An Ultra Diffuse Galaxy (UDGs) has been discovered that is the mass of the Milky Way but 99.9% dark matter. Researchers used the 8-meter Gemini North telescope to see a halo of spherical clusters of stars around the galaxy’s core, similar to the halo that surrounds our Milky Way galaxy.
Dragonfly 44’s mass is estimated to be 1 trillion times the mass of the Sun, or 2 tredecillion kilograms (a 2 followed by 42 zeros), which is similar to the mass of the Milky Way. Only 0.01% of the dark matter galaxy is in the form of stars and “normal” matter. The other 99.99% is in the form of dark matter—a hypothesized material that remains unseen but may make up more than 90% of the universe.
Finding a galaxy composed mainly of dark matter is not new. Ultra-faint dwarf galaxies have similar compositions. But those galaxies were roughly 10,000 times less massive than Dragonfly 44.
Recently a population of large, very low surface brightness, spheroidal galaxies was identified in the Coma cluster. The apparent survival of these Ultra Diffuse Galaxies (UDGs) in a rich cluster suggests that they have very high masses. Here we present the stellar kinematics of Dragonfly 44, one of the largest Coma UDGs, using a 33.5 hr integration with DEIMOS on the Keck II telescope. We find a velocity dispersion of 47 km/s, which implies a dynamical mass of M_dyn=0.7×10^10 M_sun within its deprojected half-light radius of r_1/2=4.6 kpc. The mass-to-light ratio is M/L=48 M_sun/L_sun, and the dark matter fraction is 98 percent within the half-light radius. The high mass of Dragonfly 44 is accompanied by a large globular cluster population. From deep Gemini imaging taken in 0.4″ seeing we infer that Dragonfly 44 has 94 globular clusters, similar to the counts for other galaxies in this mass range. Our results add to other recent evidence that many UDGs are “failed” galaxies, with the sizes, dark matter content, and globular cluster systems of much more luminous objects. They estimate the total dark halo mass of Dragonfly 44 by comparing the amount of dark matter within r=4.6 kpc to enclosed mass profiles of NFW halos. The enclosed mass suggests a total mass of ~10^12 M_sun, similar to the mass of the Milky Way. The existence of nearly-dark objects with this mass is unexpected, as galaxy formation is thought to be maximally-efficient in this regime.
SOURCES- Arxiv, Astrophysical Journal Letters, Gemini Telescope
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