Scientists discover that Milky Way was struck some 100 million years ago, still rings like a bell

Jun 28, 2012
Credit: Fermilab

(Phys.org) -- Our galaxy, the Milky Way, is a large spiral galaxy surrounded by dozens of smaller satellite galaxies. Scientists have long theorized that occasionally these satellites will pass through the disk of the Milky Way, perturbing both the satellite and the disk. A team of astronomers from Canada and the United States have discovered what may well be the smoking gun of such an encounter, one that occurred close to our position in the galaxy and relatively recently, at least in the cosmological sense.

“We have found evidence that our Milky Way had an encounter with a small galaxy or massive structure perhaps as recently as 100 million years ago,” said Larry Widrow, professor at Queen’s University in Canada. “We clearly observe unexpected differences in the Milky Way’s stellar distribution above and below the Galaxy’s midplane that have the appearance of a vertical wave -- something that nobody has seen before.”

The discovery is based on observations of some 300,000 nearby Milky Way stars by the Sloan Digital Sky Survey. Stars in the disk of the Milky Way move up and down at a speed of about 20-30 kilometers per second while orbiting the center of the galaxy at a brisk 220 kilometers per second. Widrow and his four collaborators from the University of Kentucky, the University of Chicago and Fermi National Accelerator Laboratory have found that the positions and motions of these nearby stars weren’t quite as regular as previously thought.

“Our part of the Milky Way is ringing like a bell,” said Brian Yanny, of the Department of Energy’s Fermilab. “But we have not been able to identify the celestial object that passed through the Milky Way. It could have been one of the small satellite that move around the center of our galaxy, or an invisible structure such as a dark matter halo.”

Adds Susan Gardner, professor of physics at the University of Kentucky: “The perturbation need not have been a single isolated event in the past, and it may even be ongoing. Additional observations may well clarify its origin.”

When the collaboration started analyzing the SDSS data on the Milky Way, they noticed a small but statistically significant difference in the distribution of stars north and south of the Milky Way's midplane. For more than a year, the team members explored various explanations of this north-south asymmetry, such as the effect of interstellar dust on distance determinations and the way the stars surveyed were selected. When those attempts failed, they began to explore the alternative explanation that the data was telling them something about recent events in the history of the Galaxy.

The scientists used computer simulations to explore what would happen if a satellite galaxy or dark matter structure passed through the disk of the Milky Way. The simulations indicate that over the next 100 million years or so, our galaxy will “stop ringing:" the north-south asymmetry will disappear and the vertical motions of stars in the solar neighborhood will revert back to their equilibrium orbits -- unless we get hit again.

The Milky Way is more than 9 billion years old with about 100 billion stars and total mass more than 300 billion times that of the sun. Most of the mass in and around the Milky Way is in the form of dark matter.

Scientists know of more than 20 visible galaxies that circle the center of the Milky Way, with masses ranging from one million to one billion solar masses. There may also be invisible satellites made of dark matter. (There is six times as much dark matter in the universe as ordinary, visible matter.) Astronomers' computer simulations have found that this invisible matter formed hundreds of massive structures that move around our Milky Way.

Because of their abundance, these dark matter satellites are more likely than the visible to cut through the ’s midplane and cause vertical waves.

“Future astronomical programs, such as the space-based Gaia Mission, will be able to map out the vertical perturbations in our galaxy in unprecedented detail,” Widrow said. “That will offer a strong test of our findings.”

The results have been published in The Astrophysical Journal Letters: iopscience.iop.org/2041-8205/7… 1-8205_750_2_L41.pdf

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verkle
2.3 / 5 (32) Jun 28, 2012
We have absolutely no proof that anything like "dark matter" exists. Yes this article makes baseless assertions that "There is six times as much dark matter in the universe as ordinary, visible matter." as it were a fact set in stone.

This is not science, but simple posturing.

eachus
5 / 5 (12) Jun 28, 2012
Sigh! The current best values for Big-Bang models give the currently accepted ratios of ordinary matter, dark matter, and dark energy. Local and distant observations of gravity at work also agree with the six to one ratio. What is dark matter? There are lots of theories, but observation of gravitational lensing can only tell us where it is, not what it is.

Same with dark energy, except that there is a consensus--but no proof that it corresponds to the energy of the vacuum. This number is easy to derive when working on particle collisions. The Feynman diagrams for collisions have to include (virtual) particles appearing from nowhere--and going back there--to get the right answers. This gives an excellent value for the energy of the vacuum near Earth. (We have no idea if this value is constant, in particular whether it has decreased with time.)
PussyCat_Eyes
1.4 / 5 (14) Jun 28, 2012
We have absolutely no proof that anything like "dark matter" exists. Yes this article makes baseless assertions that "There is six times as much dark matter in the universe as ordinary, visible matter." as it were a fact set in stone.

This is not science, but simple posturing.


- verkle

Most of science, cosmology in particular, is pure speculation. There are very few absolutes to sate our appetites for the unequivocal truth, in my generally disappointed opinion. But I suppose this new turn of events may be the beginning of a new branch of cosmology, if there isn't one already. That is, Galaxies in Collision, which has been briefly mentioned before. Velikovsky must be turning in his grave.
frajo
4 / 5 (4) Jun 28, 2012
As long as you can't tell people what that dark matter is made of, you just can't force people to believe in its existence.

They'd rather question the validity of the dominant model.
And that's perfectly legitimate in consideration of the long list of unsolved problems in physics.

To dismiss beforehand any extension to gravity might turn out to have been short sighted.
Archea
1.9 / 5 (9) Jun 28, 2012
We have absolutely no proof that anything like "dark matter" exists.
The "dark matter" is the phenomenological denomination for group of phenomena, the evidence of which is rather bulletproof. It doesn't say, all these phenomena are of material origin/nature in strict sense. You should realize, that mainstream physics ignored its existence for fifty years, because it doesn't play well with general relativity ("who ordered that?").
A2G
2.7 / 5 (7) Jun 28, 2012
Archea wrote. The "dark matter" is the phenomenological denomination for group of phenomena, the evidence of which is rather bulletproof. It doesn't say, all these phenomena are of material origin/nature in strict sense...."

But that is not the way it is written or spoken about over and over. It is treated not as what you seem to understand, but as if the is an actual substance with its own properties called "Dark MATTER" They refer to it as matter, not some unknown phenomena as it truly should be spoken of.

You Archea seem to understand this. But others are confused by the labeling.

Who really knows for sure what is causing these "effects". Until we know for certain it is misleading at the very least to call it "Dark MATTER".
NMvoiceofreason
4 / 5 (7) Jun 28, 2012
We have absolutely no proof that anything like "dark matter" exists. Yes this article makes baseless assertions that "There is six times as much dark matter in the universe as ordinary, visible matter." as it were a fact set in stone.

This is not science, but simple posturing.



http://phys.org/n...rgy.html

http://phys.org/n...sus.html

http://phys.org/n...axy.html

This is science. It is also an observationally established fact.
dogbert
2.5 / 5 (19) Jun 28, 2012
This is science. It is also an observationally established fact.


No, it is not. We see gravitational anomalies and we create this imaginary substance to account for the failure of our theories of gravity to predict what we observe.

Science would attempt to determine why our theories do not predict the observations. But it is simpler to insert a kludge into the mix. The kludge nature of dark matter is revealed in that it cannot predict. We can only assign an amount and position of dark matter after the observation. This is what a kludge does.

This dark matter is never a conceptual place holder saying "we don't know". It is always referred to an an actual substance as if it were real. The problem with kludges is that people begin to believe they are real.
nevermark
4.4 / 5 (11) Jun 29, 2012
This is science. It is also an observationally established fact.


No, it is not. We see gravitational anomalies and we create this imaginary substance to account for the failure of our theories of gravity to predict what we observe.


Dogbert,

Perhaps you are trolling but dark matter is not a gravitational anomaly at this point. Galaxies without dark matter, and no obvious collision that stripped their dark matter, are anomalies. There are other anomalies associated with dark matter. But it is reliably measurable across the visible universe, the complete opposite of an anomaly.

It is also not a kludge. Too much is now known about it, and continues to be learned about it, for it to simply be someone's unsupported fudge factor to make some isolated equation work.

You are confusing the many open questions about dark matter with the question of whether it exists as a reliably detectable phenomena.
dogbert
3.3 / 5 (13) Jun 29, 2012
You are confusing the many open questions about dark matter with the question of whether it exists as a reliably detectable phenomena.


I am not. And I have never said that the anomalies which have resulted in our creation of dark matter are not reliably detectable.

Dark matter has never been detected. Not one particle of it has ever been detected. If dark matter were real, we should be surrounded by it, yet we cannot detect it. Our sun and every other star should be full of it, but our understanding of stellar processes does not require dark matter.

It was from the beginning a kludge and it remains a kludge.
Torbjorn_Larsson_OM
4.4 / 5 (5) Jun 29, 2012
Interesting test of dark matter models. Also, good to know when they will try to sort out from which population and neighborhood our Sun originated, they still haven't got a handle on that.

As for the DM trolls (they only state the current science erroneously), of course we know it exists unambiguously. It is part of standard cosmology, and there are specific lensing observations that no other theory can predict with the same parameters - except DM. DM is recognized as an unambiguous detection, and its theory has been plenty tested. (The article describes such a test.) Cosmologists have also been able to pin down many of its properties - it is particulate, it is dark, it is cold, 80 % of matter is DM, and its energy is above some GeV. There is a new particle-antiparticle annihilation observation at 135 GeV (IIRC) from close to the galactic center which looks promising to be DM.
Torbjorn_Larsson_OM
3.7 / 5 (6) Jun 29, 2012
So we have detected it albeit those who reject science won't admit to such detections.

They discuss "direct" detection, but what is that? We never see an object as it is, merely by interactions. If we look in EM (say) at baryonic matter (which composes us), it is by interactions with atoms EM and electron fields. We don't see the nucleus, we don't see the electrons, we don't see the vacuum in between. Not "directly".

The proper concept is unambiguous observation, when no other hypothesis can describe what comes out of the interaction.

DM was never "a kludge" or "a placeholder". That is what you put in bookshelves, not parameters in equations expressing what can be predicted. DM was predicted, later it was observed - it has been tested OK.
Torbjorn_Larsson_OM
1.5 / 5 (2) Jun 29, 2012
For a revealing example, it doesn't help if you see a table "directly". You can be looking at movie emulating its optical properties on a screen - same wavelength photons from the same angles.

If OTOH we want an unambiguous observation, we can eliminate screens and the rest should be tables. Modulo hallucinations and other forms of "direct" observation outside a lab.
SteveL
4.3 / 5 (6) Jun 30, 2012
If dark matter were real, we should be surrounded by it, yet we cannot detect it.
I like most of your argument, except this part. There are an unknown number of real things out there we are unable to detect. Germs and viruses, and their effects, were just as real before we learned how to see them. While our understanding of what Dark Matter actually is may only be inferred, something or somethings exist all around us that demonstrably effect our universal environment. Dark Matter is just as good a name as any for what that something, or those somethings are.
dogbert
2.7 / 5 (9) Jun 30, 2012
SteveL,
Dark matter is supposed to be everywhere around us. It only interacts with normal matter gravitationally. It should either be accumulating on the surface of normal matter or passing into the interior of normal matter objects. The sun and planets should be accumulating dark matter. We have not noticed our sun and planets becoming more massive.

The mass of our solar system is fully accounted for in normal matter. Where then is the dark matter which should be counted in this area?

Everything about the concept of dark matter is similarly flawed.

SteveL
5 / 5 (3) Jun 30, 2012
The article didn't provide the frequency of this ringing and I don't have access to the published journal, so I'll ask here.

From an engineering perspective, what effect would galactic "ringing" have on the local solar systems and their planets? Mechanically an implied force that starts resonation within a body tends to dissipate via heat either internally or via pressure waves externally. Could the frequency of this "ringing" have imposed a heat component to solar systems in this area of the galaxy?

By now you know where I'm going with this: Could the resonant frequency of this galactic ringing coincide with solar perturbations?
Torbjorn_Larsson_OM
3 / 5 (3) Jul 01, 2012
"It should either be accumulating on the surface of normal matter or passing into the interior of normal matter objects."

It isn't accumulating or it wouldn't be EM silent dark matter. It is passing through, people are looking for it, but it is very dilute. Its annihilation isn't even enough to heat exoplanets to habitability near the MW center, someone looked if it was feasible.

The solar system is too small to have a large dark matter footprint, but it recently turned out that the MW disk which is some 1000 ly thin (as opposed to the 100 000 ly MW diameter) needs DM. Someone tried to exclude DM by modeling it, turned out they did it wrongly and the model gave the best estimate for local DM density ever. Another test DM passed, another case where DM was highly fruitful.
Torbjorn_Larsson_OM
5 / 5 (1) Jul 01, 2012
SteveL, the article gives the best description outside the papers I have seen, and it shows how the stars in the disk is displaced and how fast the collision energy dissipates.

I don't get where you are going, why wouldn't it coincide with solar perturbations? The Sun is perturbed with the disk, see the article.
SteveL
1 / 5 (1) Jul 01, 2012
SteveL, the article gives the best description outside the papers I have seen, and it shows how the stars in the disk is displaced and how fast the collision energy dissipates.

I don't get where you are going, why wouldn't it coincide with solar perturbations? The Sun is perturbed with the disk, see the article.
I'm not suggesting it wouldn't, I'm suggesting it might. If our sector of the Milky Way is "ringing", I'm asking if that frequency might carry with it physical thermaldynamic effects on ours and neighboring solar systems. Specifically I'm wondering if this frequency of the perturbation might be tied to solar activity levels.
SteveL
1 / 5 (1) Jul 01, 2012
"thermaldynamic" should have been thermodynamic. The act of perturbation is basically "work" for the purposes of thermodynamics. So is the return to equilibrium. Although the frequency is very long I'm wondering if the effect of this cyclic process wouldn't be a thermodynamic response.

Specifically, I'm thinking of the first law of thermodynamics. A force has been applied to a body (the galaxy) such that there is a remaining "ringing" that is currently expected to last at least another 100 million years. Would not conservation of energy imply that there must be an energetic or thermal response?
dogbert
2.1 / 5 (7) Jul 01, 2012
Torbjorn_Larsson_Olv,

... it recently turned out that the MW disk which is some 1000 ly thin (as opposed to the 100 000 ly MW diameter) needs DM


Recently? Dark matter was created because the movement of stars around galaxies was faster than our theories of gravitation allowed. Our galaxy was recognized as a galaxy when dark matter was created.

The fact that in our galaxy (as well as other galaxies), stars move too fast to be contained according to our theories of gravitation does not mean we need dark matter. It means we need to understand why our theories of gravitation do not predict the movement of stars.

Shelgeyr
1 / 5 (4) Jul 01, 2012
There may also be invisible satellites made of dark matter.

But there aren't.

(There is six times as much dark matter in the universe as ordinary, visible matter.)

Not there isn't, but seriously - thank you very much for quantifying the magnitude of your error!

Astronomers' computer simulations have found that...

Fraud. If they replaced "have found" with "suggest" they'd be on much firmer ground, because all the models can do is suggest. You might even be OK using the term "indicate". However, the simulations "FIND" NOTHING and it is deceptive and fraudulent to claim otherwise.

This article is unscientific garbage. My opinion regarding the work upon which this article was written is nowhere near so flattering.
Shelgeyr
1 / 5 (4) Jul 02, 2012
@NMvoiceofreason said:
This is science. It is also an observationally established fact.


Wrong, and citing erroneous articles (filled with unsupported assumptions presented as facts) to support an erroneous article doesn't exactly prove your point.

I too have seen many claims of this or that team producing "maps" or even "highly accurate maps" of dark matter. There is no such thing as a map of dark matter, or dark matter structures, despite rampant claims to the contrary.

I can say this with certainty because there isn't an actual "thing" being mapped. There is a discrepancy between what we observe, and what we calculate we should be observing, and credit the difference to the kludge called "dark matter".

That is actually what is going on. Truly.

And that is NOT how one determines observationally established fact.
Shelgeyr
2 / 5 (4) Jul 02, 2012
@Torbjorn_Larsson_OM said:
As for the DM trolls...


And this is how science falls - when those who soundly reject the standard dogma and actually insist on scientific methods, real evidence, and actual proof (when available) are labeled "trolls" (or kooks, quacks, deniers, etc.) there is scant hope that actual progress can still be made. Instead, the fairy tales grow ever more complex.

There is no evidence for dark matter. There is a mountain of written material, there are endless calculations, there are innumerable cross-citations, and absurdly bountiful grants, but they all rest upon a tissue of assumptions that the faithful dare not peer at too closely.

Never mind. Of course Dark Matter exists! Why, if it didn't, there would be far too many careers at risk!

Heh.

Again, we "dark matter deniers" don't reject science, we insist on it. That makes us the radical fringe.
nevermark
3.2 / 5 (6) Jul 02, 2012
Again, we "dark matter deniers" don't reject science, we insist on it. That makes us the radical fringe.


Your claims of scientific conspiracies are not supported by facts. If they are then go public with your evidence. If not, keep such non-logical arguments out of your posts if you want to claim you are standing up for science. Science doesn't involve vague attacks at peoples motives. If you understood science you would understand how pointless such arguments are.
Shelgeyr
2 / 5 (4) Jul 02, 2012
@nevermark said:
Your claims of scientific conspiracies...

It is not a conspiracy. It is a cabal of the like-minded.

If you understood science...

I understand science. The problem lies with what has laughingly and somewhat accurately been labeled "Science!" (complete with exclamation point). You go on to say "...you would understand how pointless such arguments are." My arguments are - to certain people - "pointless", but that is for political reasons, not scientific ones.

As for "going public", I do that all the time. It is easy. Someone is quoted in an article saying that such and such is a fact (or the article is written in a definitive fashion positing something as a fact), when they're not. I point that out, and why.

These things (in this article's case "dark matter") are conjectural, posited, hypothetical, theoretical... pick whichever non-definitive adjective you like, but to state that dark matter exists as fact is just simply - factually - wrong.
Shelgeyr
3 / 5 (4) Jul 02, 2012
I said:
It is not a conspiracy. It is a cabal of the like-minded.


Let me flesh that out a bit so that it doesn't look like I'm just trolling @nevermark here.

I did not say there is a conspiracy regarding dark matter. It is simply a matter of (and when I say "simply", I admit I'm drastically oversimplifying) too many people all reading from the same playbook, and sadly the playbook contains errors.

Example: An estimated billion years ago (or at least that's what it feels like) when I was taking basic chemistry, they were still teaching the "wafting technique" as a "safe" (ha!) way of smelling an unknown volatile substance. They were wrong. It wasn't safe. There was no conspiracy to endanger students, and that certainly was the standard procedure in universities across the land. Those procedures have since changed, and for good reason. But it took awhile, at least partially because it was contrary to the accepted dogma.

See also: Cargo Cult Science
SteveL
5 / 5 (2) Jul 02, 2012
@ barakn; If you're going to score me with 1's for asking an on topic physics question about a potential thermodynamic response to our portion of the galaxy being perturbed, at least do me the favor of posting why.
ryggesogn2
1 / 5 (1) Jul 03, 2012
What is the period of oscillation?
SteveL
not rated yet Jul 03, 2012
I wanted to know that also. They haven't said. It may be in the published article, but I don't have access.
There is the normal 11-year solar sun spot cycle. That cycle is likely completely specific to our sun. There is a longer term variation that I've seen posted to be between 40-50 years if memory serves. I poked around for more info on it but didn't really find anything of worth except a note that not enough was known about it.

I think what we would be looking for is an even longer term variation as it is supposed to effect our entire sector of space.
Fleetfoot
not rated yet Jul 04, 2012
What is the period of oscillation?


They have only got a single snapshot showing the distribution of stars is asymetric so they couldn't measure a period. The Solar System's motion through the plane is of the order of 30 million years but an oscillation of the plane as a whole would be much slower.
slayerwulfe
not rated yet Jul 10, 2012
SteveL, the article gives the best description outside the papers I have seen, and it shows how the stars in the disk is displaced and how fast the collision energy dissipates.

I don't get where you are going, why wouldn't it coincide with solar perturbations? The Sun is perturbed with the disk, see the article.


the mapping out of the perturbations at some future point is about the only chance the tourist at some future point will have in expressing an opinion about where they are going.
slayerwulfe
not rated yet Jul 10, 2012
i am becoming annoyed once again that i may have to exit. to the stevel's of this place: the earth has a hole at the poles, you drop in a weight of 100 tons it will not exit but fall back to the point it eventually rest at the center.the central plane of our galaxy is a gravitational force that holds bodies within this plane that can be displaced by a force but where they will eventually come to rest again,what does resonance have to do with this/or how does it matter??
dark matter does not reflect light do any of you as the common prudential masses realize how many asteroids do not reflect light.
please become educated before you speak but don't speak in an effort (as an argument) to become educated. slayerwulfe cave
Fleetfoot
not rated yet Aug 27, 2012
i am becoming annoyed once again that i may have to exit. to the stevel's of this place: the earth has a hole at the poles, you drop in a weight of 100 tons it will not exit but fall back to the point it eventually rest at the center.


It will oscillate between the poles with some characteristic frequency and only settle in the centre due to air drag.

the central plane of our galaxy is a gravitational force that holds bodies within this plane that can be displaced by a force but where they will eventually come to rest again,what does resonance have to do with this/or how does it matter??


It determines the frequency of the decaying oscillation.

dark matter does not reflect light do any of you as the common prudential masses realize how many asteroids do not reflect light.


Asteroids are opaque, they interact with light to block that from more distant sources. DM does not, it is perfectly transparent.

please become educated before you speak


Good advice.