Astronomers using the Dragonfly Telephoto Array and the 10m telescope at the W.M. Keck Observatory on Maunakea, Hawaiʻi, have discovered 47 so-called ultra-diffuse galaxies in the field of the Coma galactic cluster. These objects are nearly as wide as our home Milky Way Galaxy – about 60,000 light years – yet harbor only 1% as many stars.
The ultra-diffuse galaxy Dragonfly 44. Image credit: Pieter van Dokkum / Roberto Abraham / Jean Brodie.
“If the Milky Way Galaxy is a sea of stars, then these newly discovered galaxies are like wisps of clouds”, said Dr Pieter van Dokkum of Yale University, who is the lead author of the paper published in the Astrophysical Journal Letters (arXiv.org preprint).
“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.”
The newly-discovered ultra-diffuse galaxies, including Dragonfly 44 – one of the largest of these galaxies, are very large and very far away, about 300 million light-years.
The ultra-diffuse galaxy Dragonfly 17 is shown next to other types of galaxies, to scale. The giant spiral Andromeda Galaxy and the dwarf elliptical galaxy NGC 205 are also shown. Image credit: B. Schoening / V. Harvey / REU program / NOAO / AURA / NSF / Pieter van Dokkum / Hubble Space Telescope.
“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 co-author Prof Jean Brodie of the University of California, Santa Cruz.
“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,” added co-author Dr Aaron Romanowsky of San Jose State University.
“The big challenge now is to figure out where these mysterious objects came from,” said co-author Dr Roberto Abraham of the University of Toronto.
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Pieter G. van Dokkum et al. 2015. Spectroscopic Confirmation of the Existence of Large, Diffuse Galaxies in the Coma Cluster. ApJ 804, L26; doi: 10.1088/2041-8205/804/1/L26
