4.7 Article

Ultra faint dwarfs: probing early cosmic star formation

Journal

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 395, Issue 1, Pages L6-L10

Publisher

OXFORD UNIV PRESS
DOI: 10.1111/j.1745-3933.2009.00627.x

Keywords

stars: formation; stars: population II; supernovae: general; galaxies: evolution; cosmology: theory; galaxies: stellar content

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We investigate the nature of the newly discovered Ultra Faint dwarf spheroidal galaxies (UF dSphs) in a general cosmological context simultaneously accounting for various 'classical' dSphs and Milky Way properties including their metallicity distribution function (MDF). To this aim, we extend the merger tree approach previously developed to include the presence of star-forming minihaloes, and a heuristic prescription for radiative feedback. The model successfully reproduces both the observed [Fe/H]-luminosity relation and the mean MDF of UFs. In this picture, UFs are the oldest, most dark matter-dominated (M/L > 100) dSphs with a total mass M = 10(7-8) M-circle dot; they are leftovers of H-2-cooling minihaloes formed at z > 8.5, that is before reionization. Their MDF is broader (because of a more prolonged star formation) and shifted towards lower [Fe/H] (as a result of a lower gas metallicity at the time of formation) than that of classical dSphs. These systems are very ineffectively star-forming, turning into stars by z = 0 only < 3 per cent of the potentially available baryons. We provide a useful fit for the star formation efficiency of dSphs.

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