Class of their own
Some enormous galaxies are surprisingly wispy and dark
HUNDREDS OF NEWLY discovered ‘fluffy’ galaxies are baffling astronomers. The so-called ultra-diffuse galaxies are often as wide as our own Milky Way yet they contain less than one per cent as many stars. “If the Milky Way is a sea of stars, then these newly discovered galaxies are like wisps of clouds,” says Pieter van Dokkum from Yale University in Connecticut. “It’s remarkable they have survived at all.”
Using a small 10-lens telescope called the Dragonfly Telephoto Array in New Mexico, van Dokkum’s team identified more than 40 faint fuzzy objects that turned out to reside in a congregation of galaxies called the Coma Cluster (Abell 1656), which lies roughly 320 million lightyears away. That means they are very large and distant dim galaxies, rather than small objects fairly close to us.
Since then, analysis of archived data from the Japanese Subaru Telescope in Hawaii has revealed that these ultra-diffuse galaxies are far from rare – the Coma Cluster contains more than 800 of them. Jin Koda from Stony Brook University in New York and colleagues showed that many are similar in size to the Milky Way but with only 1/1,000th of the stars.
“If there are any aliens living on a planet in an ultra-diffuse galaxy, they would have no band of light across the sky, like our own Milky Way,” says van Dokkum’s colleague Aaron Romanowsky from San Jose State University in California.
How the wispy galaxies can exist is unclear. They inhabit a dense, violent region of space filled with galaxies whizzing around and could easily be disrupted by external gravitational forces.
Possibly, they’re glued together by the gravity of extreme amounts of dark matter, the unidentified invisible substance that makes up most of the mass in the Universe. “They must be cloaked in their own invisible dark matter shields that are protecting them from this intergalactic assault,” says van Dokkum.
The fluffy galaxies may turn out to be ‘failed’ galaxies that ran out of star-forming gas. Alternatively, they could be normal galaxies that have been gravitationally disturbed so often inside the Coma Cluster that they puffed up. http://subarutelescope.org