What can green pea galaxies teach us about our origins?

Green Pea Galaxies
iStock/Anastasiia Shavshyna

It is probable that primordial galaxies triggered the period in the history of the universe known as “cosmic reionisation”. The Geneva-based astronomer Anne Verhamme has succeeded in demonstrating this by studying green pea galaxies.

Following the Big Bang some 14 billion years ago, the universe gradually cooled down, allowing electrons and protons to fuse together to form hydrogen atoms. This was the beginning of the Dark Ages of the universe, which lasted until the first stars were formed. These stars must have emitted large quantities of ultraviolet radiation that was capable of ionising the hydrogen atoms, because astronomers observed that electrons and protons separated again a billion years after the Big Bang. This is what we call the cosmic reionisation period.

For a long time, astronomers could not explain where the powerful UV radiation needed for reionisation had come from. The majority of observed galaxies do not emit ionising photons and the few known exceptions emit too little to keep the universe ionised.

Anne Verhamme, professor of astronomy at the University of Geneva, proposed that green pea galaxies – a new type of galaxy discovered ten years ago – probably emit large quantities of ionising photons. This assumption was based on the highly specific properties of rays emitted by the hydrogen atoms in these galaxies, known as Lyman-alpha radiation. Astronomers believe that green pea galaxies resemble primordial galaxies as they are extremely compact, are creating their first generations of stars, and are still rich in gas.

Using data from the Hubble Space Telescope, Anne Verhamme and a international team of collaborators were able to demonstrate that green pea galaxies do indeed emit large quantities of ionising photons. If green peas are analogous to primordial galaxies, it seems very likely that it was galaxies that triggered the reionisation of the universe more than 13 billion years ago.

Anne Verhamme was able to conduct her research at the University of Geneva thanks to a Marie Heim-Vögtlin grant from the SNSF. In 2018, she was awarded an SNSF professorship and received a Starting Grant from the European Research Council (ERC). A mother of three, Verhamme will receive the Marie Heim-Vögtlin prize – which comes with prize money of CHF 25,000 – in Geneva on 16 September 2019. The event will be held as part of the welcoming ceremony for new students of the Science Faculty.

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