which occurred approximately 400 million years after the Big Bang. However, we have not yet observed the changing ionization state of the IGM directly, and theoretical models struggle to explain how the known population of galaxies at this epoch could have produced enough radiation for reionization to occur.

The problem is twofold: large numbers of faint galaxies seem to be required to supply the necessary radiation4, but photons must also be able to escape from the galaxies in which they are produced. Stars form out of cool gas, and thus star-forming galaxies are filled with neutral hydrogen. Neutral hydrogen absorbs ionizing photons and therefore, even in a galaxy whose stars produce copious amounts of energetic radiation, the proportion of this radiation that actually escapes from the galaxy (the escape fraction) may be extremely low.

The Lyman continuum radiation from the galaxies that reionized the Universe will never reach our telescopes, because the photons are absorbed by the IGM long before they get to Earth. It is possible to detect ionizing radiation from closer galaxies, those corresponding to an age of the Universe of approximately 1.6 billion to 2 billion years (see refs 5 and 6, for example), but such detections are complicated by the probability of contamination by non-ionizing radiation originating in faint galaxies along the line of sight. In some ways, galaxies in the local Universe offer the best prospect for a detailed understanding of how ionizing radiation escapes from galaxies. Complicating factors are the unknown similarities and differences between local galaxies and the galaxies of the reionization era, and the fact that Earth's atmosphere blocks the ultraviolet wavelengths of this radiation, requiring that observations be made from space.

Izotov et al. focused on the compact dwarf galaxy J0925 + 1403 — deemed likely to produce escaping ionizing radiation on the basis of properties inferred from its optical emission lines. These lines indicate an unusually high ionization state in the gas near the galaxy's star-forming regions, suggesting that the stars may make more ionizing radiation than can be absorbed by the surrounding gas. The authors' successful detection of ionizing photons is the fourth such observation from a nearby galaxy7,8,9.

Crucially, this galaxy has the highest escape fraction yet measured locally: about 8%, compared with the roughly 1–3% escape fraction measured from other nearby galaxies7,8,9. The total amount of escaped radiation is sufficient to ionize a mass of IGM gas 40 times greater than the galaxy's stellar mass. Finding Lyman continuum radiation from this galaxy therefore broadly confirms our understanding of the general conditions that may facilitate the escape of ionizing radiation.

However, much work remains to be done to understand how galaxies reionized the Universe: the current study involves a single galaxy, whereas reionization depends on the properties of a population. It is not yet clear whether or not J0925 + 1403 is typical of compact, highly ionized starbursts (galaxies with extremely high rates of star formation) in the nearby Universe. We also do not know whether this galaxy is similar to those that reionized the Universe; its small size, high ionization state and relatively low degree of enrichment by elements heavier than helium generally match the expected properties of such objects, but none of these properties has been measured for the earliest galaxies.

Izotov et al. report that J0925 + 1403 leaks a large number of ionizing photons relative to its ultraviolet luminosity. This finding will inform future theoretical models of the reionization of the Universe by faint galaxies, but it remains to be determined whether this result is typical in the local Universe or representative of galaxies in the reionization era. The authors' detection therefore emphasizes the need for additional, larger studies to develop a statistical understanding of Lyman continuum escape in the local Universe and its relationship to the properties of galaxies more generally.Footnote 1