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The First Stars and Galaxies: Challenges for the Next Decade Participants
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Talk
Title: Probing Early Cosmic Star Formation with the Faintest Dwarf Galaxies Author(s): Stefania Salvadori Abstract: Ultra faint dwarf spheroidal galaxies (UFs) are the least luminous and metal-rich stellar systems known (L ~10^(3-5) Lsun, <[Fe/H]> < -2). The Fe-Luminosity relation derived for UFs constitutes a natural extension towards lower metallicity of that of "classical" (L > 10^5 Lsun) dwarf spheroidal galaxies (dSphs), suggesting that the formation of all these Milky Way (MW) satellites has been governed by the same physical processes. However, while classical dSphs and UFs together span more than four orders of magnitude in luminosity, their total mass is roughly the same within the innermost 300 pc. Why is star formation so inefficient in UFs? Recently, observations of metal-poor stars in dSphs have revealed that [Fe/H]< -3 stars are extremely rare in classical dSphs, while they represent the 25% of the total stellar populations in UFs. When did UFs form? Are these galaxies among the first star-forming objects in the MW system? We investigate the nature of UFs in a general cosmological context, simultaneously accounting for various classical dSphs and MW properties, including their metallicity distribution function (MDF). The model successfully reproduces the observed Fe-Luminosity relation and the mean MDF of UFs. We find that UFs are leftovers of minihalos cooled by H2. They are the oldest dSphs in MW systems formed at z > 8.5, i.e. before the reionization epoch.
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