Astronomers from Durham University and the Harvard-Smithsonian Center for Astrophysics (CfA) have found evidence that the faintest satellite galaxies orbiting our own Milky Way galaxy are among the very first galaxies that formed in our Universe.
The research group’s results suggest that galaxies including Segue-1, Bootes I, Tucana II and Ursa Major I are, in fact, some of the first galaxies ever formed, thought to be over 13 billion years old. Their findings are published in The Astrophysical Journal.
When the Universe was about 380,000 years old, the very first atoms formed. These were hydrogen atoms, the simplest element in the periodic table. These atoms collected into clouds and began to cool gradually and settle into the small clumps or “halos” of dark matter that emerged from the Big Bang.
This cooling phase, known as the “cosmic dark ages,” lasted about 100 million years. Eventually, the gas that had cooled inside the halos became unstable and began to form stars. These objects are the very first galaxies ever to have formed. With the formation of the first galaxies, the Universe burst into light, bringing the cosmic dark ages to an end.
Dr. Sownak Bose of the CfA, working with Dr. Alis Deason and Professor Carlos Frenk at Durham University’s Institute for Computational Cosmology (ICC), identified two populations of satellite galaxies orbiting the Milky Way.
The first was a very faint population consisting of the galaxies that formed at the end of the “cosmic dark ages”. The second was a slightly brighter population consisting of galaxies that formed hundreds of millions of years later — once the hydrogen that had been ionized (that is, had their electrons knocked out) — by the intense ultraviolet radiation emitted by the first stars was able to cool into more massive dark matter halos. Eventually, the halos of dark matter became so massive that bright galaxies like our own Milky Way were able to form.
Remarkably, the team found that a model of galaxy formation that they had developed previously agreed perfectly with the data, allowing them to infer the formation times of the faint satellite galaxies.
Professor Frenk, Director of Durham’s ICC, said: “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. It is hugely exciting.
“Our finding supports the current model for the evolution of our Universe, the ‘Lambda-cold-dark-matter model’ in which the elementary particles that make up the dark matter drive cosmic evolution,” said Professor Frenk. In this model “Lambda” refers to dark energy, which is causing the expansion of the Universe to accelerate.
Dr. Bose, who was a PhD student at the ICC when this work began and is now a research fellow at the CfA, said: “A nice aspect of this work is that it highlights the complementarity between the predictions of a theoretical model and real data.
“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.”
Dr. Deason, who is a Royal Society University Research Fellow at the ICC said: “This is a wonderful example of how observations of the tiniest dwarf galaxies residing in our own Milky Way can be used to learn about the early Universe.”
Dr. Bose is supported through the Institute for Theory and Computation fellowship at Harvard University, while Dr. Deason is supported by a Royal Society University Research Fellowship. Professor Frenk and Dr. Deason are both supported by the Science and Technology Facilities Council Consolidated Grant for Astronomy and Durham University.
A paper describing this work appears in The Astrophysical Journal and is available online.