Physicists have revealed some of the oldest galaxies in the Universe as some of the faintest satellite galaxies orbiting our own Milky Way galaxy are amongst the very first that formed in the Universe.
Findings by a team of academics including physicists Professor Carlos Frenk and Dr Alis Deason from the Institute for Computational Cosmology (ICC) at Durham University, UK, and Dr Sownak Bose from the Harvard-Smithsonian Center for Astrophysics, USA, suggest that galaxies including Segue-1, Bootes I, Tucana II and Ursa Major I are over 13 billion years old revealing them to be amongst the oldest galaxies in the Universe.
Professor Carlos Frenk, Director of Durham University’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.”
How old are the oldest galaxies?
Cosmologists believe that when the Universe was about 380,000 years old, the very first atoms formed. These were hydrogen atoms. According to the University of Durham, 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 around 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 to ever form.
How was the study conducted?
The research team identified two populations of satellite galaxies orbiting the Milky Way:
- The first was a very faint population consisting of the galaxies that formed during 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 by the intense ultraviolet radiation emitted by the first stars was able to cool into more massive dark matter halos.
The researchers found that a model of the galaxy formation that they developed previously agreed with the data, allowing them to infer the formation times.
How was the Milky Way formed?
The intense ultraviolet radiation emitted by the first galaxies destroyed the remaining hydrogen atoms by ionizing them, making it difficult for this gas to cool and form new stars.
This process of galaxy formation stopped and no new galaxies were able to form for the next billion years or so.
Eventually, the halos of dark matter became so large that even ionized gas was able to cool. Galaxy formation resumed, concluding in the formation of spectacular bright galaxies like our own Milky Way.
This research has been published in The Astrophysical Journal.