Sometimes things are right in front of your eyes but you just can’t see them. That is the case for the latest satellite – Crater 2 – discovered in the Milky Way, which has gone completely unnoticed until now.
The big surprise wasn’t the discovery itself, but its size. Crater 2 is the fourth-largest satellite of the Milky Way, surpassed only by the Sagittarius Dwarf and the Large and Small Magellanic Clouds. Crater 2 is a faint ultra-dwarf galaxy that's located 391,000 light-years from Earth, and it’s part of the dozens of small galaxies that surround the Milky Way.
Although it is one of the largest structures around our galaxy, it was difficult to find. The stars belonging to Crater 2 are spread out from one another, making it hard for astronomers to view it as a single object.
Crater 2 has a luminosity equivalent to over 160,000 Suns, making it one of the most luminous dwarfs discovered in the last decade. Its luminosity is also used to estimate its size. As astronomical objects often lack edges, scientists measure the area from which half of its light comes from. The “half-light” radius for Crater 2 is about 3,500 light-years, which suggests that it is much larger than 7,000 light-years across.
The discovery of this galaxy, which is described in a paper published in the Monthly Notices of the Royal Astronomical Society, is important for our understanding of the evolution of the Milky Way. Galaxies evolve through “mergers,” which are when smaller galaxies are cannibalized by larger ones.
The location of Crater 2 also points to a gravitational disruption in our galaxy’s past. Crater 2 seems to be aligned with the peculiar globular cluster Crater, the ultra-faint dwarf galaxies Leo IV and Leo V, and the classical dwarf Leo II. Scientists believe these satellites were once a single object, which was disrupted by the Milky Way, and they're now a long, narrow string of objects.