Some Milky Way satellite galaxies are among the oldest in the universe

Discovery affirms scientists' model of dark matter having driven cosmic evolution.
By Laurel Kornfeld | Aug 17, 2018
Several faint dwarf galaxies that are satellites of the Milky Way are among the oldest galaxies in the universe and may be more than 13 billion years old.

A research team composed of scientists from the Institute for Computational Cosmology (ICC) at Durham Universityin northeast England and the Harvard Smithsonian Center for Astrophysics (CfA) discovered two types of ancient satellite galaxies in orbit around the Milky Way.

One is a very faint group of galaxies that date back to about 100 million years after the formation of the universe's earliest atoms, approximately 380,000 years after the Big Bang. These were atoms of hydrogen, the simplest element in existence. During a time period known as the "cosmic dark ages," these hydrogen atoms formed clouds, cooled, and settled into small clumps of dark matter.

The "cosmic dark ages" ended when the gas in these clouds became unstable and started forming the first stars that made up the earliest galaxies.

While the first group of galaxies formed after the "cosmic dark ages," the second one formed hundreds of millions of years later, after the universe's first stars ionized the early hydrogen atoms by bombarding them with intense ultraviolet radiation. These galaxies are somewhat brighter than their earlier counterparts.

The ionized hydrogen settled into larger clumps of dark matter, halting galaxy formation for about a billion years until the ionized gases were able to cool and resume star formation.

"A decade ago, the faintest galaxies in the vicinity of the Milky Way would have gone under the radar. With the increasing sensitivity of present and future galaxy censuses, a whole new trove of the tiniest galaxies has come into the light, allowing us to test theoretical models in new regimes," said Sownak Bose, now at CfA.

"Our finding supports the current model for the evolution of the universe, the 'Lambda-cold-dark-matter model,' in which the elementary particles that make up the dark matter drive cosmic evolution," explained ICC director Carlos Frenk.

"Finding some of the very first galaxies that formed in our universe orbiting in the Milky Way's own backyard is the astronomical equivalent of finding the remains of the first humans that inhabited the Earth," he emphasized.

Galaxies Segue-1, Bootes 1, Tucana II, and Ursa Major 1 were identified as probable members of the older group of Milky Way satellites.

Findings of the study have been published in the Astrophysical Journal.



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