How did the universe form is a question which is discussed even now. It formed 14 billion years ago and has several theories attached to its formation. Big Bang theory is a popular one and an international team, including researchers from the University of Geneva (UNIGE) have largely validated this hypothesis.
The theory states that after the Big Bang, the Universe expanded and, by cooling down, the matter progressively took shape. The first stars and galaxies form several hundred thousand years later. One billion year later the Universe is observed to be reheated, and hydrogen, the most abundant element, is again ionized, as it was shortly after the Big Bang.
Scientists have long suspected that galaxies were responsible for the cosmic reionization. However, to make this happen galaxies needed to eject these photons, which are easily absorbed, before they could escape. Despite twenty years of intensive research, no galaxy emitting sufficient radiation had been found.
To solve this problem, astronomers from Daniel Schaerer’s research group at the Department of Astronomy of the Faculty of Sciences, and an international team proposed to observe green pea galaxies. Discovered in 2007, these galaxies represent a special and rare class in the nearby Universe. Being very compact they could host stellar explosions or power winds strong enough to eject the ionizing photons.
Yuri Izotov, from the National Academy of Sciences of Ukraine and first author of the paper, has examined the Sloan Survey – a database of more than 1 million galaxies. From this, the team has identified approximately five thousand galaxies that match their criteria, i.e. very compact galaxies emitting very intense UV radiation. Five of those were chosen for the experiment.
Using the Hubble Space Telescope, capable of detecting UV radiation, the research team found that the green pea galaxy J0925, located at a distance of three billion light-years, was in fact ejecting ionizing photons, with unprecedented intensity.
This fundamental discovery shows that galaxies of this type could explain cosmic reionization, thus confirming the most commonly made hypothesis.
Anne Verhamme, researcher at UNIGE, has also found that this green pea galaxy shows a very peculiar signature. Indeed, its Lyman-alpha spectrum, i.e. one of the spectral lines of hydrogen it emits, is much narrower and stronger than that of most galaxies, therefore confirming her theoretical predictions. These observations provide now the base for an efficient new method to search for galaxies responsible of cosmic reionization 13 to 14 billion years ago.
These discoveries represent an important step for studies of the early Universe. Current technology only provides us with a first glimpse on galaxies during the first billion years of the Universe existence. The future James Webb Space Telescope, whose launch is planned for 2018, is expected to revolutionize the field.
The article, published in Nature, opens an important new avenue for our understanding of the early Universe.