The earliest galaxies were 10,000 times smaller than the Milky Way, but the radiation they emitted effectively precluded any more of their kind from developing. From then on, big galaxies dominated, signaling a major shift in the history of the universe. Now a team has provided the first, albeit indirect, evidence for the scenario.
Galaxies first formed about 13 billion years ago, when gas clouds began to condense under the force of gravity. Most of these early entities were tiny by today's standards, but despite their petiteness, they had a massive effect on the universe. Many theorists believe that dwarf galaxies released so much radiation, they reionized the cosmos, stripping gas atoms of their electrons (ScienceNOW, 13 January). In a twist of fate, the gas heated up during this process, causing small clouds to expand instead of collapse. As a result, only massive gas clouds had the gravity necessary to form galaxies, marking the end of the dwarf galaxy reign.
Confirming this switch has been difficult, however, as current telescopes are only now starting to see some of the galaxies from the dawn of the universe. Cosmologists Stuart Wyithe of the University of Melbourne in Australia and Abraham Loeb of Harvard University have found an indirect way to study galaxy evolution by observing the light from bright, pointlike objects called quasars. As this light traveled to us, it passed through intervening gas clouds, whose level of ionization depends on the number of bright nearby galaxies. If there are lots of galaxies spread around, then the ionization level should be fairly constant. But Wyithe and Loeb find this isn't the case: In the period after cosmic reionization, there are large fluctuations in the ionization level, implying that galaxies are less common. Using a model for the mass distribution of galaxies, the authors calculate that the dominant galaxies in the post-reionization epoch have 100 times the mass of dwarf galaxies, they report today in Nature.
The lack of new dwarf galaxies after reionization could resolve a long-standing problem in galaxy formation theory, which predicts more small galaxies in our local neighborhood than are observed, says cosmologist James Bullock of the University of California, Irvine. Confirmation of this result could come from the next generation of telescopes that should be able to see the dwarf galaxies themselves.
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