The results of a new study suggest that a number of small satellite galaxies orbiting the Milky Way are actually some of the first to come into existence following the creation of the universe. The birth of these dim, early galaxies represents a vital step in the evolution of our universe, which would eventually allow for the creation of the colossal, breath-taking galaxies that astronomers observe today.
In the study, which was a joint venture between Durham University and the Harvard Smithsonian Center for Astrophysics, scientists identified two distinct populations of relic satellite galaxies tracking a course around the Milky Way.
The first group was made up of extremely dim galaxies, while the second set was a little brighter. The individual galaxies that make up the two populations are very small compared to spiral galaxies such as our Milky Way.
The team discovered that data collected on the two populations agreed well with a theoretical model on galaxy formation that they had developed previously. The scientists were able to use their model to infer the times at which the satellite galaxies coalesced.
When the cosmos was only 380,000 years old, the first atoms began to form. These were hydrogen atoms. Hydrogen is the simplest of the known elements and continues to be the most prevalent element in the current-day universe.
In the 100 million years that followed the creation of the first atoms – a period referred to by astronomers as the "cosmic dark ages" – the hydrogen slowly cooled and settled into vast haloes made up of dark matter. Once gathered inside the haloes, the vast clouds of hydrogen gradually became unstable and collapsed to form the first stars, and subsequently, the first galaxies.
This dramatic period of creation would not continue unabated. The energetic newly-created stars blasted out enormous quantities of ultraviolet radiation that interacted with the surrounding matter, ionizing the hydrogen clouds that had yet to coalesce, stripping them of their electrons.
Galaxy creation stalled for roughly a billion years as the ionized hydrogen atoms cooled back down to the point where they could settle into larger dark matter haloes, and be used up in the creation of further stars and galaxies.
The new study suggests that the first population of dimmer Milky Way satellites are among the oldest galaxies in the universe, which, with the light of their stars, brought an end to the cosmic dark ages. The slightly brighter population of galaxies identified by the team are still phenomenally old, but formed hundreds of millions of years later from hydrogen atoms that had been ionized by the radiation pouring forth from the earliest galaxies.
"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," said Professor Carlos Frenk, Director of Durham University's Institute for Computational Cosmology. "It is hugely exciting."
A paper detailing the research has been published in The Astrophysical Journal.
Source: Durham University
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