A group of astronomers, led by Dr Jin Koda of the Stony Brook University, has discovered 854 ultra-diffuse galaxies (UDGs) in the Coma cluster using images obtained with the Subaru telescope. These galaxies are very far away, about 300 million light-years, and many of them (332 galaxies) are Milky Way-sized.
This composite image shows ultra-diffuse galaxies in the Coma cluster: yellow circles show two of the 47 Dragonfly galaxies, and green circles are the ones discovered by the Subaru telescope. Image credit: NAOJ.
Dr Koda and colleagues said their study was motivated by the recent discovery of 47 UDGs in the Coma cluster by astronomers using the Dragonfly Telephoto Array.
The Subaru galaxies show a distribution concentrated around the center of the cluster, strongly suggesting that the great majority are cluster members.
“They are a passively evolving population, lying along the red sequence in the color-magnitude diagram with no signature of H-alpha emission. Star formation was, therefore, quenched in the past,” the astronomers said.
Many UDGs are similar in size to our home Milky Way Galaxy, but have only 1/1,000 of stars that our Galaxy does.
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.
Scientists speculate that these galaxies probably have very high dark matter fractions as they have survived in the strong tidal field of the cluster.
“Not only these galaxies appear very diffuse, but they are very likely enveloped by something very massive,” said Dr Koda, lead author on the study accepted for publication in the Astrophysical Journal Letters (arXiv.org preprint).
The amount of visible matter UDGs contain, less than 1 percent, is extremely low compared to the average fraction within the Universe.
“These UDGs may offer insights into the model of galaxy formation. However, more work needs to be done to understand them and their place in the standard picture of galaxy formation,” the scientists said.
“Follow-up spectroscopic observations in the future may reveal the history of star formation in these ‘dark’ galaxies.”
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Jin Koda et al. 2015. Approximately a Thousand Ultra Diffuse Galaxies in the Coma cluster. ApJL, accepted for publication, arXiv: 1506.01712
