A team of astronomers has discovered a galaxy – the first of its kind – that appears to contain virtually no dark matter. It is an exceptional find since galaxies are commonly thought to contain more dark matter than the ordinary matter that makes up a galaxy’s stars, gas and dust.
Also, it is generally accepted that galaxies first formed from concentrations of dark matter that act like “galaxy starters.” They gravitationally attract ordinary, or baryonic, matter that eventually settles within the extant cloud of dark matter.
Finding a galaxy with no dark matter raises the question: If the galaxy has no dark matter, how did it form?
“We thought all galaxies were made up of stars, gas and dark matter mixed together, but with dark matter always dominating,” says Roberto Abraham, a professor in the University of Toronto's department of astronomy and astrophysics, and co-author of the Nature paper describing the discovery. “Now it seems that at least some galaxies exist with lots of stars and gas and hardly any dark matter. It is pretty bizarre.”
According to the paper’s lead author, Pieter van Dokkum, a professor of astronomy and physics at Yale University, “It challenges the standard ideas of how we think galaxies work. This result also suggests that there may be more than one way to form a galaxy.”
The paper appears in the March 29 issue of the journal Nature.
U of T Professor Robert Abraham (left) and Yale Professor Pieter van Dokkum (right), with their team of graduate students from both universities. They are standing with one-half of the 48-lens Dragonfly array at its home site in New Mexico
The newly discovered galaxy is called NGC1052-DF2, or DF2 for short. In addition to the dearth of dark matter, DF2 is unusual in another way: It is roughly the size of our Milky Way Galaxy, but contains only 1/200 of the number of stars.
It was first identified as a peculiar object using the Dragonfly Telescope Array, a groundbreaking instrument for detecting very faint astronomical objects, conceived of and built by van Dokkum and Abraham.