A team of US scientists, including an Indian origin, Sownak Bose, have spotted some of the earliest galaxies formed when the universe burst into light. Segue-1, Bootes I, Tucana II and Ursa Major I, thought to be over 13 billion years old, are some of the first galaxies ever formed, the scientists say.
“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 Bose, a research fellow at the Harvard-Smithsonian Center for Astrophysics in the US.
The faintest satellite galaxies
Scientists found evidence that the faintest satellite galaxies orbiting the earth’s Milky Way galaxy are among the earliest galaxies in the universe.
The finding was published in the Astrophysical Journal.
The scientists found it “hugely exciting” as they explained that identifying some of the universe’s oldest galaxies orbiting the Milky Way is “equivalent to finding the remains of the first humans that inhabited the Earth.”
The Halos of dark matter
The first atom formed when the universe was about 380,000 years old were hydrogen atoms, the simplest element in the periodic table. These atoms collected into clouds and began to cool slowly and settle into small clumps or “halos” of dark matter that emerged from the Big Bang.
It took about 100 million years for the cooling period called “Cosmic dark ages”. Eventually, the gas that had cooled inside the halos began to form stars - these objects are the very first galaxies ever to have formed, the scientists explained.
When the universe burst into light
With the formation of the first galaxies, the universe burst into light, ending the cosmic dark ages. There were two populations of satellite galaxies orbiting the Milky Way, the scientists say. The first was a group of very faint 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 ionised by the intense ultraviolet radiation emitted by the first stars was able to cool into more massive dark matter halos.
The team says that a model of galaxy formation that they had formed previously fit perfectly with the data, allowing them to infer the formation times of the satellite galaxies.
The Lambda-cold-dark-matter model
“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 Carlos Frenk, a professor at Durham University in the UK.
The first galaxies emitted intense ultraviolet radiation that destroyed the remaining hydrogen atoms by ionising them (knocking out their electrons).
It made it difficult for this gas to cool and form new stars. The process of galaxy formation stopped and no new galaxies were able to form for the next billion years or so. Gradually, the halos of dark matter grew so massive that even ionised gas was able to cool. Galaxy formation resumed, culminating in the formation of spectacular bright galaxies like our own Milky Way, the scientists added.
(With inputs from PTI)