Starless Galaxies: BBC Report on Dark Matter Discovery

[ohwilleke]

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.

 

[turbo]

Thank you for the heads-up. Paper here.

http://xxx.lanl.gov/PS_cache/astro-ph/pdf/0502/0502312.pdf

 

[Chronos]

That is a potentially important find. Thanks for the links.

 

[Erich]

Hey Folks, they found a whole galaxy of it

http://www.jb.man.ac.uk/news/darkgalaxy/

Erich

 

[RoboSapien]

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

 

[Ola]

Very interesting!
Check this:http://www.universetoday.com/am/publish/dark_matter_galaxy.html?2322005

 

[bayan]

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]

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.

 

[member 11137]

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.

 

[member 11137]

Oh I forgot, you have a lot of links concerning this dark energy and this dark matter: www.astro.uni-bonn.de/webiaef/outreach/posters/darkenergy or www.space.com/scienceastronomy/generalscience/darkenergy_folo_010410.html

 

[wolram]

http://arxiv.org/pdf/astro-ph/0501514
This paper states there are no starless galaxies.

 

[cepheid]

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...

 

[RoboSapien]

... 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...

Is that the reason I am unable to see any galaxy in this picture http://newsimg.bbc.co.uk/media/images/40855000/jpg/_40855587_astronomy300.jpg

of the article posted by ohwilleke

 

[cepheid]

Is that the reason I am unable to see any galaxy in this picture http://newsimg.bbc.co.uk/media/images/40855000/jpg/_40855587_astronomy300.jpg

of the article posted by ohwilleke

There's a caption underneath that photo. It reads:

"The invisible galaxy could only be "seen" using radio waves"

 

[RoboSapien]

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]

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]

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.

 

[moving finger]

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.

 

[RoboSapien]

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.

 

[RoboSapien]

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 into a black hole ?

 

[SpaceTiger]

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 into 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.

 

[RoboSapien]

Thanks for those revealing answers.

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 ?

 

[Entropy]

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.

 

[SpaceTiger]

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

 

[RoboSapien]

... 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 ?

 

[SpaceTiger]

And why not non observable ? What if the radiation is not EM at all ?

Perhaps it's a possibility, but it requires even more arbitrary assumptions than the normal dark matter model (such as new physics), so I don't think it would be taken seriously.

 

[tdunc]

Is it not completely obvious to anyone that what this is talking about is a black hole? Oh my god. Galaxy? Starless Galaxy? Holy crap you got to be kidding me.

-Hydrogen gas rotating around a central point
-The whole thing is X times more massive than what can be accounted for by the hyrogen alone, gee I wonder why, could there be a black hole? Oh you duh. Maybe that's why its rotating? "Shush! don't tell anyone were so eager to make a new discovery I wonder if people will buy it?"

 

[matt.o]

you have no idea. do you think you are smarter than the people who wrote the paper? do you think they didn't consider this. sheesh, some people...

 

[RoboSapien]

you have no idea. do you think you are smarter than the people who wrote the paper? do you think they didn't consider this. sheesh, some people...

We need people like U here, U r the man.

 

[SpaceTiger]

-The whole thing is X times more massive than what can be accounted for by the hyrogen alone, gee I wonder why, could there be a black hole?

If this galaxy's mass were dominated by a single, massive black hole, it would imply the existence of an alternative gravity model other than MOND, which is designed to make luminous matter create the rotation curves. In Newtonian/Einsteinian gravity, the rotation curve around a black hole wouldn't be flat at large distances.

 

[tdunc]

I don't follow you on the luminous matter thing. So are you saying matter that is equally massive yet does not emit light per say would not achieve the same effect?
Also the rotation curve around a black hole Would Absolutly be Flatter at larger distances...

 

[SpaceTiger]

I don't follow you on the luminous matter thing. So are you saying matter that is equally massive yet does not emit light per say would not achieve the same effect?

The rotation curve depends on the mass as a function of radius. MOND is designed to make it so that we can simply say that the amount of mass is proportional to the amount of light we see and still reproduce the shapes of observed rotation curves. Black holes have basically a delta function mass profile (it's all at the center), so this isn't the same thing.

Also the rotation curve around a black hole Would Absolutly be Flatter at larger distances...

In Newtonian/Einsteinian gravity, the rotation curve would go as:

$$v=\sqrt{\frac{GM}{r}}$$

a large distance from the black hole. This isn't flat.

 

[tdunc]

I read the paper a second time, what they are really talking about is the discovery of a Dark Matter Halo, not a galaxy per say which would be contained within that halo. A halo is supposedly around every galaxy to explain for the flat rotation curve. Basically the rotation of stars are not what they think they should be according to gravitational tides from a central black hole and visible stars alone, so they introduce a halo a very large distance from the edge of the galaxy itself to explain why the stars near the outter edges are rotating so fast - effectivly giving a flat rotation curve. Personally I still don't understand the dynamics of how that would be.

First of all what is a rotation curve? Here is a diagram Fig.2 with a short description.
http://en.wikipedia.org/wiki/MOND

Fig.3 is shows where the Dark matter halo would be.

MOND attempts to do away with the need for a Dark matter halo and explain the flat rotation curve by a modification to Newtons 2nd law of motion which gives a change to acceleration in situations where very large distances between bodies are present therefore the effects of gravitational pull are lower overall than say here on Earth or around our Solar system.

Have I got this right?

SpaceTiger
"a large distance from the black hole. This isn't flat."
I still do not know why you would say this. Every galaxy has a black hole at the center, it is known the rotation curve a "large distance from the black hole" remains flat to the outter edges. What do you consider a large distance from center? I would say a 3rd maybe 4th from the center, that is well within the range of the observed flat rotation curve. In MOND theory "Consequently, the velocity of stars on a circular orbit far from the center is a constant, and doesn't depend on the distance r: the rotation curve is flat."

But that's not quite relevant now, also I don't recall anything said in the paper about such rotation curvature, whether is would be flat or not the point is mute because the paper is about the exsistence of the halo not the exsistence of the galaxy which is what you need to have in order to observe said rotation curve. Of course I might have missed that if they mentioned it with regards to the hydrogen that is located where the galaxy should be, and its rotation, but that doesn't make much sense because rotation of stars and rotation of an isotropic hydrogen are two different dynamics.

In any case I guess I'm still not convinced a black hole is not or could not be there for the sake of argument. Or even that what they have discovered is a Dark Matter halo which are up to this point as it would seem somewhat theoretical.

 

[SpaceTiger]

Personally I still don't understand the dynamics of how that would be.

One simple way to produce a flat rotation curve is with a simple isothermal sphere mass profile:

$$\rho \propto \frac{1}{r^2}$$

We used this as a fit to galaxy dark matter profiles for a long time, but recent evidence indicates that it's not quite as simple as that. We need another parameter to fit the observed velocity profiles.

Have I got this right?

Everything you've said up to this point looks right.

"a large distance from the black hole. This isn't flat."
I still do not know why you would say this. Every galaxy has a black hole at the center, it is known the rotation curve a "large distance from the black hole" remains flat to the outter edges.

Your theory, as I understood it, was that black hole was itself causing these motions. Current theory says that's not the case anywhere except very close to the black hole. The two observations you cite are roughly accurate, but they are not related to one another. Even just the luminous matter in a galaxy is several orders of magnitude more massive the central black hole.

In MOND theory "Consequently, the velocity of stars on a circular orbit far from the center is a constant, and doesn't depend on the distance r: the rotation curve is flat."

I didn't realize you were using MOND to make that statement. In MOND, you're right that the rotation curve would be flat a large distance from a central black hole. However, if you assume the mass is entirely in a central black hole and use MOND, then the rotation curve you get will be monotonically decreasing, with a Newtonian dependence at the center and flatness at the outskirts. This is not what we observe. In fact, the rotation curve increases from the center of the galaxy and then becomes flat on the outskirts.

In any case I guess I'm still not convinced a black hole is not or could not be there for the sake of argument. Or even that what they have discovered is a Dark Matter halo which are up to this point as it would seem somewhat theoretical.

The black holes exist, but we're sure they aren't the dominant source of gravity in most of the galaxies we see. I'm not sure if that's what you're trying to say, so if I'm misinterpreting you, my apologies.