Faint Stars in the Ursa Minor Dwarf Spheroidal Galaxy: Implications for the Stellar IMF
The stellar Initial Mass Function (IMF) is a crucial ingredient in models of star formation and chemical evolution. Star counts in systems with simple star-formation histories are particularly straightforward to interpret. We present the faint stellar luminosity function in an external galaxy, the Ursa Minor dwarf Spheroidal (dSph). This relatively-nearby 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, i.e. old and metal-poor. The integrated luminosity of the Ursa Minor dSph is also similar to that of a globular cluster, but its central surface brightness is many orders of magnitude lower. Further, again in contrast to globular clusters, the internal dynamics of the Ursa Minor dSph are apparently dominated by dark matter, with(M/L)v ~ 80(M/L) ⊙ .Perhaps surprisingly, we determine for the Ursa Minor dSph a stellar luminosity function, and inferred IMF, that is identical, down to our completeness limit of ~0.4M ® to that of the globular clusters of similar age and metallicity. Thus the stellar IMF long ago and far away, in a dark-matter-dominated galaxy, was the same as that in star clusters in the Milky Way.
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