Astronomers Find Star-Less Galaxy 608
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."
FYI (Score:5, Informative)
Re:Black holes? (Score:3, Informative)
Re:Black holes? (Score:5, Informative)
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.
Re:Black holes? (Score:5, Informative)
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.
FYI: What is Dark Matter (Score:5, Informative)
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 [wikipedia.org] provides more information.
Get the paper here (Score:3, Informative)
Re:Dark Matter (Score:4, Informative)
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.
Re:Black holes? (Score:5, Informative)
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.
Re:Black holes? (Score:5, Informative)
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.
Re: Dyson/Matroska Spheres? (Score:5, Informative)
> 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.
More detailed info (Score:5, Informative)
Find it here [adelaide.edu.au].
The Beeb is slashdotted? (Score:2, Informative)
To get this out of the way: (Score:5, Informative)
Can't dark matter just be brown dwarves or black holes or something? Why do scientists postulate crazy exotic invisible particles?
Dark matter [wikipedia.org] 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 [umd.edu] 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 [arxiv.org] 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
Re:Not Black holes (Score:2, Informative)
Re:How can it be detected (Score:3, Informative)
--LWM
Re:What really happened: (Score:3, Informative)
I presume the process of discovery was that they found a large, rotating disc of cold hydrogen gas (you can measure velocities through the red- and blue-shifts of a particular emission frequency). The distance, angular size and radio strength of the disc gives you its approximate mass, and you can compare that with the mass it should have for that particular speed of rotation, thanks to gravity holding it all together. With that, you can calculate the amount of 'dark matter' - subtract the mass of hydrogen from the total mass, and what's left is presumably still there, just in an undetectable form.
A disclaimer: IANARABIDAUEAJBAPTAS*.
(* I am not a radio astronomer, but I did an undergraduate experiment at Jodrell Bank and pointed telescopes and stuff!)
Re:Meanwhile, on VIRGOHI21... (Score:4, Informative)
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
Link to the science article (Score:3, Informative)
http://xxx.lanl.gov/abs/astro-ph/0502312 [lanl.gov]
(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).
More on Hawking radiation (Score:5, Informative)
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.
Re:Anyone Question the Existence of Dark Matter? (Score:3, Informative)
How?
By observing supernova. The immense amount of light given off by a super novae explosion actually illuminates this "Dark Matter" which is merely diffuse hydrogen uneavenly spread throughout the universe and allows us to actually see parts of it for a small period of time. "Proving" the existance of Dark Matter is one of the many things the Hubble is credited with being responsible for.
There have also been many other forms of indirect evidence that have all pointed to the same conclusion over the past 2-3 years.
The certainty of the existance and the makeup of what Dark Matter is made a giant leap in the confidence level in recent years and can be talked about with a lot more certainty that you are giving it.
Now as to the subject of Dark Energy...
Re:Anyone Question the Existence of Dark Matter? (Score:1, Informative)
Re:Anyone Question the Existence of Dark Matter? (Score:1, Informative)
The previous poster. But it doesn't matter.
No, that is way, way wrong. Even taking interstellar hydrogen into account leaves 80+% of the matter unexplained.
Re: Dyson/Matroska Spheres? (Score:4, Informative)
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...
Re:Black holes? (Score:5, Informative)
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.
Re:Black holes? (Score:5, Informative)
Re:Dark matter is sciences god (Score:5, Informative)
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 [devine-ent.com].
How about evolution.. oh, wait, no.. the fundamentalists [drdino.com] and literalists [creationism.org] won't have any of that.
Okay, how about something really simple - the lightning conductor. Oh, no, wait.. churches originally considered lightning conductors blasphemy [atheistalliance.org] as they attempted to counter god's will - some went as far as to blame them for earthquakes.
I call moron (Score:3, Informative)
So where is the false color image of this galaxy?
The availability or otherwise of a false color image reflects only on how the researchers chose to present their data; has no bearing whatsoever on the existence of the galaxy. In this particular case, it would in fact be unusual to present a false color image, since radio data are more commonly illustrated using contour maps.
How do they know it's rotating like a galaxy?
From the radio observations, which pick up 21cm emission from cold, neutral hydrogen gas. Doppler shifts of the 21cm line allow them to establish a rotation curve for the galaxy.
They haven't shown any sort of evidence of the real matter they claim to have detected.
No, in fact they have presented evidence for the real matter (neutral hydrogen), in the form of the 21cm emission.
To post a picture of empty space and say it's full of dark matter is just stupid.
No, it's quite significant: based on the radio emission, we would expect a population of stars, that would show up in the optical image. The actual absence of these stars, as evidenced by the 'empty space', is the whole reason that this is news.
I think the only dark matter this article shows is in the astronomers head.
By totally misunderstanding every aspect of the story, you have effctively stood up in front of the /. community, and loudly proclaimed 'I'm dumb as shit'. Congratulations.
Re:Black holes? (Score:3, Informative)
Re:Black holes? (Score:5, Informative)
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 [adelaide.edu.au]. They argue that the low surface density of gas prevents fragmentation of hte gas, and hence stars not forming.
5) This is total crap.
Re:Black holes? (Score:3, Informative)
Re:"neutral hydrogen gas" != HI (Score:4, Informative)
You just missed the class. (Score:3, Informative)
Earth is pre-Type I; Sagan apparently calculated us at about 0.7 on the Kardashev Scale [wikipedia.org].
Re:Not Black holes (Score:5, Informative)
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. [uchicago.edu]
Re:But is it really Dark Matter? (Score:1, Informative)
Re:I've figured out dark matter (Score:3, Informative)
No, light reaching our eyes after travelling near a black hole is redshifted, decreasing its energy. See this Wikipedia article [wikipedia.org] (search for "red-shifted") if you're unable to reason about it yourself: intuitively, a light wave coming at us from the vicinity of a black hole (where the gravity is significantly stronger at the "tail" of the light wave than at the "head" of the light wave) would be stretched out, not squished together. Hence the redshift.
There goes that plan.
Jeremy
P.S. "...and it doesn't require exotic quarks, leptons, or baryons to work." doesn't mean much when your alternative is to posit the existence of a type III civilization.
Re:I've figured out dark matter (Score:3, Informative)
Re:Not Black holes (Score:4, Informative)
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.
Re:Not Black holes (Score:4, Informative)
The descreased amplitude of the second peak arises from an effect called baryon loading explained here [uchicago.edu]. The suppression arises from a coupling of the barynons to the plasma prior to recombination. The non-baryonic matter is transparent.
Re:I've figured out dark matter (Score:3, Informative)
so there's no problem with that plan at all.
Re:But is it really Dark Matter? (Score:3, Informative)
[TMB]