Because light does not travel instantaneously through space, when we look towards distant objects in the universe we actually see them as they appeared in their past. By using this property, astronomers are able to observe how galaxies looked, back in the early days of the universe. Just recently, they discovered what seem to be two classes of unique galaxies.
While studying distant galaxies, a research team observed a galaxy that appeared relatively old in relation to the age of the universe,
meaning that it must have been one of the first galaxies to form. Yet another researching team found galaxies from a period in the life of the universe when it experienced the most active star formation process.
"They're already kind of dying. They've mostly finished forming stars," said Ph.D student at the University of Nottingham, Will Hartley.
Rapid star formation Although they were dated to 4 billion years after the Big Bang event, the galaxies appeared to be very old and inactive, evidence given away by the large population of reddish stars they house. "It is a puzzle and a challenge for us to explain how these galaxies formed their stars so quickly that they are already dead galaxies when we observe them," says Hartley.
They must have been amongst the first objects ever to form in the universe. Astronomers believe that they were created out of high-density matter existing through the smooth spread of matter at that time and could be considerably heavier than the galaxies that formed afterwards. They are probably some of the largest galaxies existing even today.
"Here we've maybe discovered a population of galaxies that is very, very vigorously forming stars, but maybe for a short period of time," said Max Planck Institute for Radio Astronomy researcher Frank Bertoldi.
It is widely believed that, while stars formed inside them at a moderate rate, the fact that this process took place in an extremely active star formation period of the universe could explain why they look so old to us now. "Maybe only ten percent of their lives are they in such a state of high star formation," added Bertoldi.
Gravitational interactions Alternatively, extremely high star formation processes could be generated by gravitational interactions between individual galaxies and even collisions and mergers. It is well known that galaxies collide rather frequently, however in the early days of the universe when matter was denser, galaxies could have been colliding at an even faster rate than they do today.
"This is one class of objects, a particular phase in evolution, that has been missed so far. We hadn't discovered them because we had a bias in our selection," said Bertoldi.
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