800+ ultra-diffuse galaxies in Coma cluster

[marcus]

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 r_e>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 H_alpha signature. Star formation was, therefore, quenched in the past. They have exponential light profiles, effective radii r_e ~ 800 pc- 5 kpc, effective surface brightnesses mu_e(R)=25-28 mag arcsec^-2, and stellar masses ~1x10^7 - 5x10^8M_sun. 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

 

[marcus]

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.

 

[marcus]

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.