The Milky Way - a not so friendly neighbour?
French researchers have for the first time discovered how intense ultraviolet (UV) light from stars in the Milky Way during their formation dispersed gas in neighbouring galaxies and put an end to their ability to form stars too. As part of the 'Light in the Dark Ages of the Universe' (LIDAU) collaboration, a three-year project that aims to reveal more about the Universe's evolution, researchers Pierre Ocvirk and Dominique Aubert, from the Observatoire Astronomique de Strasbourg in France, explain in the journal Monthly Notices of the Royal Astronomical Society why some of these other galaxies were killed off, while stars continued to form in more distant objects. It was about 150 million years after the Big Bang when the first stars of the Universe appeared and back then the hydrogen and helium gas filling the Universe was cold enough for its atoms to be electrically neutral. As the UV light of the first stars propagated through this gas, it broke apart the proton-electron pairs that make up hydrogen atoms, returning them to the so-called plasma state they would have experienced in the first moments of the Universe. As a result of this process, known as reionisation, significant heating occurred and the gas became so hot that it escaped the weak gravity of the lowest mass galaxies which deprived them of the material needed to form stars. Scientists believe that this process is the reason behind the small number and large ages of the stars seen in the faintest dwarf galaxy satellites of the Milky Way. The satellite galaxies are also relatively close, from 30,000 to 900,000 light years away, which means scientists can study them in greater detail. By comparing the population of stars in each galaxy with its position scientists hope to better understand the structure of the UV radiation emitted from the earliest stars in the Milky Way. However, until now models for this process assumed that the radiation leading to the removal of gas from galaxy satellites was produced collectively by all the large galaxies nearby, resulting in a uniform background of UV light; but the French space researchers have made new ground by looking at the way the invisible 'dark matter' that makes up about 23% of the Universe structured itself with the stars in our galaxy and its environs from shortly after the Big Bang to the present day. Pierre Ocvirk comments: 'This is the first time that a model accounts for the effect of the radiation emitted by the first stars formed at the centre of the Milky Way on its satellite galaxies. In contrast to previous models, the radiation field produced is not uniform, but decreases in intensity as one moves away from the centre of the Milky Way. The satellite galaxies close to the galactic centre see their gas evaporate very quickly. They form so few stars that they can be undetectable with current telescopes. At the same time, the more remote satellite galaxies experience on average a weaker irradiation. Therefore they manage to keep their gas longer, and form more stars. As a consequence they are easier to detect and appear more numerous.'For more information, please visit:Observatoire Astronomique de Strasbourghttp://astro.u-strasbg.fr/
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