Ultra-faint Companions to the Milky Way: A Gallery

Ultra-faint galaxies, the subject of Keith Bechtol's feature article in the March 2017 issue of Sky & Telescope, are not easy to find. While Harlow Shapley spotted the Fornax and Sculptor dwarf galaxies in 1938 by examining photographic plates, the darker companions to the Milky Way Galaxy have required modern, statistical methods to suss out. Dominated by dark matter, the smallest of these have only a few hundred stars that we can see, and those must be picked out from a foreground rich with our galaxy's stellar population.

We present here a gallery of some of the larger and more easily spotted ultra-faint galaxies. For more on how these galaxies are shedding light on dark matter, the first stars, and the nature of galaxy evolution, read the March 2017 issue.

Dwarf galaxies discovered in Dark Energy Survey

The Dark Energy Surey uses the Auxiliary Telescopes at the Paranal Observatory in the Atacama Desert in Chile. Six ultra-faint galaxies are present in this image. The Magellanic Clouds can also be seen on the sky. The insets, each 13 x 13 arcminutes on the sky (or 3,000 by 3,000 pixels in the Dark Energy Camera) show images of the three most visible objects: Eridanus 1, Horologium 1 and Pictoris 1.
V. Belokurov, S. Koposov (Institute of Astrophysics, Cambridge). Photo: Y. Beletsky (Carnegie Observatories)

Eridanus II ultra-faint dwarf galaxy

Eridanus II
V. Belokurov & S. Koposov

Horologium I ultra-faint dwarf galaxy

Horologium I
V. Belokurov & S. Koposov

Pictoris I ultra-faint dwarf galaxy

Pictoris I
V. Belokurov & S. Koposov

Reticulum II ultra-faint dwarf galaxy

These two images demonstrate why dwarf galaxies are so difficult to see. The left image shows the Dark Energy Survey discovery image of ultra-faint dwarf galaxy Reticulum II. However, most of the visible stars lie in the foreground; only about 300 stars belong to the dwarf itself. The image at right shows only the dwarf galaxy's stars, with all the other stars masked out. Spectroscopic follow-up has since revealed that these stars survived a rare explosion, possibly from a neutron star merger, which enriched their surfaces in certain heavy elements. Click image for larger view.
Fermilab / Dark Energy Survey

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