800+ ultra-diffuse galaxies in Coma cluster

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In summary, the recently discovered ultra-diffuse galaxies in the Coma cluster have very little ordinary matter, estimated to be only 1% of their total mass. This indicates that star formation in these galaxies has ceased and they are a passively evolving population. They have large sizes and are mainly concentrated around the cluster center, suggesting they are longtime cluster members. The presence of nucleated UDGs and their survival in the strong tidal field indicate a high dark matter fraction protecting the diffuse stellar component. The gas removal mechanism is believed to be related to the cluster environment, such as hot ionized gas or shock waves. The discovery of over 1000 UDGs in the Coma cluster, including some as large as the Milky Way
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The galaxies are unusually depleted of ordinary matter---estimated 99% dark matter---and star-formation in them has stopped (they've run out of gas or it has been drawn out of them by other galaxies in the cluster.)

http://arxiv.org/abs/1506.01712
Approximately A Thousand Ultra Diffuse Galaxies in the Coma cluster
Jin Koda (1), Masafumi Yagi (2, 3), Hitomi Yamanoi (2), Yutaka Komiyama (2,4) ((1) Stony Brook, (2) NAOJ, (3) Hosei Univ., (4) SOKENDAI)
(Submitted on 4 Jun 2015)
We report the discovery of 854 ultra diffuse galaxies (UDGs) in the Coma cluster using deep R band images, with partial B, i, and Halpha band coverage, obtained with the Subaru telescope. Many of them (332) are Milky Way-sized with very large effective radii of re>1.5kpc. This study was motivated by the recent discovery of 47 UDGs by van-Dokkum et al. (2015); our discovery suggests >1,000 UDGs after accounting for the smaller Subaru field. The new UDGs show a distribution concentrated around the cluster center, strongly suggesting that the great majority are (likely longtime) cluster members. They are a passively evolving population, lying along the red sequence in the CM diagram with no Halpha signature. Star formation was, therefore, quenched in the past. They have exponential light profiles, effective radii re ~ 800 pc- 5 kpc, effective surface brightnesses mue(R)=25-28 mag arcsec-2, and stellar masses ~1x10^7 - 5x10^8Msun. There is also a population of nucleated UDGs. Some MW-sized UDGs appear closer to the cluster center than previously reported; their survival in the strong tidal field, despite their large sizes, possibly indicates a large dark matter fraction protecting the diffuse stellar component. The indicated baryon fraction ~<1% is less than the cosmic average, and thus the gas must have been removed from the possibly massive dark halo. The UDG population appears to be elevated in the Coma cluster compared to the field, indicating that the gas removal mechanism is related primarily to the cluster environment.
7 pages, 5 figures. Accepted for publication in ApJ Letters. 7 pages, 5 figures
 
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Reading about all these galaxies which are "dead" in the sense of star formation got me thinking about our galaxy. How much longer is it expected to stay in the star-forming stage. I looked it up and the estimate was around 5 billion years. Not a terribly long time.
It is in the transitional stage called the "green valley" which galaxies don't stay in very long. They are mainly in two groups: blue and red.
"Blue" galaxies have UV and blue emitting clouds where star formation is occurring.
"Red" galaxies no longer have these clouds and their light consists of light from older (therefore redder) stars.
Apparently Milky is in the transition phase.
 
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Subaru is an 8 meter optical-infrared telescope on Mauna Kea---that's what they were using.
the press release is informative:
http://subarutelescope.org/Pressrelease/2015/06/22/index.html

It's interesting that these ultra-diffuse "dark" galaxies have so little ordinary matter---only 1% which is below the norm of about 4%.
So something drove out their ordinary matter. Various guesses include gusts of intergalactic wind. Hot ionized gas from other galaxies in the Coma cluster. And maybe shock waves from this wind could have triggered formation of massive stars and subsequent supernova explosions could have contributed to driving out the ordinary matter.

It's hard to imagine how you can get galaxies (some as large as the Milky Way galaxy in spatial extent but with only 1/1000 as much visible matter) which are almost entirely made of dark matter.

The press release has a picture of a section of Coma cluster with the dark galaxies circled, so you can see what they look like in context.
 

FAQ: 800+ ultra-diffuse galaxies in Coma cluster

What are ultra-diffuse galaxies?

Ultra-diffuse galaxies are a type of galaxy that have a very low surface brightness, making them difficult to detect and study. They are typically larger and have lower concentrations of stars compared to other types of galaxies.

What is the significance of finding 800+ ultra-diffuse galaxies in the Coma cluster?

The Coma cluster is a large grouping of galaxies, and the discovery of 800+ ultra-diffuse galaxies within it is significant because it provides further evidence for the existence of these elusive galaxies. It also suggests that ultra-diffuse galaxies may be more abundant than previously thought.

How were these ultra-diffuse galaxies identified and studied?

These galaxies were identified through deep imaging data from the Dragonfly Telephoto Array. Follow-up observations were then done using the Hubble Space Telescope to obtain more detailed information about their properties and formation.

What do these ultra-diffuse galaxies tell us about the Coma cluster?

The presence of these galaxies in the Coma cluster suggests that it is a dynamic and diverse environment, with a mix of different types of galaxies. This can provide valuable insights into the formation and evolution of galaxy clusters.

What are the next steps in studying these ultra-diffuse galaxies?

Future studies will focus on obtaining more detailed information about the physical properties of these galaxies, such as their mass and star formation history. This will help us better understand the role of ultra-diffuse galaxies in the larger context of galaxy evolution.

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