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Star Formation Histories of Nearby Galaxies and the Connection to High Redshift

Eline Tolstoy
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Abstract

It is an obvious statement that all the galaxies we see today in and around our Local Group have been forming and evolving for a significant fraction of the age of the Universe. It is not a great leap of logic to further state that the manner in which they have formed and evolved must be fairly representative of these processes in general. Unless of course we would like to assume that our local region of space is in some way peculiar for which there is no evidence. In other words, if we are able to determine accurate star formation histories for the nearby galaxies back to the ages of the oldest globular clusters then we will also obtain a representative picture of how galaxies have evolved from the earliest times, and predict what nearby galaxies looked like at intermediate and high redshifts. Deep, precision, multi-colour photometry of resolved stellar populations in external galaxies can uniquely determine the star formation histories of nearby galaxies going back many Gyrs. Hubble Space Telescope and high quality ground based imaging have recently resulted in dramatic Colour-Magnitude Diagrams of the faint old resolved stellar populations in nearby galaxies. Although data deep enough to unambiguously trace back to the very oldest populations does not yet exist (except for a few small dSphs in the Galaxy Halo), these preliminary studies make the potential for deeper data for a range of galaxy types look very exciting. For example, recent results on the very low metallicity dwarf irregular galaxy Leo A suggests we have found a predominantly, if not entirely, young galaxy, less than 2 Gyr old, in the Local Group. Nearby faint star Colour-Magnitude Diagrams thus provide an important and independent method to confirm the high redshift galaxy survey predictions of galactic evolution.

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Eline Tolstoy. 1998-07-15. Star Formation Histories of Nearby Galaxies and the Connection to High Redshift. https://arxiv.org/abs/astro-ph/9807154

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