400 million stars in a super-dense dwarf galaxy

M60-UCD1 is the galaxy with the highest stellar density in the local universe

Credit: NASA, ESA, CXC, J. Strader — Highlighted by the circle, M60-UCD1 is an ultra-compact dwarf galaxy, a satellite of the giant elliptical M60, visible in the upper left

Messier 60, also known as NGC 4649, is a massive elliptical galaxy, the third brightest in the Virgo cluster. Located about 54 million light-years from Earth, it has a diameter of at least 120,000 light-years and a mass of about one trillion suns. Some 5,000 globular clusters surround it, compared to 150 or so that orbit the Milky Way.

At an apparent distance of only 25,000 light-years from the center of this galaxy, there is a truly unusual object. Dubbed M60-UCD1, the object was discovered by a team of researchers led by Jay Strader, a Michigan State University astronomer with a penchant for goats. The acronym UCD stands for ultra-compact dwarf; the number 1 after UCD indicates that it is the first ultra-compact dwarf galaxy satellite of Messier 60. The study describing its characteristics was published in September 2013 in Astrophysical Journal Letters.

Ultra-compact dwarf galaxies are a cross between a large globular cluster and a small elliptical galaxy. Discovered since the late 90s of the last century, they have cores with a very high stellar density, radii not exceeding 300 light-years, absolute visual magnitudes between −9 and −14, and masses of a few million solar masses.