The Low-Mass Stellar IMF at High Redshift: Faint Stars in the Ursa Minor Dwarf Spheroidal Galaxy

Details The Low-Mass Stellar IMF at High Redshift: Faint Stars in the Ursa Minor Dwarf Spheroidal Galaxy The Low-Mass Stellar IMF at High Redshift: Faint Stars in the Ursa Minor Dwarf Spheroidal Galaxy

1999-11-18

arxiv.org/abs/astro-ph/9911357v1

Astronomy and Astrophysics

Astrophysics

4 pages, 2 embedded figures, uses paspconf.sty. To appear in `The Hy-Redshift Universe: Galaxy Formation & Evolution at High R

Scientific paper

Low-mass stars, those with main-sequence lifetimes that are of order the age of the Universe, provide unique constraints on the Initial Mass Function (IMF) when they formed. Star counts in systems with simple star-formation histories are particularly straightforward to interpret, and those in old systems allow one to determine the low-mass stellar IMF at large look-back times and thus at high redshift. We present the faint stellar luminosity function (based on optical HST data) in an external galaxy, the Ursa Minor dwarf Spheroidal (dSph). This relatively-nearby (distance 70kpc) companion galaxy to the Milky Way has a stellar population with narrow distributions of age and of metallicity, remarkably similar to that of a classical halo globular cluster such as M92 or M15, i.e. old and metal-poor. Contrasting with globular clusters, the internal velocity dispersion of the Ursa Minor dSph indicates the presence of significant amounts of dark matter. We find that the main sequence stellar luminosity function of the Ursa Minor dSph, and implied IMF, down to 0.4 of a solar mass is indistinguishable from that of the halo globular clusters M92 and M15. Thus the low mass stellar IMF for stars that formed at high redshift is invariant in going from a low-surface-brightness, dark-matter-dominated external galaxy, to a globular cluster within the Milky Way.

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