Galaxy Sans Dark Matter

ChromaticDragon writes "Astronomers have crunched some numbers on a galaxy to discover that its rotation can be fully explained by the gravity of the observable matter — in effect, this galaxy seems to lack dark matter. This shouldn't come as a total surprise given that one of the stronger observations of Dark Matter was the Bullet Cluster where supposedly a good deal of Dark Matter and good old fashion regular matter had separated."

Comments

A good link

[The Empiricist]

I'm not sure if it is the story the submitter was trying to link to, but this article seems to cover the subject.

Re:A good link

[Architect_sasyr]

It's truly indicative of how well the firehose and editorial system really does work here on /. when we can get something as glaringly obvious as a screwed link through to the front page...

Back of the Galaxy

[chill]

All the Dark Matter is there, it just was told to move to the back of the galaxy.

Another argument for variability of "constants"

[Ungrounded Lightning]

No (or negligible) dark matter in our galaxy, eh?

When we're looking farther away, we're looking back in time, too. So perhaps the observations could be explained by "constants" of physics (notably the gravitational constant) varying with the age of the universe, rather than by the gravitational pull of some otherwise-unobservable dark matter.

Let's see if "dark matter" is "more dense" the farther away we look... B-)

Re:Another argument for variability of "constants"

[arotenbe]

No (or negligible) dark matter in our galaxy, eh? That's what I thought when I first read this story, too. The summary is confusing:

Astronomers have crunched some numbers on a galaxy to discover that its rotation can be fully explained by the gravity of the observable matter -- in effect, this galaxy seems to lack dark matter. Here, "this galaxy" refers not to the Milky Way but to the other galaxy mentioned in the first part of the sentence.

Impact on gravity theories

[Dice]

This is another nail in the "dark matter can be solved with a modified theory of gravity" coffin. If we can find a galaxy composed of stars whose observed motion is entirely explained by the mass of those stars and known theories of gravity (Newton, Einstein) that's a serious blow to theories like MOND.

Re:Impact on gravity theories

[syousef]

I'm not the GP but can answer your question.

You measure the dopler shift of the stars on each side of the galaxy. Waves from stars travelling towards you compress, waves travelling away from you so it helps if galaxy is seen more edge on than top or bottom towards us.

(Of course it won't be perfectly edge on so you have to calculate the component that is edge on to work out actual speeds around the galactic center. The less edge on the more accurate you can be because the component that's edge on is larger). ...which leads to how do you measure doppler shifts.

One way is to look at the spectral lines of light in a star (ie split the light through a prism or diffraction grating). Chemicals that make up the star's surface absorb at precisely known wavelengths. It's actually really easy to do some calculation once you know what wavelength these lines have shifted to. (I did it when I did my astronomy masters. It's basic algebra andsimple equations). The difficult part is building equipment that can measure spectra so accurately. In the early days they'd be literally measuring the difference between wavelengths on glass plates.

http://aether.lbl.gov/www/science/galrotcurve.html
"To make a rotation curve one calculates the rotational velocity of stars along the length of a galaxy by measuring their Doppler shifts, and then plots this quantity versus their respective distance away from the galactic center."

Re:Fascinating!

[tirerim]

Sure. But most of the galaxies we have observed seem to be made up of a mixture of dark matter and visible matter, given their gravitational characteristics. Finding a galaxy with no matter isn't any sort of physical impossibility, but it's surprising because it's not the norm. If most galaxies contain dark matter, then something unusual must have happened to this one for it not to contain dark matter, and that's interesting; beyond that, the fact that such a thing even could happen may give us insights into the nature of dark matter.

Re:Broken link?

[RuBLed]

Broken link? I don't think so..

It says 404.. Not Found.. Pretty much in line with TFS...

Re:simplest thing ever

[logicnazi]

Umm, no.

There are some good reasons to believe it isn't normal matter that isn't making light. For starters one would still expect it to absorb light and thus be observable. Additionally our models of galaxy formation would suggest it should have a certain distribution which doesn't conform with what is necessery to explain the rotation behavior. In fact it may even need to be relatively free from interactions to be as spread out as needed. Most relevantly the observations that suggest that dark matter doesn't collide with itself or normal gas when galaxies collide suggests it isn't normal matter.

Of course your general sentiment is right. There are reasons to believe dark matter isn't made up of neutrinos but it isn't any more mysterious than they are. It is probably just some weakly interacting particle much like those we have already discovered.

History of Dark Matter

[foxpaws]

I thought I could contribute a bit to the discussion by giving some background on why the theory of dark matter came about.

Vera Rubin's work on galaxy rotation rates is still pretty compelling evidence for dark matter... OR, at least, it shows us that all galaxies do not behave they way we think they should, to be more accurate. People much smarter than my own self have decided that "dark matter" or some sort of mass/force/something that does not emit light or radio waves, etc. (which is why we never noticed it before) must be responsible.

When we look at a solar system like ours, we see that the farther a planet is from the sun, the slower it travels. Not only does it have a much longer way to go, but it doesn't - and according to what we understand, shouldn't - travel as fast.

Vera Rubins decided to check a whole galaxy. What she found did not hold with our understanding. The solar systems, stars and other observable matter near the outside were traveling faster than expected.

Vera Rubin's work, combined with the discovery that the univers appears to be expanding at an accelerating rate, rather than slowing down, kind of kicked off the whole dark matter/dark energy thing.