Two researchers from Observatoire Astronomique de Strasbourg have revealed for the first time the existence of a new signature of the birth of our galaxy's first stars. More than 12 billion years ago, their intense light dispersed the gas of the Milky Way's satellite galaxies. By computing the observable consequences of this process, Pierre Ocvirk and Dominique Aubert demonstrated their prevailing role. This result confirms that reionisation is indeed an essential process in the standard model of galaxy formation. The study took place within the LIDAU collaboration (Light In the Dark Ages of the Universe). It is published in the october issue of the letters of the Monthly Notices of the Royal Astronomical Society.
The first stars of the Universe appeared about 150 million years after the Big Bang. Back then, the hydrogen and helium gas filling the universe was cold enough to have its atoms be electrically neutral. As the intense light of the first stars propagated through this gas, it broke the hydrogen atoms, returning them to the plasma state they experienced in the first moments of the Universe. This process, known as reionisation, also results in significant heating, which can have dramatic consequences: the gas becomes so hot that it escapes the weak gravity of the lowest mass galaxies, thereby depriving them of the material needed to form stars. It is now widely admitted that this photo-evaporation process explains the small number and large ages of the stars seen in the dwarf galaxies satellites of the Milky Way. It also offers a credible solution to the missing satellites problem. On the other hand, their sensitivity to UV radiation means satellite galaxies are good probes of the reionisation epoch. Moreover, they are relatively nearby, from 30000 to 900000 light-years, which allows us to study them in great details, especially with the forthcoming generation of telescopes. In particular, the study of their stellar content with respect to their position could give us precious insight into the structure of the local UV radiation field during the reionisation epoch.
Until now, satellite galaxies models assumed that the radiation leading to the photo-evaporation of their gas was produced collectively by the large galaxies nearby, resulting in a quasi-uniform background at the scale of the Milky Way. The new model built by the two french researchers proves this assumption wrong. It is based on a high resolution numerical simulation (the Via Lactea II) describing the dynamics of the dark matter haloes that populated our galaxy and its neigbourhood from the Big Bang to present times. This dataset is completed by a description of the formation of stars from the gas trapped in these haloes, and in paricular a detailed model of the reaction of this gas to UV radiation.
It is the first time that a model accounts for the effect of the radiation emitted by the first stars formed at the center of the Milky way, on its satellite galaxies. Indeed, contrary to previous models, the radiation field produced in this configuration is not uniform, but decreases in intensity as one moves away from the source. On one hand, the satellite galaxies close to the galactic center see their gas evaporate very quickly. They form so few stars that they can be undetectable with current telescopes. On the other hand, 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.
Previous models assumed a uniform UV background during reionisation. In contrast, the influence of the first stars of the Milky Way results in fewer satellite galaxies in the inner parts of our galaxy, and an excess in the outer parts. Comparing the observed spatial distribution of the satellite galaxies with the predictions of the new model, it appears that the latter matches the observations much better than older models. This suggests that the first stars of our galaxy played a major role in the photo-evaporation of the satellite galaxies' gas. It is not the large nearby galaxies, but our own, who caused the demise of her tiny sisters, asphyxiating them through her intense radiation.
This new scenario has deep consequences on the formation of galaxies and the interpretation of the large astronomicals surveys to come. Indeed, satellite galaxies are affected by our galaxy's tidal field, and can be slowly digested into our galaxy's stellar halo. They can also be stretched into filaments and form stellar streams. These will be the main science goals of the Gaia space mission, scheduled for launch in 2013. Therefore we need to understand as soon as possible how they are affected by radiative processes during reionisation.
Notes
The LIDAU project is funded by the french Agence Nationale pour la Recherche (ANR). The collaboration comprises the 2 researchers from Observatoire Astronomique de Strasbourg, as well as Benoit Semelin, Patrick Vonlanthen et Kenji Hasegawa, who belong to LERMA (Observatoire de Paris).
The missing satellties problem was formulated about 10 years ago, as a disagreement between the expected and observed numbers of satellite galaxies of the Milky Way. While standard numerical simulations predicted as many as 500, only 10 of them were known, and still only about 20 at present. This means that, either thes galaxies do not exist, thereby ruling out the standard cosmological model, or they do exist, but are rendered undetectable for some unknow reason. This problem finds a credible solution in the processes resulting from the UV background pervading the Universe during reionization. Its intensity may be sufficient to photo-evaporate the gas of low mass satellite galaxies and stop their star formation very early on. The paucity of their stars eventually makes them difficult to detect today, and explains why we see so few of them.
A signature of the internal reionisation of the Milky Way?, Pierre Ocvirk, Dominique Aubert, in press in Monthly Notices of the Royal Astronomical Society - Letters : http://arxiv.org/abs/1108.1193
