Scattered among the galaxies of the Coma cluster are dozens of objects that are as large as the Milky Way but shine only 1/100 as bright. The stability of those ultradiffuse galaxies (UDGs) in the tidally disruptive Coma environment suggests that they may be held together by an unusually large component of dark matter. Now a research team led by Pieter van Dokkum of Yale University has measured the mass of one UDG, Dragonfly 44, and found a value similar to that of our Milky Way. Combining mass and luminosity determinations, the group concluded that Dragonfly 44 is 99.99% dark matter.
The group’s key measurement was of the UDG’s spectrum near the hydrogen-alpha absorption line at 656 nm. Because different stars in the galaxy move with different speeds, the nominal absorption line is broadened to a peak whose width can be related to the mass of the central, star-containing portion...
