Today the sparse distribution of hot gas between galaxies, known as the interstellar medium (IGM), is found to be ionized. The early universe was extremely hot at first, but it quickly expanded and cooled, allowing its main constituent hydrogen to combine to form neutral atoms. When and how did these neutral atoms reionize to form the interstellar medium we see today? Astronomers think that ultraviolet light from massive young stars did the job once stars began to form and glow in the cosmic age named for the event, the "reionization age."

One of the key steps in the reionization of the interstellar medium is the escape of ultraviolet radiation from the galaxy to the interstellar medium, but this is not well understood. Astronomers only know that it has to be effective, because starlight can only do the job if the escaping part is high enough. Star-forming galaxies, however, are rich in dense molecular gas and dust, which also absorb a lot of ultraviolet radiation. This suggests that a number of other important sources of ionizing radiation are needed, presumably including the possible presence of exotic objects such as quasars, X-ray binaries, and possibly even decay/annihilation particles.

So far, however, there is little evidence that any of them is rich enough or capable of doing the job. Rohan Naidu, Sandro Tacchella, Charlotte Mason, Sownak Bose and Charlie Conroy, astronomers at the Harvard-Smithsonian Center for Astrophysics, led a study to better estimate the most uncertain parameter of the puzzle (and the most difficult to measure directly) : the escape fraction of ionized photons. The study compared measurements and models of two other key processes, the rate of star formation in the galaxy and the number of ultraviolet photons produced.So far, however, there is little evidence that any of them is rich enough or capable of doing the job. Rohan Naidu, Sandro Tacchella, Charlotte Mason, Sownak Bose and Charlie Conroy, astronomers at the Harvard-Smithsonian Center for Astrophysics, led a study to better estimate the most uncertain parameter of the puzzle (and the most difficult to measure directly) : the escape fraction of ionized photons. The study compared measurements and models of two other key processes, the rate of star formation in the galaxy and the number of ultraviolet photons produced.

Examine what these must be applied to limit escape scores in order to keep the modeling consistent. The measurements are not controversial, but the model is different, and the scientists choose between two types: one where the escape fraction is constant during reionization, and one that depends on the rate of star formation. The astronomers reached several important conclusions: Escape scores (at least for bright galaxies) would need to reach about 20 percent in the early universe, about twice as high as they had before. This may be because concentrated areas of star formation can blow away channels through which ultraviolet light escapes.

Using cosmological simulations, the study also found that the young universe went from being 90% neutral to only 10% in just 300 million years. Most importantly, most of the reionization is done by a small number of the most massive and luminous galaxies that the researchers call "oligopolies." Previous studies have shown that there are plenty of dark galaxies that can do this, but the new results disagree and conclude that such a population should have been detected, according to a study published in the Astrophysical Journal.