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Astronomers Find Star-Less Galaxy
Wohngebaeudeversicherung writes "Astronomers have discovered a galaxy about 50 million lightyears away from earth that appears to be composed entirly of dark matter. This galaxy, dubbed VIRGOHI21 is rotating like a real galaxy, at speeds only explainable through massive amounts of matter, thought no single visible star could be detected."
Each black hole is practically a point-like source, not good at blanketting to shield off the light from a bunch of stars all over the place. A thick smoke screen (like hydrogen) is better at doing that.
Besides, black holes may be bright in X-rays and other wavelengths. They should've been detected a long ago, if it were a full of BHs.
It doesn't have anything to do with black holes.
If it was a black hole, it would be detected by the movement of visible objects around it, or x-ray and gamma-ray bursts from acceleration jets and from energy emitted by the accretion disk.
Dark Matter is simply "missing matter", or matter that cannot be detected through emitted radiation. It can, however, be detected through its (gravitational) effects on surrounding bodies.
Vivin Suresh Paliath
http://vivin.net
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Dark Matter is matter that cannot be directly detected through emitted radiation. But you can detect it through its effect on surrounding bodies. The effect is usually gravitational.
The concept of Dark Matter evolved from the "missing mass problem". You can estimate the amount of mass in a cluster of galaxies based on the motions of other objects around the object in question. When you compare this mass to the mass based on the total brightness (visible mass) of the galaxy, you can find a huge discrepancy. This is the "missing mass".
Wikipedia provides more information.
Vivin Suresh Paliath
http://vivin.net
I like
The preprint.
All galaxies must spin, otherwise they would collapse.
As for how they tell how much it is spinning -- one side is spinning towards us, the other is spinning away. Thus the spectrum of radiation from the side spinning toward us is blue-shifted relative to the side spinning away from us. By measuring the amount of blue-shift they can figure out the speed at which it rotates.
Nothing escapes the event horizon. Not even "invisible" radiation, whatever that is.
Black holes shine (at extremely high energies) because of the matter falling into the accretion disk. That traffic jam of matter that's fallen deep into a gravity well heats it up to phenomenal temperatures. The disks are part of what you might call a black hole system, but they are no more part of the black hole than the earth is part of the sun.
For every complex problem there is an answer that is clear, simple, and wrong. -- H L Mencken
No, black holes by themselves do not emit light since nothing can escape from beyond the event horizon. The light is just a small part of a large range of electromagnetic radiation released by the black hole. This radiation comes from the accretion disk around a black hole, where matter that is spiralling into the black hole starts heating up immensely due to friction. Occasionally, matter escapes (from above the event horizon) in the form of bipolar acceleration jets. Scientists are not sure exactly why this happens.
The other form of radiation emitted by black holes is Hawking Radiation. Space is teeming with particle-antiparticle pairs that are constantly created and annhilated. In the vicinity of a black hole, one member of the pair can be sucked in (consequently annhilating its evil twin inside the black hole) while the other escapes. This gives the impression of the black-hole emitting radiation. Hawking came up with this theory when it was found that black-holes have temperature. That would seem preposterous since it means that the black hole was emitting energy, which it shouldn't.
Vivin Suresh Paliath
http://vivin.net
I like
> Isn't this what they've been telling us to look for for years now - the entire energy output of a galaxy caught and channelled for use by an intelligence that has spread throughout it's own galaxy?
Such spheres still have to radiate heat, or else the inside of the sphere would become as hot as the star. The Wikipedia article says it would show up as stars emitting radiation with the blackbody spectrum.
Sheesh, evil *and* a jerk. -- Jade
More detailed information can be found in the paper, which has been accepted for publication in a letter to the Astrophysical Journal.
Find it here.
Some common objections to dark matter I constantly see whenever the topic comes up on Slashdot:
...
Can't dark matter just be brown dwarves or black holes or something? Why do scientists postulate crazy exotic invisible particles?
Dark matter is postulated to come in two kinds, Massive Compact Halo Objects (MACHOs) and Weakly Interacting Massive Particles (WIMPs). MACHOs are things like brown dwarves, etc.; WIMPs are the new kind of matter. We have already detected some MACHOs through gravitational microlensing experiments (looking for them by how they gravitationally deflect light). But if all the dark matter were MACHOs or something else mundane and baryonic, we would have detected more of them by now. That leaves WIMPs. Also, MACHOs and WIMPs have different physical properties (e.g., they cluster differently, and thus seed the formation of the large-scale galactic clusters we see today in different manners), and an all-MACHO universe doesn't cluster right, though it works out if you let some WIMPs into the mixture.
Ordinary neutrinos don't do the trick, either; we evidently need some new kind of particle. We don't know what WIMPs are, but some have postulated axions, neutralinos or other supersymmetric particles, WIMPZILLAs, solitons, sterile neutriono (that only interact gravitationally),
Dark matter is unscientific; it can't be tested or falsified.
Dark matter theories can be tested indirectly by observing the different predictions they make for galactic rotation curves, early-universe structure formation, cosmological expansion, etc. Already such observations have excluded a number of dark matter theories. And there are experiments underway that try to directly detect them, similarly to how we detect neutrinos.
Dark matter is just epicycles all over again, a fudge factor to preserve a wrong theory of gravity.
Once upon a time, irregularities were noted in the orbit of Uranus. It could have been postulated that the laws of gravity were wrong. Instead, it was postulated that an unseen bulk of matter was perturbing Uranus's orbit. Eventually, that bulk of matter was seen: the planet Neptune.
On the other hand, once upon a time, irregularities were noted in the orbit of Mercury. It was postulated that maybe a new planet caused them (Vulcan), but that turned out to be wrong; instead, a new theory of gravity was needed (general relativity).
The moral: you can attempt to explain away the observations with either dark matter or a new theory of gravity; both are scientifically valid approach. The problem with the latter is that it has proven extraordinarily difficult to produce a modified theory of gravity that is consistent with all observations, whereas there are dark matter theories that appear to do the job. Believe me, scientists don't ignore the possibility of a new theory of gravity any more than they ignore the possibility of a new type of matter; it's just that new theories of gravity don't seem to work as well as new theories of matter in explaining the observations.
What about MOND?
MOdified Newtonian Dynamics is the leading candidate for a non-dark matter alternative, modifying the laws of gravity. (Note that this page is by MOND's inventor, and may be biased.) However, it has had trouble with a number of observational tests; you can search the astro-ph arXiv for critiques of MOND. In particular, although it seems to work for galactic rotation curves, it's hard to get it to also work for cosmological expansion and structure formation. It's also very difficult to make it into a theory compatible with observed tests of relativity.
What about Bekenstein's MOND theory?
Bekenstein recently proposed a relativistic version of MOND called
Any astronomer could tell you that the Milky Way does have dark matter. The rotational curve of the galaxy does not match what we would expect from a purely baryonic galaxy of our size. The closest thing to a baryonic "galaxy" would be a globular cluster.
Shit, I am such a f*cking geek.
JMD
When all else fails, feel free to panic.
Here is the accepted Astrophysical Journal Letter regarding this discovery.
http://xxx.lanl.gov/abs/astro-ph/0502312
(Note: Be on guard for confusing astronomical conventions, like measuring almost everything logrithmically with decreasing numbers representing increasing brightnesses.)
To sum up: Astronomers discovered a large mass of rotating Hydrogen gas towards the Virgo Cluster. From the gas dynamics they were able to estimate the mass of the system, and found it to be comparible to the mass of a galaxy. When they went to look at the optical light given off by stars, they found they couldn't find nearly the amount they should for a normal galaxy, hence the 'star-less galaxy' title.
Current Cold Dark Matter (CMD) models of galaxy formation predict that these 'star-less' masses of dark matter should exist in the universe. While other candidates have been discovered in the past, this is the only (currently) viable candidate now known. If it holds up to subsequent analysis, it will provide observational support for the CDM formation models.
A few quick points --
- Dark matter is simply non-luminous matter (matter that does not emit light at any wavelength).
- Yes, black holes are a form of dark matter (baryonic).
- No, this is not an 'anti-matter' galaxy.
- Current Dark Matter theories lean towards it having a non-baryonic source (i.e. not being made up of 'normal' matter).
Furthermore, Hawking radiation is inversely proportional to the mass of the black hole. In order for the amount of Hawking radiation to exceed the cosmic microwave background (CMB) radiation, the black hole must have a mass significantly less than our sun. A super-massive black hole would emit a miniscule fraction of the CMB, and hence would be black for all intents and purposes.
Ben Hocking
Need a professional organizer?
Slightly more seriously, though - if you did want to use this technique to move a star around, it would be more complex. If you just did the procedure described you would smash your sphere into the star - so you would need to reflect the energy back into the star in all directions except one.
Anyway, here are the design calculations so you can visit your girl - a sun-like star puts out 386,000,000,000,000,000,000 MW, dividing by the speed of light (300,000,000 m/s) yields the force of about 1,000,000,000,000,000,000 kgm/s^2. Since a sun-like star has a mass of 2x10^30 kg, your acceleration is 5x10^-12 m/s2.
So it may take a while...
while (sig==sig) sig=!sig;
First off, someone please mod up the parent. Good reply, and I bow to obvious facts that contradict my statement.
However, your point about hubble is mis-placed. Hubble can't resolve this kind of image any better than ground-based AO scopes at this point (not because the atmosphere poses no obsticle, but because AO allows better than default resolution, and technology has advanced since Hubble was sent up).
As others have pointed out to me here on Slashdot, the reason that Hubble is useful is that certain wavelengths simply don't get through our atmosphere, so while pictures like the one you link to could be taken from the ground today, a great deal of research cannot.
Personally, I'd love to see a ground-based scope on the far side of the moon to replace hubble, but I'm probably just dreaming.
There is both baryonic and non-baryonic dark matter. Astronomers distinguish between the two types, and try to study/understand both. We don't know what the major non-baryonic dark matter is, but we know some of its properties (how it clumps on various scales), and we know it doesn't readily interact with baryonic matter. There are candidate particles. Neutrinos apparently have a mass, and likely make up a small fraction of it, but for the most part, no, we don't know what it is.
Professor of Astronomy, Author of Spider Star & Star Dragon (Tor)
There is even a school of thought that says without Christianity a lot of Scientific discoveries would have been a really late in coming
Like heliocentricism, for example? Oh, wait.. wrong way round, the church battled that one for 300 years, finally pardoning Galileo for his 'crimes' in 1992.
How about evolution.. oh, wait, no.. the fundamentalists and literalists won't have any of that.
Okay, how about something really simple - the lightning conductor. Oh, no, wait.. churches originally considered lightning conductors blasphemy as they attempted to counter god's will - some went as far as to blame them for earthquakes.
Curiosity was framed. Ignorance killed the cat.
1) You definitely wouldn't see single stars. We'd see only the integrated light from a whole population of stars.
2) The numbers are already done for us. From the paper: 'We conclude that there is no optical counterpart to VIRGOHI21 down to a B-band surface-brightness limit of 27.5 B mag/arcsec^2. This is less than 1 solar luminosity pc^-2, giving a maximum luminosity in stars of less than 10^8 solar luminosities if a diameter of 16 kpc is assumed.'
3) M31 isn't far away at all. In fact, its the closest large galaxy to the MW. HST can resolve individual stars there, allowing us to measure the brightnesses and construct helpful "colour-magnitude diagrams" for instance.
4) No. Read the paper. They argue that the low surface density of gas prevents fragmentation of hte gas, and hence stars not forming.
5) This is total crap.
sorry, wrong again. HI, as defined and used by every astronomer on the planet, it neutral hydrogen. That's a H with a roman I next to it. HII is ionised hydrogen (H+ to chemists). H_2 is molecular hydrogen.
Yes, right, neutrinos can only contribute a tiny amount.
Similar to galactic rotation curves, galaxy velocities in clusters are too high without large amounts of dark matter.
The best evidence at this stage probably comes from the microwave background acoustic peaks. The amplitudes of the second and third peaks depend on the amount of baryonic matter (second peak) and the total amount of matter (third peak), and indicate about six times as much non-bayonic matter as baryonic matter. We still don't know what it is, but know how much there is to two significant figures.
I've alerady linked to it already in this thread, but I'll do it again because it is a very nice pedagogical website about these results. Check out Wayne Hu's webapages.
Professor of Astronomy, Author of Spider Star & Star Dragon (Tor)
The maximum size sphere you describe would require 1.6x10^15 jupiter masses of carbon. If every star in our galaxy (200 billion if I remember right) had a solar system, and every solar system had 10 (!) planets the mass of jupiter, and every planet was made of pure carbon, you would need all the carbon of about 80 such galaxies to build that bubble. Then again if there were that many such planets in the universe, it'd be obvious what "dark matter" was.
The second peak depends on the sound speed, which depends on the temperature and density of ONLY THE BARYONIC matter. It is a true acoustic peak, and so depends on how fast sound waves travel, which is not something affected by the non-baryonic matter. The number you get is completely consistent with the number you get from the light element abundances and big bang nucleosynthesis theory.
The universe is almost certainly exactly flat. Flatness is expected from inflation, but, more tellingingly, is the fact that if it weren't exactly flat (to within 40 orders of magnitude), it shouldn't be close to anything flat today.
Professor of Astronomy, Author of Spider Star & Star Dragon (Tor)
Sorry, my off-the-cuff statement about the sound speed and baryon density wasn't really right. Certainly there is some effect there, but it isn't the important issue in determining the amplitude of the second peak. I teach this stuff, but I don't do research in it and I do need to look up the details sometimes.
The descreased amplitude of the second peak arises from an effect called baryon loading explained here. The suppression arises from a coupling of the barynons to the plasma prior to recombination. The non-baryonic matter is transparent.
Professor of Astronomy, Author of Spider Star & Star Dragon (Tor)