What is a Galaxy?

In the past six years, more than two dozen dwarf galaxies have been discovered around the Milky Way and M31. Many of these discoveries are 100 times less luminous than any galaxy previously known, and a million times less luminous than the Milky Way itself. These discoveries hint that "ultra-faint" galaxies are the most numerous type of galaxy in the universe. The Milky Way's ultra-faint dwarf population is currently our best tracer of dark matter on subgalactic scales, making a well-defined census and study of these objects a fundamental test for cold dark matter models on such scales. This talk will highlight i. the properties of the few Milky Way companions with only one ten-millionth of the Milky Way's own luminosity, ii. the definition of the term "galaxy", and iii. the cosmological implications (and uncertainties) of the Milky Way's dwarf population.

Followed by wine and cheese in Pupin 1402.

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