Australian Observatory Takes Us A Big Step Closer To Studying The First Stars In The Universe

Simulation of the Epoch of Reionisation. Neutral hydrogen (red) is gradually ionized by the first stars (white). The image was made by the University of Melbourne’s Dark-ages Reionisation And Galaxy Observables from Numerical Simulations (DRAGONS) program. Paul Geil and Simon Mutch

There are still many things that we do not understand about the first few hundred million years after the Big Bang, and one particular observation that is lacking is seeing the first stars. We have some ideas of what they might be like but as of yet, we haven’t seen them directly.

They lived too far in the past and died too quickly. Our instruments are not yet good enough to spot them. In the meantime, however, we are improving how to spot them indirectly. The Murchinson Widefield Array, a radio observatory in Australia, has seen a 10-fold improvement on the data from the early universe. This improvement is reported in a paper currently available to read on the preprint server ArXiv, soon to be published in The Astrophysical Journal.

The observatory was designed with one goal in mind: to study the Epoch of Reionization (EoR). This is the period when the formation of first stars happened, named after what happened to hydrogen during this time.

About 380,000 years after the Big Bang, the universe was finally cool enough for the protons and electrons to form hydrogen atoms. Over hundreds of millions of years (the so-called cosmic dark ages), hydrogen gathered in clouds until it eventually collapsed into the first stars. The light of the first stars was so intense that it stripped the electrons from the hydrogen (a process called ionization), for the second time in the history of the universe.

"Defining the evolution of the EoR is extremely important for our understanding of astrophysics and cosmology," lead author Dr Nichole Barry, from the University of Melbourne, said in a statement. "So far, though, no one has been able to observe it. These results take us a lot closer to that goal."

The team is looking at a specific emission of neutral hydrogen known as the 21-centimeter line. The signal is difficult to capture as it is very weak and there are a lot of other galaxies and objects between its sources and us.

The team collected 21 hours of raw data from the observatory and applied novel techniques to refine the analysis. Part of the focus was to make sure that potential sources of contamination (including signals from Earth) where detected and excluded with high accuracy. The result significantly improves what we know about this mysterious epoch.

"We can't really say that this paper gets us closer to precisely dating the start or finish of the EoR, but it does rule out some of the more extreme models," added co-author Professor Cathryn Trott, from the International Centre for Radio Astronomy Research at Curtin University in Western Australia. "That it happened very rapidly is now ruled out. That the conditions were very cold is now also ruled out."

Future observatories will be able to study the first generation of stars better and help answers the many mysteries left to solve. These first stars are expected to be hundreds, if not thousands, of times the size of our Sun and when they went supernova they may have even produced the seeds for the supermassive black holes now found at the core of galaxies. 

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