“Hobbit galaxies” are even smaller than dwarf galaxies, containing only a few hundred thousand stars. Leo T is the only known hobbit galaxy, with a luminosity equal to 50,000 Suns and a diameter of 600 light-years. It contains young stars and is embedded within a large mass of dark matter. The existence of more dwarf galaxies is predicted.
The term “hobbit galaxy” has recently been used to refer to galaxies even smaller than dwarf galaxies, which contain a few million stars. Hobbit galaxies contain only a few hundred thousand stars, similar to a large star cluster within a galaxy. By comparison, our Milky Way contains 200-400 billion stars, more stars than there are galaxies in the universe.
The term “hobbit galaxy”, being new, has not yet acquired a distinct definition. The only galaxy that unquestionably fits the label is Leo T, a galaxy discovered in 2007 during the Sloan Digital Sky Survey. The “T” designation indicates that Leo is a transitional object between one galaxy and something else. Located 1.4 million light-years away, Leo T is beyond the gravitational influence of the Milky Way. Leo T has a luminosity equal to only 50,000 Suns, unprecedented for a galaxy.
The Milky Way is also orbited by at least seven small galaxies usually called dwarf galaxies, which might actually qualify for hobbit galaxy status. As our telescopes become more sensitive, the instrumentation available to the field of extragalactic astronomy improves, and we learn more about the fainter and darker types of galaxies. Current cosmological theories predict the existence of 5 to 10 times as many dwarf galaxies in our Local Group as we currently observe.
Two qualities of Leo T in particular are relatively unusual: first, its diameter is about 600 light-years, very large compared to the diameter of a cluster of stars of similar mass within a galaxy: only about 100 light-years . This indicates that the stars in Leo T are embedded within a large mass of dark matter. The second unusual thing about Leo T is that it contains relatively young stars, unusual for previously observed dwarf galaxies that typically host ancient stars. Leone T also asks the question: how small can a galaxy be?
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