New class of galaxy has been discovered, made almost entirely of dark matter

https://www.washingtonpost.com/news/speaking-of-science/wp/2016/08/25/a-new-class-of-galaxy-has-been-discovered-one-made-almost-entirely-of-dark-matter/

by daegloe • 10 years ago

288 139 10 years ago

washingtonpost.com

Florin_Andrei4• 10 years ago

This is significant. Now astronomers can study dark matter with less interference from the regular matter background. So you have a more broad range of objects to study, from galaxies with lots of regular matter, to galaxies with almost none of it.

Hopefully this will accelerate DM research. It will certainly provide lots more data.

mordocai• 10 years ago

One thing I don't understand and maybe someone can take the time to explain.

Why does it have to be this "Dark Matter" causing the mass differences we've been seeing? How do we know it isn't undetectable-through-normal-means normal matter, or aliens, or mistakes in our calculations/observations, or anything else?

Since we haven't been able to detect dark matter, it seems from my layman point of view that any of the above "hypotheses" are equally viable.

What am I missing that makes astronomers and physicists so sure that this "dark matter" exists?

Edit: Thanks for all the great responses! I'm not going to respond to each response, but the discussion has been enlightening.

burkaman• 10 years ago

I think dark matter is a synonym for "undetectable-through-normal-means matter". It's just a description of something mysterious causing effects that some kind of matter would cause. It could be a mix of different kinds of undetectable matter, or some fundamental flaw in our understanding of matter and gravity, or something else.

The arguments against aliens and mistakes are presumably that the effects appear consistently across the whole visible universe, and through many different measurements.

krastanov• 10 years ago

"undetectable-through-normal-means matter" is by definition not normal matter. We do not know what it is, so we have a placeholder name for it: "dark matter", i.e. matter that does not shine in the EM spectrum. We have theories about what it is, but most of the mundane ideas (like cold dust) have been ruled out by observations. Now we are left with all these stranger ideas. "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."

empath75• 10 years ago

undetectable-through-normal-means normal matter

I assume by this you mean dust and planets and such. https://medium.com/starts-with-a-bang/could-dark-matter-just...

That also goes for 'aliens', because presumably, they'd also be made of normal matter, and if they're not, then we're back to some exotic form of matter like dark matter.

As far as being a mistake, it could be possible that we are fundamentally misunderstanding something about how gravity works. That would be as interesting a result as dark matter, to be honest.

Florin_Andrei4• 10 years ago

The only thing we're sure it exists is - there's something out there that acts exactly like normal matter with mass, and it interacts gravitationally with itself and with regular matter, but it doesn't seem to interact in any other way.

That's it. The name "dark matter" is a bit optimistic, because we don't even know if it's anything like "matter".

It's not a fluke in the equations, it's not a fifth fundamental force. All these alternatives have been shown to be very unlikely.

It's not "normal" matter because that one would interact in ways other than gravity, too, and it would tend to make "clumps" (stars, planets). This one doesn't make clumps.

So, yes, it's pretty certain that it exists, but nobody knows what it is. Its only attribute observed so far is its mass. Maybe we should call it "dark mass" instead.

semaphoreP2• 10 years ago

Basically, dark matter is something that has gravitational pull, but shows no other evidence of its existence. We have seen evidence for dark matter in galaxy rotation curves, galaxy cluster interactions, and the cosmic microwave background fluctuations, so we know it's not a mistake (each of these measurements are independent and do not share common systematic errors). We know it's not normal matter (e.g. black holes), because we at least somewhat understand the physics of how normal matter works, and we would expect to see something observationally if it is just normal matter. Since it is not normal matter, using Occam's razor, we can only postulate it is something that has gravitational pull but doesn't interact otherwise. Perhaps once we figure out how to study dark matter, we can tease out some of its properties and it won't just be all lumped together as "dark matter".

symmetricsaurus• 10 years ago

Dark matter can explain: the large scale evolution of the universe (together with dark energy but whatever), rotational velocities of galaxies, gravitational lensing and other phenomena. No other theory explains all of these things as well or simply.

Also, dark matter is not really so exotic and weird. We already know of one type of dark matter particle, neutrinos. To think that there are other more massive particles that only interact through gravity and the weak force isn't much of a strectch in that light.

T0T0R07• 10 years ago

Exoplanets are dark matter. They do not emit light, so they are dark. You can't see an exoplanet, you have to look for stars that wobble.

Asteroid belts are also dark matter. If a remote star has an asteroid belt, it wouldn't be easily detectable, since it probably wouldn't elicit the kepler-style wobbles and flickers, or if it did, perhaps less obvious/detectable events.

Based on this, the astronomical fad terminology is flawed, since it seems to claim that only stars matter because only stars are matter (and the luminous gas of nebulae too, of course). The layperson finds intrigue in the term Dark Matter, because journalists are trying to sell a story.

mikeash8• 10 years ago

"Dark matter" just means "stuff we can't detect except by gravity" so that covers your first case.

There's a theory that "dark matter" isn't matter at all, but some modification to the currently accepted laws of physics. Wikipedia lists some of these ideas:

https://en.wikipedia.org/wiki/Dark_matter#Alternative_theori...

Ultimately, it's generally assumed to be some sort of actual matter because that's what best fits the observations. No alternative explanation has been raised that better fits the observations than the explanation that it's some unknown form of matter that doesn't interact with electromagnetic radiation very much, but still participates in gravity.

As for what those observations are, galaxies are observed to rotate much differently than if they only had the mass that was visible, galaxies cluster as if there was more mass than can be seen, gravitational lensing indicates that galaxies mass more than what's observed with light, the cosmic microwave background spectrum is different from what you'd expect from only the stuff we can see, and a fair bit more mentioned on that Wikipedia page.

Keep in mind that any alternative explanation needs to not only explain the fact that these things are different, but also the exact quantity by which they are different. So far, "dark matter" is the best explanation anyone's come up with.

tomr_stargazer2• 10 years ago

(PhD student in astrophysics answering here.)

This is a great question! You're addressing the idea (roughly along the lines of Occam's razor) that if there's any way to explain the "extra mass" without invoking a new form of matter, it should be preferred.

Most of these other possibilities have been largely ruled out via careful observations, which are detailed here: https://en.wikipedia.org/wiki/Dark_matter#Composition

One of these possibilities, for example, is that the missing matter is actually contained in a great many small, dark, massive objects scattered throughout galaxies -- such as failed stars, planets, or even black holes -- rather than in a diffuse, invisible material. This possibility has actually been largely ruled out through a number of statistical "microlensing" surveys that are sensitive specifically to the presence of massive, dark bodies (via their gravitational lensing of background stars -- a rare event, but measurable statistically).

https://en.wikipedia.org/wiki/Massive_compact_halo_object#De...

I think the wikipedia Dark Matter article is actually super well written and should address these issues, too!

maverick_iceman2• 10 years ago

Big Bang Nucleosynthesis (BBN) says that the amount of baryonic (i.e. usual) matter cannot be much higher than what we directly observe. However, it accounts for only ~16% of the mass necessary to account for galactic rotation curves, gravitational lensing by galaxies etc. So the leftover must be non-baryonic dark matter.

One can of course say that BBN is wrong but that's very unlikely as it has been tested to high precision from CMB study.

grahamburger• 10 years ago

How do we know it isn't undetectable-through-normal-means normal matter

As I understand this is kind of what we think it is, which is why it's called 'dark.'

stouset• 10 years ago

Why does it have to be this "Dark Matter" causing the mass differences we've been seeing? How do we know it isn't undetectable-through-normal-means normal matter, or aliens, or mistakes in our calculations/observations, or anything else?

"Dark matter" is just a name for "a phenomenon that only appears to interact with the rest of the universe through gravitational fields, and not through elecromagnetic fields, the weak, or the strong nuclear force". There are many competing theories for what dark matter actually "is".

The name is independent of any particular theory that describes its true nature, whether that's an exotic form of matter, aliens, or mistakes in our observation.

SmellTheGlove• 10 years ago

undetectable-through-normal-means normal matter

I'm not a physicist, but my understanding was that dark matter is undetectable-through-normal-means because it doesn't interact with normal matter. If it were undetectable-through-normal-means normal matter, it would be detectable through normal means.

yincrash• 10 years ago

Matt O'Dowd, an astrophysics professor, hosts a YouTube series called PBS Space Time that does a really effective job if explaining the evidence of dark matter in this episode: https://youtu.be/z3rgl-_a5C0

the8472• 10 years ago

It's more localized, but we've seen similar things before at a larger scale, e.g. the bullet cluster.

https://en.wikipedia.org/wiki/Bullet_Cluster

jomamaxx3• 10 years ago

" Now astronomers can study dark matter with less interference from the regular matter background"

There is no such thing as 'Dark Matter' yet.

Would you people please stop talking about it as though it exists?

It is, at this stage, a very crude idea.

There is no evidence of its existence.

We assume 'Dark Matter' exists because our relativistic equations are broken and it's the easiest thing we can imagine to 'fix' the problem - and we conveniently use the theory to make up for some other flaws as well.

There could be many other reasons or characterizations for those phenomena.

Maybe we should wait until there are some nice direct experiments that strongly confirm that the 'nothing there' is actually 'something' :)

If fear 'Dark Matter' is the 21st century equivalent of 'aether'.

Retra2• 10 years ago

It is, at this stage, a very crude idea.

It is an idea. Ideas exist. It does exist. There's nothing here for you to argue with.

teh_klev• 10 years ago

Archive.is linkage for those who can't get past the paywall:

http://archive.is/b4e7X

semaphoreP2• 10 years ago

Here is the pre-print version of the paper: https://arxiv.org/abs/1606.06291

pc2g4d• 10 years ago

I find myself wondering if the type of inference required to make this claim could be automated. Given the right observations, couldn't dark matter be detected (or at least hypothesized) algorithmically? Couldn't this be used to create a "dark matter scope" by which the dark matter in the universe can be "seen" and visualized?

tombone12• 10 years ago

Not everything can be automated, but there is certainly at least some machine learning in all of the larger surveys these days. As to dark matter surveys explicitly, I found this one: http://kids.strw.leidenuniv.nl/pr_july2015.php

gus_massa3• 10 years ago

I'm guessing here, but they probably detected this automatically. There are millions of millions of galaxies and you can't put a even graduate student to look at each one.

My guess is that they were preparing a boring paper, like "Analysis of Normal/Dark Matter ratio in WhAtEvEr type galaxies near SoMeWhErE". They put a telescope, some processing and then they transfer the data to Excel to make a nice graphic. Then got some outliers, and with more analysis they were discarded as error. But they got one nasty outlier that were not easy to kill. They measure it again, and again, and probably made another team double check it.

(Perhaps they were looking for faint galaxies, and it was not too much luck.)

Anyway, probably most of the calculation was automated, and probably now some other teams will try to find similar objects.

gammarator2• 10 years ago

This was definitely a high-priority, by-hand, single object reduction. They put 33 hours of Keck time into it--you don't do that unless you're certain you have something very interesting.

(For reference, the capitalized value of one night (~8 hours) of 10-m telescope time is roughly $100k.)

Exras• 10 years ago

"The researchers who found Dragonfly 44 weren't looking for a dark galaxy. Another surprise: They found it using a telescope built of camera parts. The Dragonfly Telephoto Array was built by a group of astronomers at Yale University and the University of Toronto who realized that telephoto lenses — so often used for nature photography and sporting events — were well-suited for spotting the kind of large, dim objects that pose problems for typical telescopes."

I like it, reminds me of the discovery of the Cosmic background radiation by Penzias and Wilson with the Holmdel Horn Antenna. Accept this time nobody had to shovel bird shit :-)

SolarNet• 10 years ago

Is it possible that an alien race has simply covered that galaxy with dyson spheres? That life is more common than we thought - or perhaps more rapidly advancing once intelligence is hit - and most galaxies are "dark" from their interference?

aab0• 10 years ago

No infrared. Already ruled out by earlier infrared surveys looking for Dyson spheres.

drjesusphd• 10 years ago

Those were only looking in our stellar neighborhood though, right? Would a galaxy of infrared "stars" be detectable?

bjelkeman-again• 10 years ago

We often seem to assume we know "everything", and we often find out there is more to learn. It would be an interesting universe if we found dark matter to be ancient civilisations. Using up much of the mass and energy (even the infrared).

3pt14159• 10 years ago

Ruled out? From Wikipedia:

Identifying one of the many infrared sources as a Dyson sphere would require improved techniques for discriminating between a Dyson sphere and natural sources. Fermilab discovered 17 potential "ambiguous" candidates, of which four have been named "amusing but still questionable". Other searches also resulted in several candidates, which are, however, unconfirmed.

perseusprime11• 10 years ago

I always thought that dark matter is a trick played by gravity. I wish we can get to these things faster.

dghughes• 10 years ago

Sometimes I have to wonder maybe we are the weird ones and dark matter being more common is normal.

Maybe there are trillions of beings looking at us and our weird matter and are amazed we can survive.

sevenless• 10 years ago

If they can't interact with normal matter or light, we're the dark matter to them.

hinkley2• 10 years ago

From the article it seems to me that it's not the telephoto lenses that were the breakthrough here but the lens coating on the lenses. So is the next step to work with the manufacturer to get those coatings available on purpose-built astronomy equipment?

And is it only applicable to refractory telescopes?

gammarator2• 10 years ago

Large astronomical telescopes are necessarily reflecting: it's more cost effective to build large mirrors than large lenses. But that necessarily implies that there will be obscuration (secondary mirrors, etc.) in the light path, which complicates the focused image and makes it difficult to look for diffuse, low-surface brightness features.

The other challenge in looking for these sorts of objects is scattered light. What van Dokkum & co realized was that commercial telephoto lenses are exquisitely designed to minimize scattered light, and as refractors have no central obscuration. So a small array of such lenses is actually more sensitive to faint, diffuse features than much larger telescopes. (Plus cheaper!)

officialjunk• 10 years ago

came across this paper that investigates how the EM propulsion drives might generate thrust, and as a side effect, the theory explains certain phenomena that we attribute to dark matter and dark energy. i only have undergrad physics degree, but it sounds interesting. anyone with more experience have any thoughts about this?

http://arxiv.org/pdf/1604.03449v1.pdf >McCulloch (2007) has proposed a new model for inertia (MiHsC) that assumes that the inertia of an object is due to the Unruh radiation it sees when it accelerates, radiation which is also subject to a Hubble-scale Casimir effect. In this model only Unruh wavelengths that fit exactly into twice the Hubble diameter are allowed, so that a greater proportion of the waves are disallowed for low accelerations (which see longer Unruh waves) leading to a gradual new loss of inertia as accelerations become tiny. MiHsC modifies the standard inertial mass (m) to a modified one (m_i) as follows:

m_i = m (1-(2c^2)/(|a|Θ)) = m (1 - λ/4Θ) (1) where c is the speed of light, Θ is twice the Hubble distance, ’|a|’ is the mag- nitude of the relative acceleration of the object relative to surrounding matter and λ is the peak wavelength of the Unruh radiation it sees. Eq. 1 predicts that for terrestrial accelerations (eg: 9.8m/s2) the second term in the bracket is tiny and standard inertia is recovered, but in low acceleration environments, for example at the edges of galaxies (when a is small and λ is large) the sec- ond term in the bracket becomes larger and the inertial mass decreases in a new way so that MiHsC can explain galaxy rotation without the need for dark matter (McCulloch, 2012) and cosmic acceleration without the need for dark energy (McCulloch, 2007, 2010).

geertj• 10 years ago

Would this mean that there's a lot more dark matter in the universe than we previously thought? And consequently that we may escape a terrible heat death but instead get some real action near the end?

JackFr2• 10 years ago

It's Dyson spheres.

JackFr2• 10 years ago

Not sure I get the downvotes. Because I'm not elevating the discussion? I was trying to be funny.

wallace_f4• 10 years ago

I sometimes enjoy to daydream about the possibilities of dark matter, but probably the simplest explanation, and the least spectacular one, is the answer here: dark matter likely is a product of black holes, and probably just an even distribution of black holes. There is already growing a little bit of evidence that this is the case. I would be thrilled to learn about gravity leaking from other dimensions or alternate forms of matter, but it is probably just normal matter trapped inside black holes.

andrepd• 10 years ago

I don't think anything in your post makes the least bit of sense.

asimuvPR• 10 years ago

Perhaps you could tell why?

zouhair• 10 years ago

Dark Matter is really a bad name. Too confusing.

JumpCrisscross2• 10 years ago

How do we rule out dust between the galaxy and us in cases like this?

okket3• 10 years ago

The amount needed would be huge and we could see the effects. This "dark dust" would dim/absorb the light emitted by stars behind it and turn it into infrared light, that we could see.

Also, since it is regular ("baryonic") dusk, the slightest distortion would make it clump together, create stars, turn into visible matter.

The last reason is the cosmic microwave background (CMB). We can infer from it how much baryonic mass in the universe should be and the huge amount of dark dusk does not fit in this calculation by a factor of roughly four.

New class of galaxy has been discovered, made almost entirely of dark matter

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