Monday, December 7, 2015

An Extreme Dark Matter Dominated Galaxy

A cluster of stars can be considered a galaxy if it shows evidence for dark matter, where the stars in the cluster are moving too fast for the gravitational field from just the stars alone to hold the cluster together. Additionally, a cluster of stars can also be considered a galaxy if the stars exhibit different metallicities (i.e. different abundances of elements heavier than hydrogen and helium), which indicates multiple episodes of star formation. Dwarf galaxies tend to contain very few stars but a lot of dark matter. Triangulum II is a dwarf galaxy located just beyond the edge of the Milky Way. It contains only ~1000 stars and it is a satellite galaxy of the Milky Way.

By observing the motion of six stars in Triangulum II, the gravitational force acting on the stars can be measured. This technique allows the mass of the galaxy to be estimated. The total mass of Triangulum II is found to be much greater than the mass of all its stars. Although the galaxy contains ~2 million times the Sun’s mass, its total luminosity is only ~450 times the Sun’s luminosity. In fact, Triangulum II has one of the largest mass-to-light ratios of any galaxy and it is the most dark matter dominated galaxy currently known.

Furthermore, the stars in Triangulum II are observed to have different metallicities, indicating multiple episodes of star formation. This shows that the galaxy has to be many times more massive than the total mass of all its stars in order for its gravity to keep in all the gas and dust that were dispersed from past episodes of star formation to form new generations of stars.

Although dark matter is few times more abundant than ordinary matter in the Universe, it has never been directly observed. The existence of dark matter is inferred by its gravitational influence in galaxies and clusters of galaxies. It is believed that the particles that make up dark matter can annihilate one another when they collide to produce gamma rays that can be detected. However, detecting these gamma ray signals is challenging because other astrophysical objects and phenomena also generate gamma rays.

Triangulum II is a very quiet galaxy and it is not forming any new stars. As a result, Triangulum II, with its high concentration of dark matter, may be a good and pristine place to search for gamma ray signals from annihilating dark matter particles, hopefully shedding more light on the nature of dark matter.

Kirby et al. (2015), “Triangulum II: Possibly a Very Dense Ultra-Faint Dwarf Galaxy”, arXiv:1510.03856 [astro-ph.GA]