Discovery of a Large and Diffuse Dwarf Galaxy

Torrealba et al. (2016) present the discovery of a dwarf galaxy identified as Crater 2. It is a satellite galaxy of the Milky Way that is estimated to lie ~380,000 light years from the Sun. The half-light radius of Crater 2 is ~3500 light years in size. A dwarf galaxy does not have a clear boundary since the density of stars gradually decreases towards the outer regions. For this reason, the size of a dwarf galaxy is measured by its half-light radius, which is basically the distance from the center of the dwarf galaxy, whereby within this radius, half the total brightness of the galaxy is emitted. In fact, the sizes of other types of stellar systems such as globular clusters are also denoted by the half-light radius.

Crater 2 has an extremely low surface brightness of only ~31 mag/arcsec². The discovery of Crater 2 indicates that a substantial number of extremely low surface brightness dwarf galaxies have yet to be detected. Crater 2 is a remarkable dwarf galaxy because it is the largest known ultra-faint satellite galaxy of the Milky Way and the fourth largest satellite galaxy, surpassed only by the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC) and the Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph).

Crater 2 appears to be in alignment with the globular cluster Crater, the pair of ultra-faint dwarf galaxies Leo IV and Leo V, and the dwarf galaxy Leo II. The alignment seems to be statistically significant enough to suggest that the Leo-Crater group was once a more cohesive stellar system that has since dissipated into a stream of multiple stellar systems due to tidal disruption from the massive Milky Way galaxy.

This diagram shows absolute magnitude versus half-light radius, whereby dwarf galaxies that are satellites of the Milky Way are denoted by red open circles, dwarf galaxies that are satellites of the Andromeda galaxy are denoted by black unfilled triangles, and other nearby galaxies are denoted by gray crosses. Black dots indicate globular clusters of the Milky Way and grey dots indicate extended objects with half-light radii smaller than ~325 light years. Crater 2 is marked with a filled red circle. The black solid line and the black dashed line, respectively, correspond to the surface brightness levels of 31 and 30 mag/arcsec². Torrealba et al. (2016)

Reference:
Torrealba et al. (2016), “The feeble giant. Discovery of a large and diffuse Milky Way dwarf galaxy in the constellation of Crater”, arXiv:1601.07178 [astro-ph.GA]