Slashdot Mirror
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."
I suggest we donate one of our stars. How about Ben Affleck?
...that I click on "Read More" to find out about matter that's invisible to us and all I get is:
"Nothing for you to see here. Please move along."
Brilliant.
"Backups are for wimps. Real men upload their data to an FTP site and have everyone else mirror it." -- Linus Torvalds
Astronomers have discovered a galaxy about 50 million darkyears away from Virgo that appears to be composed entirly of light matter. This galaxy, dubbed EARTHHI21 is rotating like a real galaxy, at speeds only explainable through massive amounts of matter, thought no single dark mass could be detected."
MORTAR COMBAT!
Should't that be 50 million darkyears?
A feeling of having made the same mistake before: Deja Foobar
Maybe the entire galaxy is surrounded by particles of dust from a long-destroyed supercomputer?
You probably shouldn't click this.
Aha! It's intelligent life! They must have engineered millions of Dyson Spheres over all the stars of their galaxy!
Do you like Japanese imports?
My favorite part of the article: Someone thought that circling the invisible galaxy in the picture was a helpful move.
Personally, I think articles with discoveries this exciting need to be written with more enthusiasm
You are checking your backups, aren't you?
"Um, Bill? The lenscap is still on..."
>
Nope, Goatse Galaxies have Goatse Stars
Sheesh, evil *and* a jerk. -- Jade
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.
...and it doesn't require exotic quarks, leptons, or baryons to work.
Okay, that's an enormous (and highly unlikely) exaggeration, but I *have* thought of an interesting possibility. A Dyson Sphere surrounding several stars (or in a Type 3 civilization, an entire galaxy) would block visible light - the problem is it would glow in the infrared, so it wouldn't really be dark. Black holes are dark, but they tend to fling stuff around, and matter sucked into them gives off bursts of energy before they disappear.
The solution: a dark bubble. At the center of our galaxy there is a supermassive black hole, which is (according to some estimates) roughly three million solar masses. A civilization putting a bubble around it would have 1 (earth) gravity a little beyond the orbit of Pluto, perhaps 40-45 A.U. or so. The problem is that you still would need to stick some stars around it to supply energy, and a Klemperer rosette would be pretty noticeable.
Well, light falling onto a blackhole blue shifts, increasing its energy. Increase the bubble enough (remember, we're talking a civilization that can harness the energy of a galaxy), and the mass of the bubble itself starts to warp space around it. There comes a point where the size of the bubble and the mass that makes it up can be just under the Schwarzschild limit - a bit more massive and it would be a black hole - even without a central singularity. For humans, we'd want a bubble that has a surface gravity equal to earth's, and a blue-shifted energy equal to the average output from our sun.
As a back-of-the envelope calculation, using v^2=2*g*R, where v is the escape velocity, g is the gravitational attraction at the earth's surface, and R is the radius from the center of mass, and setting v=c (the speed of light) for the maximum size, you get a bubble with a diameter just a bit under a light-year across (354 light days, if I figured correctly). The surface area would be about 3 square light-years, 2.6 x 10^26 square kilometers, or 5.2 x 10^17 times the surface area of the earth. The mass would be equivalent to 1.5 trillion suns - roughly twice the mass of our galaxy. Assuming you use buckytubes as the material of choice, you'd have a shell 7000 kilometers thick of solid buckminsterfullerene.
Of course, this is the absolute maximum size and mass just before it becomes a black hole, so the actual construct would be a bit smaller and less massive, balancing surface gravity and blue-shifted energy hitting the surface. You'd also want to carve out mountain ranges and oceans for a bit of variety - a galactic Kansas would be kind of boring. For safety reasons, you would have to stick these bubbles in the empty space between galaxies, or just use all of the mass in one large galaxy (you'd have to be careful, though, to keep relativistic rocks from flying at the completed project). You'd have a sky that would look kind of like a slow-moving aurora, perhaps -- infrared would be shifted into visible light, visible stars would have their peak shifted to ultraviolet -- especially since the gravitational warping would slow down time considerably compared to the rest of the galaxy.
To detect them, you'd have to aim telescopes at the "empty" parts of the sky and see if there was any gravitational lensing. If something was there that was far too massive to be a neutron star but didn't have the characteristics of a supermassive black hole, that could be a sign of it. The largest ones would have the gravitational mass of a large galaxy, so if a supercluster appears to be missing a galaxy's worth of stars that stellar motions demand, it might not be exotic matter but instead bubbles of normal matter from some vast engineering project.
Of course, it might be too early in the evolution of the universe for a type 3 civilization to appear, or you might not be able to make a buckytube bubble big enough that would also support its own weight, so exotic forms of matter might still be necessary. One thing's for certain, though - a bubble like this would make Ringworld look as spacious as a phone booth.
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
I like
It was easy to disprove the existence of aether with the Michelson-Morley experiment. Had that experiment not been possible it would have been very premature to jump to the conclusion that there is no aether. When it comes to dark matter, there is no easy experiment to disprove its existence and so it would be very rash to conclude that our understanding of gravity, which has worked extremely well for us for hundreds of years, is wrong.
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
Having never seen an invisible galaxy before, I'm glad they circled it...
Sometimes I get the feeling that scientists are just fucking with me.
They observed the neutral hydrogen gas (HI), which emits radio waves at the well-known 21cm wavelength. This is not dark at all. From the rotation of the gas, we can work out, with a few assumptions, how big the gravitational potentional would be required in order that the gas is bound. This extra mass is assumed to be dark matter.
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
There are a few mistakes in your conjectures. First off, we're not talking about "a star". You would most likely not be able to make out a single star in such a remote galaxy unless it were astoundingly bright.
;-)
What this tells us is that the density of visible stars in that galaxy (assuming a normal distribution of magnitudes) is low enough that we cannot detect any of them. Someone else care to do the math and tell us what that density threshold is?
When you see "stars" in distant galaxies like Andromeda, what you're really seeing are clusters of stars, though perhaps modern technology has allowed us to resolve single very bright stars, I'm not sure.
As for something blocking our view... that's unlikely, as the dark galaxy was detected by viewing its hydrogen signature in radio wavelengths, so there's no problem seeing it in the correct wavelength.
Most likely (my untrained opinion), this is a galaxy composed of either very small stars or very old (burned out) stars. I'm sure there are good models for describing either. In the first case, for example, I would think that a low initial density of stellar material (mostly hydrogen) would lead to the formation of smaller-than-average stars.
What I think this observation proves is that galactic magnitudes can dip below our viewing threshold in the visible spectrum, and therefore any estimates of the mass of the universe based on visual surveys can be discounted. This makes the closed theory of universal expansion far more likely (e.g. that the universe will expand to a certain point, and then begin to contract until it collapses back into a singularity from which a new Big Bang would arise).
Ok, real astronomers ready your red ink!
> 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.
Obligatory "farside" caption: "They knew they had the telescope pointed in the right direction, but forgot to remove the lenscap"
People who think they know everything are a great annoyance to those of us who do.
...our understanding of gravity, which has worked extremely well for us for hundreds of years...
Keep in mind that our understanding of gravity is that we have no clue what it is. However, our understanding of the effect of gravity has been working fine. The effect of gravity and gravity are two different things. It could very well be that there is absolutely no such thing as gravity and the effect of gravity is actually a side-effect multi-dimensional distortion, or subatomic radiation, or pure heavenly magic. That is why there is a 'theory of gravity' and a set of 'laws of the force of gravity'.
The previous comment is purposely vague and generalized, but all of the facts are completely true.
Congressman (skeptical): Well, I dunno...I don't really see anything there.
Astronomer: Oh, one moment...let me circle it for you!
Congressman: Yes, yes I see it!
Astronomer: Now I was wondering, Congressman...how much additional funding might we get for this discovery?
Congressman: Hm. I'm not sure we have additional funds for such an admittedly amazing find. Now, if you had TWO dark matter galaxies, we'd have something to discuss.
Astronomer (uncapping pen): Funny you should mention that...
...with my tax return?
IRS Auditor: We've added up all of the income your employers have reported for you and it is much greater than what is reported on your tax form. How do you explain that.
Me: While you can usually detect income through tax forms, some types simply don't register. I believe that it is called...dark income.
IRS Auditor: I believe that it is called...tax evasion.
Me: gulp...
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
This may or may not be true - for example, for my personal Dyson sphere I was displeased by the loss of energy caused by alowing the radiatated energy from the sun to spread over the large volume of my sphere (The effective temperature goes down as you get farther away from the sun), so I made my Dyson's sphere reflective on the inside - focusing the light towards two points on the top and bottom of the sphere. That lead my sphere to emit strongly from the top and bottom, but not at all from the sides.
It increased thermal conversion effeciency by 50%, making me the envy of all the other Spheriods.
while (sig==sig) sig=!sig;
If you then made one of those points perfectly reflective and radiated all extra energy out the one point, you could use it as a method of propulsion. You'll that the fastest spheroid around.
I'm sure I speak for many here when I say I was extremely reluctant to click on that link.
-
- - You can't take something off the Internet! That's like trying to take pee out of a swimming pool.
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?
TFA says this object is "a mass of hydrogen atoms a hundred million times the mass of the Sun."
It may be unusual that none of this hydrogen has ignited in a fusion reaction, but that doesn't change the fact that hydrogen atoms are baryonic matter, quite common here on earth. (There are quadrillions of them in my body right now.)
Later, TFA says "according to cosmological models, dark matter is five times more abundant than the ordinary (baryonic) matter that makes up everything we can see and touch."
So this object is "dark" in the sense that it doesn't emit visible light, but it's not Dark Matter.
Or am I missing something here?
That that is is that that that that is not is not.
Still seeing only "clusters" of stars? Check out this view from our old friend Hubble!
This image and the TERAbytes of data like it that have been collected over such a short time are testimony to why losing Hubble is going to be such a tragedy -- whether or not we understand or accept the reasons it's going to happen.
This space intentionally left (almost) blank.
Only on Slashdot will you see calculations for accelerating a Dyson sphere with starlight so you can go visit your girlfriend in another part of the galaxy.
The global economy is a great thing until you feel it locally.
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.
I'm sorry if I sound aggressive. I'm not dismissing the idea of the supremacy of Christianity as an ideology altogether, I just find it very, very arrogant that someone would support that without extremely good scientific proof.
The grandparent didn't express his views with good manners, I'll give you that - but the core idea of his post is still true: The church has through the years made up 'truths' and tried suppress scientific research that tests those 'truths'. Isaac Newton or other christian scientists might have believed in the scientific method, but it seems that the church as an entity does not...
What I find most interesting about the "dark galaxy" is that it's got plenty of hydrogen but it somehow has not managed to form stars.
Sustainability and energy independence essay
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.
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)