Astronomers discover more than 800 dark galaxies in the famous Coma Cluster

Astronomers discover more than 800 dark galaxies in the famous Coma Cluster
A color image made with B, R, and i-band images from the Subaru Telescope. A small region of 6 x 6 arcmin is cut out from large Coma Cluster images. Yellow circles show two of the 47 dark galaxies discovered last year, and green circles are the ones discovered in this new study. Credit: NAOJ

A group of researchers from the Stony Brook University (the State University of New York) and the National Astronomical Observatory of Japan has discovered 854 "ultra dark galaxies" in the Coma Cluster by analyzing archival data from the Subaru Telescope. The discovery of 47 such mysterious dark galaxies was a surprising find in 2014, and the new discovery of more than 800 suggests galaxy clusters as the key environment for the evolution of these mysterious dark galaxies. "Not only these galaxies appear very diffuse," said Jin Koda, principal investigator of the study, "but they are very likely enveloped by something very massive."

These galaxies appear very diffuse and remarkably extended as seen by the light of the stars they contain. Many are similar in size to the Milky Way, but have only 1/1,000 of stars that our galaxy does. The stellar population within such fluffy extended galaxies is subject to rapid disruption due to a strong tidal force detected within the cluster. Something invisible must be protecting the fragile star systems of these galaxies, something with a high mass. That "something" is very likely an excessive amount of dark matter. The component of visible matter, such as stars, is calculated to contribute only 1% or less to the total mass of each galaxy. The rest - dark matter - accounts for more than 99%.

The Subaru Telescope, with its large-aperture and wide-field camera, used under excellent seeing conditions, revealed that these dark galaxies contain old stellar populations and shows a spatial distribution similar to those of other brighter galaxies in the Coma Cluster. That suggests they have been a long-lived population of galaxies within the cluster. The amount of visible matter they contain, less than 1%, is extremely low compared to the average fraction within the universe.

Why are these galaxies dark? Somehow, they lost gas needed to create new stars during or after their largely unknown formation process billions of years ago. From their preferential presence within the cluster, it's likely that the cluster environment played a key role in the loss of gas, which affects star formation within the galaxy. Several loss mechanisms are possible, including ram-pressure stripping by intra-cluster gas, gravitational interactions with other galaxies within the cluster, and gas outflows due to simultaneous supernova explosions triggered, e.g., by the ram pressure or gravitational encounters.

These dark galaxies may offer another insight into the model of galaxy formation. However, according to Dr. Jin Koda more work needs to be done to understand them and their place in the standard picture of galaxy formation. "Follow-up spectroscopic observations in the future may reveal the history of star formation in these dark galaxies," he said.

In addition to research into galaxies' stellar populations, further investigation of the large dark matter component of the galaxies is essential. Dark matter is invisible, but measurements of stellar motions may expose the distribution of in these galaxies. Such a dream measurement may not be immediately possible, because they are so faint. It is difficult to measure the detailed motions of stars, even with the Subaru Telescope. The construction of Thirty Meter Telescope (TMT) by an international partnership of institutions, including the National Astronomical Observatory of Japan may well reveal the mystery of the dark galaxies in near future.

The National Astronomical Observatory of Japan has maintained all the data obtained with the Subaru Telescope since its very first light observations 16 years ago (in 1999). All archive data are made available to the community one and half years from the night of the observation. This new discovery is made possible thanks to the availability of abundant archival Subaru data. Re-analyses of archival data have often resulted in new discoveries and publications. The Subaru data archive continuously offers "treasure hunting" opportunities

Explore further

VLT discovers new kind of globular star cluster

More information: This discovery will be published on June 24, 2015 in the Astrophysical Journal Letters by the American Astronomical Society (Koda et al. 2015, "Approximately A Thousand Ultra Diffuse Galaxies in the Coma cluster"). The preprint is available at
Journal information: Astrophysical Journal Letters

Provided by National Institutes of Natural Sciences
Citation: Astronomers discover more than 800 dark galaxies in the famous Coma Cluster (2015, June 22) retrieved 21 July 2019 from
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.

Feedback to editors

User comments

Jun 23, 2015
Dreamers still dreaming of dark matter...

These darker galaxies likely simply have a somewhat smaller core star, ejecting new matter therefrom much less frequently than other galaxies whose core has grown larger over time. Once it grows larger, it becomes more active, ejecting more material, lighting up the galaxy with more star production. The final growth stage then becomes the quasar.

Simple really. No need for 'dark' mysterious explanations. A bit of logic is all that is needed, but is so sorely lacking in this community.

Jun 24, 2015

I have a lot of questions about your "model" because I don't see how it's more elegant or more explanatory:

What mechanism allows this "core" to violate conservation so flagrantly?
In the life cycle of this "core star" what process leads to its formation?
What prevents the collapse of the system?
Where is the evidence for this?

Even if quasars were a kind of whitehole that wouldn't obviate Dark Matter. Something still has to hold these galaxies together. Or if these galaxies are not being held together but are spiralling out there should be a plethora of doppler effects all over the Universe to show that, right?

Shouldn't we also then find late-stage torus shaped galaxies where the "core" has been expended and the matter continues on the course set by its angular momentum?

I just would like to understand this idea and why you cleave to it despite it being at odds with what our best minds have to say about the Universe. Any insight would be awesome. Thanks Tux.

Jun 24, 2015
I read the:

I was hoping to get some better insight on how the DM managed to stay together while the visible matter didn't. Left out of this article is the inclusion of AGNs/galactic winds contributing. Other than that I was trying to figure out(not with math lol) how tidal forces would act like a DM sieve. <-- Would be a peculiar and interesting mechanic, no?

Other things of note was that the method of reduction for this sounds pretty painstaking and tedious although much smarter than anything I would have come up with. Rather than finding needles in a haystack they systematically removed the hay piece by piece leaving only the needles.

Jun 24, 2015
Thanks for interest. Too few dare, as they are afraid of being called a fool.

It is NOT my model. It is LaViolette's. And I took a year to change my attitude after a careful read of his work, especially SQK. Having a solid history of closing servo loops in the aerospace industry, perhaps, enabled me to see the implications, eventually. Physicists do not have these skills, so it is a loosing battle.

After many questions with LaViolette, I finally understood. First law is NOT violated, when you understand that our universe is simply an open subset of a more comprehensive structure occupying the same space as our universe. We are the salt in the ocean.

Under proper diffusive conditions of the elements of the substructure, the salt is formed. Conditions are most ripe within the core stars. Since increasing 'genic' energy (photon blue shifting) is also produced therein, collapse is eventually halted, and quasars result.

Jun 24, 2015
Dark matter is likely mostly a consequence of the sub-structure refracting light, which also get's tired in intergalactic space in this model. It also get's tired when climbing out of a gravity well.
And BTW, gravity is a consequence of the diffusive substructure too, which is not precisely concentrated the same everywhere, due to diffusive flow of various sub-elements cause by the local sinks of matter. This influence likely has a limit, according to LaViolette, perhaps around 10K Lyrs. Thus, the influence of gravity has limit. Isn't this just logical?

Jun 25, 2015
This comment has been removed by a moderator.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more