Faint stars in the Ursa Minor dwarf spheroidal galaxy: implications for the low-mass stellar initial mass function at high redshift |
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| Institution: | 1. Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA;2. School of Physics and Astronomy, University of St. Andrews, St. Andrews, UK;3. Astrophysics, Oxford University, Oxford, UK;4. Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK;5. Lund Observatory, Box 43, 221 00 Lund, Sweden;6. Department of Astronomy, University of Wisconsin, Madison, WI 53706, USA;7. Department of Physics & Astronomy, University of California, Irvine, CA 92697, USA |
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| Abstract: | The stellar initial mass function at high redshift is an important defining property of the first stellar systems to form and may also play a role in various dark matter problems. We here determine the faint stellar luminosity function in an apparently dark-matter-dominated external galaxy in which the stars formed at high redshift. The Ursa Minor dwarf spheroidal galaxy is a system with a particularly simple stellar population—all of the stars being old and metal-poor—similar to that of a classical halo globular cluster. A direct comparison of the faint luminosity functions of the UMi dSph and of similar metallicity, old globular clusters is equivalent to a comparison of the initial mass functions and is presented here, based on deep HST WFPC2 and STIS imaging data. We find that these luminosity functions are indistinguishable, down to a luminosity corresponding to ～0.3 M⊙. Our results show that the low-mass stellar IMF for stars that formed at very high redshift is apparently invariant across environments as diverse as those of an extremely low-surface-brightness, dark-matter-dominated dwarf galaxy and a dark-matter-free, high-density globular cluster within the Milky Way. |
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