The Washington Post reports about research on a galaxy called Dragonfly 44 which is believed to contain about the same mass as the Milky Way but is only 1% as bright. The low ratio of luminosity to mass is characteristic of ultra diffuse galaxies (UDGs). The galaxy is believed to lie 101 megaparsecs (329 million light years) away. The researchers offer explanations for the dimness of UDGs:
[...] it may be that UDGs are "failed" galaxies that were prevented from building a normal stellar population, because of extreme feedback from supernovae and young stars (Agertz & Kravtsov 2015; Calura et al. 2015), gas stripping (Fujita 2004; Yozin & Bekki 2015), AGN feedback (Reines et al. 2013), or other effects.
"AGN" is short for active galactic nucleus — where matter falls into a supermassive black hole. The citation is to "Dwarf Galaxies with Optical Signatures of Active Massive Black Holes" (open, DOI: 10.1088/0004-637X/775/2/116) (DX).
Previously: Huge Population of "Ultra-Dark Galaxies" Discovered
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About 321 million light-years away from us is the Coma Cluster, a massive grouping of more than 1,000 galaxies. Some of its galaxies are a little unusual, however: they're incredibly dim. So dim, in fact, that they have earned the title of "Ultra-Dark Galaxies" (UDGs). (The term is actually "Ultra-Diffuse Galaxies", as their visible matter is thinly spread, though "ultra-dark" has been used by some sources and, let's face it, sounds a lot better). This was discovered earlier this year in a study that identified 47 such galaxies.
Dimness isn't necessarily unusual in a galaxy. Most of a galaxy's light comes from its stars, so the smaller a galaxy is (and thus the fewer stars it has), the dimmer it will be. We've found many dwarf galaxies that are significantly dimmer than their larger cousins.
What was so unusual about these 47 is that they're not small enough to account for their dimness. In fact, many of them are roughly the size of our own Milky Way (ranging in diameter from 1.5 to 4.6 kiloparsecs, compared with the Milky Way's roughly 3.6) but have only roughly one thousandth of the Milky Way's stars. The authors of the recent study interpret this to mean that these galaxies must be even more dominated by dark matter than are ordinary galaxies.
Original Submission
(Score: 2) by wonkey_monkey on Friday August 26 2016, @03:02PM
Is the fact that it's believed to be 99.99% dark matter not interesting enough to put in the summary?
http://www.keckobservatory.org/recent/entry/scientists_discover_massive_galaxy_made_of_99.99_percent_dark_matter [keckobservatory.org]
systemd is Roko's Basilisk
(Score: 2) by Ken_g6 on Friday August 26 2016, @03:43PM
"Dark matter" just means "not stars", basically. Gas and dust clouds are dark matter. We're sitting on dark matter. It's a very general term.
You're probably thinking of nonbaryonic dark matter. [wikipedia.org] But the galaxy isn't made of 99.99% nonbaryonic dark matter.
(Score: 1) by nitehawk214 on Friday August 26 2016, @04:15PM
The article does not say that at all. They don't say which "kind" of dark matter.
"Don't you ever miss the days when you used to be nostalgic?" -Loiosh
(Score: 2) by number6x on Friday August 26 2016, @04:20PM
Exactly this! For the most part the term 'dark matter' means ordinary everyday space dust that is not:
If it were doing those things we would be able to more easily observe it. Since it is not doing those things we do not see it in the bandwidths our telescopes 'see' in, but we can infer its presence because of the behaviour of the matter we do see. This 'not lit up' matter effects matter around it through gravity.
This is astronomer's dark matter, not cosmologist's dark matter.
(Score: 2) by wonkey_monkey on Friday August 26 2016, @04:41PM
Exactly this! For the most part the term 'dark matter' means ordinary everyday space dust that is not:
The majority of dark matter is not just ordinary matter that's not being lit.
This is astronomer's dark matter, not cosmologist's dark matter.
I've never heard of this distinction before. Anyway, the article makes it pretty clear that the "dark matter" they refer to is not "normal matter."
systemd is Roko's Basilisk
(Score: 3, Informative) by wonkey_monkey on Friday August 26 2016, @04:37PM
It's a very general term.
It's really not:
Dark matter is an unidentified type of matter [...] that is not accounted for by dark energy, baryonic matter (ordinary matter), and neutrinos.
And from the article:
However, only one hundredth of one percent of that is in the form of stars and "normal" matter; the other 99.99 percent is in the form of dark matter.
systemd is Roko's Basilisk
(Score: 2) by frojack on Friday August 26 2016, @06:30PM
Its also probably wise to point out that to-date, nobody has been able to detect dark matter, otherh than indirectly, (like needing a fudge factor to get an equasion to balance).
Even recent reports of dark matter detection earlier this year are highly suspect [sciencealert.com].
Dark matter remains undefined to the best minds in physics. Here is a list of possible theories [sciencealert.com]. Nobody can say what it is. But we know a lot about what it isn't.
No, you are mistaken. I've always had this sig.
(Score: 2) by wonkey_monkey on Friday August 26 2016, @06:37PM
(like needing a fudge factor to get an equasion to balance)
Hey, look, science would love to hear if you have an idea that better fits the observations.
systemd is Roko's Basilisk
(Score: 2) by frojack on Friday August 26 2016, @07:10PM
And science would like to hear from you if you have ANY independently verifiable evidence of Dark Matter that exists in in the real workd other than a theoritical mathimatical model.
No, you are mistaken. I've always had this sig.
(Score: 2) by aristarchus on Friday August 26 2016, @10:19PM
exists in in the real workd other than a theoritical mathimatical model.
Seems that whenever anyone challenges the froj, the quality of his typing/spelling seriously degrades. I suspect Dark Matter is responsible.
(Score: 2) by wonkey_monkey on Friday August 26 2016, @11:02PM
There is independently verifiable evidence that there is additional mass out there which can't be accounted for by normal matter. Is it proven? No. But no-one's come up with a better explanation yet, so it's reasonable to stick with it for now.
systemd is Roko's Basilisk
(Score: 2) by NotSanguine on Friday August 26 2016, @05:17PM
That's right. Only 27% of the universe is made up of non-baryonic dark matter [wikipedia.org], as compared with 4.9% of ordinary matter [stackexchange.com].
What's more, no one suggests that matter, whether it's baryonic, non-baryonic or dark or not is evenly distributed. As such, what's your point?
No, no, you're not thinking; you're just being logical. --Niels Bohr
(Score: 2) by butthurt on Friday August 26 2016, @07:18PM
I thought that
[...] is believed to contain about the same mass as the Milky Way but is only 1% as bright. The low ratio of luminosity to mass [...] explanations for the dimness [...]
would convey the idea that the galaxy is unusually dim for the amount of mass it contains. The 99.99% figure given by the Washington Post is at odds with the research that's being reported on, the abstract for which says
The mass-to-light ratio is M/L=48 M_sun/L_sun, and the dark matter fraction is 98 percent within the half-light radius.
(Score: 1, Informative) by Anonymous Coward on Friday August 26 2016, @04:29PM
AGN is a preferred term because it is much less laden with theoretical assumptions. It simply describes what people see.
(Score: 2) by martyb on Saturday August 27 2016, @01:03AM
You are correct; story updated. Thanks!
Wit is intellect, dancing.
(Score: 0) by Anonymous Coward on Friday August 26 2016, @04:53PM
They've already completed Dyson spheres around 99% of their stars. We've got to get it together and catch up!
(Score: 2) by bob_super on Friday August 26 2016, @05:16PM
It takes six months to budget filling a pothole...
(Score: 1, Touché) by Anonymous Coward on Saturday August 27 2016, @09:42AM
Did they know terrorists can hide in potholes? Let them know then there will be money and military contractors lining up around the block.