What do you call an absence of darkness? Dark matter is supposed to be spread throughout the universe, but a new study reports a spiral galaxy that seems to be empty of the stuff, and astrophysicists cannot easily explain why.
In the outer regions of most galaxies, stars orbit around the centre so fast that they should fly away. The combined mass of all the observable inner stars and gas does not exert strong enough gravity to hold onto these speeding outliers, suggesting some mass is missing.
Most astronomers believe that the missing mass is made up of some exotic invisible substance, labelled dark matter, which forms vast spherical halos around each galaxy. Another possibility is that the force of gravity behaves in an unexpected way, a theory known as modified Newtonian dynamics, or MOND.
In the spiral galaxy NGC 4736, however, the rotation slows down as you move farther out from the crowded inner reaches of the galaxy. At first glance, that declining rotation curve is just what you would expect if there is no extended halo of dark matter, and no modification to gravity. As you move far away from the swarming stars of the inner galaxy, gravity becomes weaker, and so motions become more sedate.
The rotation measurements only stretch 35,000 light years out from the galactic centre, which is not far enough to confirm that first impression. So a team of astronomers in Poland developed a more sophisticated analysis.
Joanna Jalocha, Lukasz Bratek and Marek Kutschera of the Polish Academy of Science in Krakow have found a way to splice the rotation curve together with another measurement: the density of hydrogen gas far from the galactic centre.
According to their combined mathematical model, ordinary luminous stars and gas can indeed account for all the mass in NGC 4736.
Sceptical response
"If this paper is correct, then this galaxy contains very little or no dark matter," says astrophysicist Jürg Diemand of the University of California, Santa Cruz, US, who is not a member of the team. "That is surprising."
Diemand says numerous other techniques - including studies of how galaxies move inside clusters and measurements of the big bang's afterglow - all show evidence for dark matter.
So could the new analysis be faulty? "One really needs excellent data to pull this off," says Stacy McGaugh of the University of Maryland in College Park, US, an expert in galaxy formation and evolution. "I'm afraid my grumpy first impression is that I just don't buy it."
Great puzzle
McGaugh points out that other galaxies have shown declining rotation curves, but later observations have always shown that beyond a certain distance, they flatten out, which can't be explained by ordinary gravity from visible stars and gas. "If we believe this decline, it seems like the exception and not the rule," he says.
Even then, one exceptional dark-matter-less galaxy would be a great puzzle. "The current picture is that galaxies form inside of dark matter halos," Diemand told New Scientist. The dark matter's gravity attracts ordinary gas, which can then coagulate into stars.
"It is unclear how one would form a galaxy without a dark halo, or how one could remove the halo without destroying the galaxy," says Diemand. "A galaxy without dark matter really does not fit into our current understanding of cosmology and galaxy formation."
Nor can galaxies with declining rotation curves be easily explained by MOND, says McGaugh. So for now, it seems that some of our missing mass is missing.
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Journal reference: Astrophysical Journal (forthcoming)
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No Dm Galaxies Exist
Thu Feb 28 22:25:20 GMT 2008 by Tissa Perera
Per my hypothesis, mimicking DM starts beyond
a certain radial distance R, which also depends on the
lookback era in time. There is no DM below R, and
NGC4736 appears to be small enough to fit the bill.
See my website:
cosmicdarkmatter.com
Rubbish Paper
Fri Feb 29 17:43:00 GMT 2008 by Sa
Why did ApJ accept it?
Rubbish Paper
Tue May 27 17:55:54 BST 2008 by As
Maybe because they couldn't disprove it
No Dark Matter
Tue Mar 11 19:19:59 GMT 2008 by Roy G. Reid
NGC 4736 rotation indicates to me that it lacks a black hole to dialate time to any significant degree. Gravatational time dialation could distort our observation of rotational rates in galaxies resulting in miscalculations that look like the galaxy is spinning to fast in the outer regions.
There are many opprotunitys presented to us when we classify dark mater as being made of Santas and Easter bunnies. We can map the gavitational time dialation curve by comapring the observed rotation to the rotation nessisary to maintain stable orbits. That would allow us to estimate the mass of black holes, detemine the age of the universe, discover true cosmological time, and have a new view of a universe.This would lead to entirely new fields in physics.
I personaly lack the resorces to tackle all these calculations. If you have the resorces to investigate this hypotisis then take a chance(privately of course) on becoming a great name in history.
No Dark Matter
Thu Jun 26 14:35:14 BST 2008 by Mr. Jamahl Peavey
The problem is conceptual. Most cosmological theories including General Relativity were developed before we had technology like Hubble and deep space probes. If we continue to teach this branch of science as if the current theories were developed beyound ous soalr system then we will continue to have problems. Classical physics was abandoned in the early 1900's by quantum physicist. Now we are paying the price unitl then science grow as an expansion of knowledge not abandonment.
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