Starless Galaxies?

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ohwilleke

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This bbc report http://news.bbc.co.uk/1/hi/wales/south_east/4288633.stm states that scientists believe that they have found a galactric scale mass of hydrogen gas without any apparent stars.

If explained by dark matter, it would need thousands of times the visible hydrogen mass to hold together. The article notes that previous proposed "starless galaxies" have turned out not to be starless.
 

Chronos

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That is a potentially important find. Thanks for the links.
 
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Is there an explenation for dark matter? I only know that it is not visible but it is detectable by its gravitational field and I know that it makes more than 90% of the universe.

Is there more to it for a simple understanding?
 

selfAdjoint

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bayan said:
Is there an explenation for dark matter? I only know that it is not visible but it is detectable by its gravitational field and I know that it makes more than 90% of the universe.

Is there more to it for a simple understanding?
There is great interest in what kind of particle it might be. One question is whether the unknown particles are "hot" (fast moving) or "cold" (slow). Very hot seems to have been ruled out by observation, so the particles can't be neutrinos, for example (they move at close to the speed of light). Various theoretical particles have been proposed, but so far no smoking gun.
 
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selfAdjoint said:
There is great interest in what kind of particle it might be. One question is whether the unknown particles are "hot" (fast moving) or "cold" (slow). Very hot seems to have been ruled out by observation, so the particles can't be neutrinos, for example (they move at close to the speed of light). Various theoretical particles have been proposed, but so far no smoking gun.
Suppose that each forum needs his poet and admit for a while that I am this man here. There is not only a great interest, there is one of the biggest interrogation about our scientific knowledges according to the fact that Dark energy + dark matter = 97% of everything ! As poet I should ask two stupid questions; has still someone imagine a short-life particle whose role would be to generate local and provisory torsions or spinning in the vacuum that is normally considered as holonomic fluid (as one can see in Lichnerowicz; 1955)? Or has some one imagine that the c-speed limit could be a modulo limit,[0-c[, [c, 2c[,a.s.a. until[N.c, (N+1).c[ are in fact equivalent for an observer that must only be in one of the intervalle, because this fluid has several parallel strata? Naturally nobody must take these questions seriously; I just want to animate the discussion about a really strange phenomenon of the nature.
 

cepheid

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RoboSapien said:
Isnt the below picture very old , Is that dark matter seen in it ?

http://imgsrc.hubblesite.org/hu/db/2003/28/images/a/formats/1280_wallpaper.jpg

That's just dust sillouhetted by the stars in that galaxy...so it is ordinary matter. Dark matter cannot be "seen" in the conventional sense. That is the whole point of it. It is not simply black matter that reflects no visible light, for that is easily detectable by other means. That picture is from late 2003 I think. It's a beautiful pic of the Sombrero galaxy. To me, it conveyed an astonishing sense of depth...
 

cepheid

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Does it mean that if there is a normal galaxy just behind the center of a DarkMatter Galaxy we should not be able to see the DMG even though its obstructing the visible one ?
 

cepheid

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I think so. That's why all of the speculative candidates for dark matter so far have been forms of "matter" that would be completely invisible. For example, neutrino particles are almost impossible to detect because almost never interact with normal matter. Billions (or is it trillions?) of neutrino particles from the sun pass through your body every second, and you don't even know it. They had to build neutrino observatories kilometres underground in abandoned mineshafts, just to detect photons (flashes of light) emitted in the very rare instances that neutrinos actually impacted more common particles of ordinary matter. That's all I know on that subject. But as somebody noted above, I guess neutrinos have been ruled out as viable candidates for dark matter. But I'm assuming any future candidates would be similarly ephemeral?
 

SpaceTiger

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cepheid said:
That's why all of the speculative candidates for dark matter so far have been forms of "matter" that would be completely invisible.
Well, they can't be completely invisible, as that would imply that they couldn't interact with radiation and, therefore, couldn't have formed in the first place. In our common sense notion of "visible", however, that's true of most of the dark matter candidates.

Not all of them however. In an exam, a professor here once asked a Ph.D. student whether it was possible that the dark matter is made up of physics textbooks (based on observational data). At the time, the answer was yes, because they would be completely unobservable (too dim), yet would still contribute mass. For various reasons, however, we now think that the dark matter is made up of weakly-interacting massive particles (WIMPs), like neutrinos.


But as somebody noted above, I guess neutrinos have been ruled out as viable candidates for dark matter. But I'm assuming any future candidates would be similarly ephemeral?
They've been ruled at as the dominant source of dark matter, but they still contribute a non-negligible amount. Most other candidates would have similar "ephemeral" properties, but would likely be more massive than a neutrino.
 
RoboSapien said:
Does it mean that if there is a normal galaxy just behind the center of a DarkMatter Galaxy we should not be able to see the DMG even though its obstructing the visible one ?
That is correct. Current theory is that Dark Matter interacts so weakly (or not at all) with visible matter (except via gravitation) that a galaxy of Dark Matter would not obscure a background galaxy of visible matter (ie the visible matter photons would pass right through the Dark Matter as if it were not there). The only effect the Dark matter would have on the visible photons is a slight gravitational deflection of the light (which may or may not be detectable, depending on how massive the Dark Matter galaxy is).

In this sense, Dark Matter is even "darker" than a black hole.
 
All this seems to make me believe that invisible man is a possibility after all.

if photons are passing through dark matter then what must be the size of the DM atoms ? I mean larger or smaller. A completely new particle science.
 
We have not yet found Anti Matter.

Can Dark Matter be infact the missing antimatter ? How can we know for sure ?

What if DM falls in to a black hole ?
 

SpaceTiger

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RoboSapien said:
All this seems to make me believe that invisible man is a possibility after all.
Dark matter is almost completely non-interacting, so there's no way that a living organism could be composed of it.


if photons are passing through dark matter then what must be the size of the DM atoms ? I mean larger or smaller. A completely new particle science.
You can't really think of particle interactions in this way. The interactions are quantum mechanical, so our normal notions of "size" don't apply when calculating cross sections. However, "effective" cross sections can be calculated which depend on the particle you're trying to get it to interact with. I'm afraid I don't know the required numbers for dark matter off the top of my head.


We have not yet found Anti Matter.
Not true. Anti-matter has been observed, though it's hard to keep around for very long.


Can Dark Matter be infact the missing antimatter ? How can we know for sure ?
Well, I don't really think we're "missing" anti-matter necessarily, but you raise an interesting question. I've never heard the possibility discussed, so I assume it's ruled out for some reason, but if the particles had a low enough interaction rate, I suppose it might be possible to have "dark" anti-matter.


What if DM falls in to a black hole ?
Just that, it falls in. Unlike gas, it doesn't interact, so there's no way for it to emit radiation as it falls in. In fact, it's rather difficult to get dark matter to accrete onto black holes because even the smallest amount of angular momentum will result in an orbit around the black hole. Since it can't interact, there's no way for this orbit to decay, it just keeps going around and around.
 
Thanks for those revealing answers.

SpaceTiger said:
Dark matter is almost completely non-interacting, so there's no way that a living organism could be composed of it.
How about some dark matter aliens ? will we be able to see them ?

Now that we know how to detect DM then why not ?
 
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That's just dust sillouhetted by the stars in that galaxy...so it is ordinary matter. Dark matter cannot be "seen" in the conventional sense. That is the whole point of it. It is not simply black matter that reflects no visible light, for that is easily detectable by other means. That picture is from late 2003 I think. It's a beautiful pic of the Sombrero galaxy. To me, it conveyed an astonishing sense of depth...
Damnit, I got my hopes up after hearing all this buzz about dark matter. Oh well.

Now that we know how to detect DM then why not ?
We can't detect DM. We don't even know what it is.

Dark matter is almost completely non-interacting, so there's no way that a living organism could be composed of it.
Non-interacting with "normal" matter. That doesn't mean dark matter can't interact with other dark matter.
 
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SpaceTiger

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Entropy said:
Non-interacting with "normal" matter. That doesn't mean dark matter can't interact with other dark matter.
Dark matter isn't a special category of matter (like anti-matter), it's just an observational quality that arises from the fact that it doesn't interact much. Most dark matter candidates would produce observable radiation even if they interacted with themselves:

Example
 
SpaceTiger said:
... Most dark matter candidates would produce observable radiation even if they interacted with themselves:
And why not non observable ? What if the radiation is not EM at all ?

Just like G ?
 

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