A universe with oligarchs: Era of reionization likely the work of the most massive, luminous galaxies

The sparsely distributed hot gas found today between galaxies, the intergalactic medium (IGM), is ionized. The early universe started off hot, but then it rapidly expanded and cooled allowing its main constituent, hydrogen, to combine to form neutral atoms. When and how did these neutral atoms become reionized to compose the IGM we see today? Astronomers think that ultraviolet radiation emitted by massive young stars did this work once stars began to form and shine during the cosmic era named after this activity, the "era of reionization."

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