An international team of researchers led by Pieter van Dokkum at Yale University have used the W. M. Keck Observatory to confirm the existence of the most diffuse class of galaxies known in the universe. These "fluffiest galaxies" are nearly as wide as our own Milky Way galaxy -- about 60,000 light-years -- yet harbor only one percent as many stars. The findings were recently published in the Astrophysical Journal Letters.
"If the Milky Way is a sea of stars, then these newly discovered galaxies are like wisps of clouds," said van Dokkum, according to a press release. "We are beginning to form some ideas about how they were born and it's remarkable they have survived at all. They are found in a dense, violent region of space filled with dark matter and galaxies whizzing around, so we think they must be cloaked in their own invisible dark matter 'shields' that are protecting them from this intergalactic assault."
Finding the distance was the clinching evidence. The data from Keck Observatory showed the diffuse "blobs" are very large and very far away, about 300 million light-years, rather than small and close by. The blobs can now safely be called Ultra Diffuse Galaxies (UDGs).
"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, to tell them they were living in a galaxy. The night sky would be much emptier of stars," said team member Aaron Romanowsky, of San Jose State University.
The UDGs were found in an area of the sky called the Coma cluster, where thousands of galaxies have been drawn together in a mutual gravitational dance. "Our fluffy objects add to the great diversity of galaxies that were previously known, from giant ellipticals that outshine the Milky Way, to ultra compact dwarfs," said Jean Brodie, a professor at the University of California, Santa Cruz, according to the press release.
"The big challenge now is to figure out where these mysterious objects came from," said Roberto Abraham, of the University of Toronto. "Are they 'failed galaxies' that started off well and then ran out of gas? Were they once normal galaxies that got knocked around so much inside the Coma cluster that they puffed up? Or are they bits of galaxies that were pulled off and then got lost in space?"
The key next step in understanding UDGs is to pin down exactly how much dark matter they have.
Reference:
"Spectroscopic Confirmation of the Existence of Large, Diffuse Galaxies in the Coma Cluster," Pieter van Dokkum et al., Astrophysical Journal Letters, 2015 May 1, Vol. 804, No. 1, L26 [https://dx.doi.org/10.1088/2041-8205/804/1/L26, preprint: https://arxiv.org/abs/1504.03320].
