Thursday, October 23, 2014

An Ancient Relic of the Universe’s First Galaxies

Ultra-faint dwarf galaxies (UFDs) are the faintest galaxies known in the universe. These galaxies are smaller than 1,000 light years in radius and have very low metallicities. As a result, UFDs are believed to represent the first generation of galaxies in the universe. UFDs are the most dark matter dominated galaxies since nearly all their mass is admittedly in the form of dark matter. In fact, the total mass of all stars in a UFD is typically less than a million solar masses. For comparison, the Milky Way galaxy contains 200 to 400 billion stars. UFDs are also much smaller and fainter than classical dwarf spheroidal galaxies. So far, UFDs have only been discovered around the Milky Way galaxy and the neighbouring Andromeda galaxy.

Using images acquired by the Hubble Space Telescope, Jang & Lee (2014) reported the discovery of a UFD in a region of space far from any massive galaxy in the Virgo Cluster. This newly discovered UFD is named Virgo UFD1. Observations indicate that Virgo UFD1 contains low metallicity red-giant-branch (RGB) stars but no asymptotic-giant-branch (AGB) stars. It means that Virgo UFD1 is very old, most likely older than 10 billion years, because AGB stars mark the final evolution of stars that are at least as massive as the Sun and such stars do not live more than about 10 billion years.

The stars in Virgo UFD1 probably formed in a single burst of star formation more than 10 billion years ago. As a consequence, the absence of subsequent generations of stars to fuse hydrogen and helium into heavier elements results in the low metallicity of Virgo UFD1. There is also no sign that Virgo UFD1 experienced tidal interaction with any galaxy. Estimates place Virgo UFD1 at a distance of more than 50 million light years away. The old age, low metallicity and large distance from any massive galaxy suggest Virgo UFD1 may represent an ancient relic of the universe’s first galaxies.

Left panel: Effective radius versus absolute total magnitude of Virgo UFD1 (large starlet symbol) in comparison with other stellar systems. Right panel: The central surface brightness versus absolute total magnitude of Virgo UFD1 in comparison with other stellar systems. Note: Circles and lenticular symbols for the giant ellipticals and bulges in spiral galaxies, downward triangles for the UCDs, pentagons for the Milky Way galaxy globular clusters, squares and diamonds for the Local Group satellite galaxies and UFDs, and upward triangles for the dwarf galaxies in M81 and M106 and the low surface brightness galaxies in M101, and small yellow starlet for Virgo dSph-D07. Jang & Lee (2014).

Jang & Lee (2014), “Discovery of an Ultra-Faint Dwarf Galaxy in the Intracluster Field of the Virgo Center: A fossil of the First Galaxies”, arXiv:1410.2247 [astro-ph.GA]