Milky Way May Have Dark Matter Satellite Galaxies

rubycodez writes "Berkeley astronomer Sukanya Chakrabarti has detected perturbations in the gases surrounding our Milky Way and concludes there is a satellite 'Galaxy X' 250,000 light years away that is mostly dark matter, but that may contain dwarf stars visible in infrared. She expects many more such dark matter satellites to the Milky Way to be discovered using her technique."

Comments

This sounds like a sci-fi blockbuster

[roc97007]

"The creature from invisible Galaxy X"

Re:This sounds like a sci-fi blockbuster

[Black Parrot]

"The creature from invisible Galaxy X"

There was an interesting musing by the author of a recent Scientific American about how dark matter may interact with its own kind by forces other than the ones that cause normal matter to interact with its own kind. According to the musing (which the author rejects), dark matter operating under such forces could form complex systems, maybe even an unseen parallel universe where "people" live lives like ours, as unaware of us as we are of them. All undetectable, except by their gravitational attraction on us.

Re:This sounds like a sci-fi blockbuster

[jbeaupre]

parallel universe where "people" live lives like ours, as unaware of us as we are of them. All undetectable, except by their gravitational attraction on us.

Are you talking about women? I've actually seen them. No joke!

Dark matter vs black holes

[Fractal Dice]

How do you tell the difference between a blob of dark matter and a black hole? With all the small galaxies the Milky Way has swallowed over its lifetime, would it not be reasonable to find some relic black holes that have swung back out after being stripped of most of their surrounding gas/stars? Or, when "dark matter" is being talked about in this situation, is a black hole simply one of the possible candidates to supply the mystery mass?

Re:Dark matter vs black holes

[Colonel Korn]

How do you tell the difference between a blob of dark matter and a black hole? With all the small galaxies the Milky Way has swallowed over its lifetime, would it not be reasonable to find some relic black holes that have swung back out after being stripped of most of their surrounding gas/stars? Or, when "dark matter" is being talked about in this situation, is a black hole simply one of the possible candidates to supply the mystery mass?

I think we'd expect to see the kind of supermassive black hole that could be mistaken for a dwarf galaxy. The processes that form black holes of that size mean that there would probably still be a lot of material in the vicinity, if not actively accreting then still getting pulled around, compressed, and prompting star formation. Also, I think nearby galactic-sized black holes would probably make for some pretty wicked and obvious gravitational lensing.

Alternatively, the detected mass might be a large number of small black holes. I doubt it, but I'm not an astronomer. Luckily, further observation will give us answers.

Re:Dark matter vs black holes

[Monkeedude1212]

How do you tell the difference between a blob of dark matter and a black hole?

Gravitational pull is probably the biggest factor. A black hole simply gets so massive that at one point the gravitational pull is so strong that not even light can escape. It will have objects orbitting around it like planets orbit stars except at distances far greater than a star would normally hold.

Dark Matter, on the other hand, simply seems to have the gravitational pull of a regular star, but doesn't emit any light.

One thing to note is that when we observe things out there, it's not just a 2D plane we're observing but a great deal of depth is involved. When observing a black hole, the light behind the black hole will get sucked into the black hole if it happens to cross the event horizon. This will create a nice black circle in the sky. However dark matter, on the other hand, would not stop the light behind it from reaching our eyes, it might bend it a little but nothing too extreme.

Re:Dark matter vs black holes

[Tim C]

A black hole simply gets so massive that at one point the gravitational pull is so strong that not even light can escape.

Actually that's a good working definition of a black hole - if its gravity weren't that strong, it wouldn't be one.

It will have objects orbitting around it like planets orbit stars

Yes...

except at distances far greater than a star would normally hold.
 
...and no, not necessarily. That depends entirely upon the mass of the hole. The gravitational field of a black hole at a given distance is no different than that generated by a star of the same mass at the same distance; the difference is that the hole is so much smaller that you can get much closer to its centre. That vastly reduces the r in GM/(r^2), thus increasing the maximum gravity that can be experienced.

When observing a black hole, the light behind the black hole will get sucked into the black hole if it happens to cross the event horizon. This will create a nice black circle in the sky.

The situation is a little more complicated than that thanks to gravitational lensing, but essentially you're correct - a black hole will block light, while dark matter does not.

Re:Dark matter vs black holes

[c 0xFF]

Say what?

First off, the GPP has a decent question. The largest supermassive black holes are on the order of 10^9 solar masses, about the same mass as what was calculated for this satellite galaxy. So, I suppose it's at least plausible that it's a single black hole, if unlikely.

But remember and repeat after me: a black hole has no more gravity than any other object of the same mass. As long as you stay away from the event horizon, that is. You need to rethink your first paragraph with that in mind.

So, how would we tell the difference? Well, an X-ray source from the same location would be a good clue that it's a black hole, which says that it's feeding off of something. You should also be able to tell from the gravitational lensing -- dark matter is incredibly diffuse compared to a black hole. It would still bend light, but not quite in the same way, especially considering the distances involved.

But what about a black circle in the sky? Well, the even horizon for such a black hole has the same diameter as the orbit of Pluto, if I remember right. Detectable, maybe, under the right conditions (but not by Hubble -- you'd need something with about 20x better resolution ... if I did the math right, which I probably didn't). But we have to capture it overlapping with some other body, such as a background galaxy. By then you'd be better off looking at the lensing effect, anyway. Here is a classic simulation of what I'm talking about.

Re:A galaxy of what? Dark stars?

[rainmouse]

What is the form of the dark matter?

Assuming it exists at all. There is much circumstantial evidence but some argue no direct proof yet (though NASA believe the have proof). Still this excerpt from NASA seems to imply that dark matter does not interact with matter except through gravity.

"The hot gas in this collision was slowed by a drag force, similar to air resistance. In contrast, the dark matter was not slowed by the impact, because it does not interact directly with itself or the gas except through gravity. "

Source: http://www.nasa.gov/home/hqnews/2006/aug/HQ_06297_CHANDRA_Dark_Matter.html

Re:Mark my words

[Beelzebud]

I don't think that's the case at all. The only reason we have "dark matter" is because of astronomical observations. That is classic science. Make an observation, and then come up with a theory to explain it. From observations we know that there is some type of mass out there affecting gravity. We call it 'dark matter' because we don't know what it is. This isn't an aether theory, it's based on real observations.

Re:Mark my words

[Black Parrot]

One day we'll find out why we're having to explain shit with "dark matter", and the stupid concept will be laughed at like the Luminiferous aether is now.

Yeah, like neutrinos and X-rays and all that other weird shit people made up to explain problems away.

Re:Mark my words

[Anonymous Coward]

One day we'll find out why we're having to explain shit with "dark matter", and the stupid concept will be laughed at like the Luminiferous aether is now.

I don't think that's the case at all. The only reason we have "dark matter" is because of astronomical observations. That is classic science. Make an observation, and then come up with a theory to explain it. From observations we know that there is some type of mass out there affecting gravity. We call it 'dark matter' because we don't know what it is. This isn't an aether theory, it's based on real observations.

I think we actually have enough evidence for dark matter but, to be fair, the luminiferous aether was also based on real observations. There used to be a great deal of controversy over whether light was a particle or a wave. If it was a particle, it didn't need a medium to travel through, but it became very hard to explain refraction and diffraction properties. If it was a wave, then it needs a medium. We know from experiments that the medium wasn't something we could normally interact with. You can pump air out of a glass container with something that is making noise inside it, and you will no longer be able to hear the noise, because the sound waves have no medium to travel through. You'll still be able to see the object, though. Therefore, it was easy to conclude, from experimental data, that if light was a wave, there had to be some medium for light to travel through, permeating through everything, but that we couldn't interact with (pump out).

We had a bunch of observations that really demanded the presence of the Aether, until Einstein came along with his photoelectric effect papers and came up with the real solution: you know what? Light is both a particle AND a wave. That was completely unlike what we normally experience, so it's not the interpretation people were coming up with to explain their observations. A whole lot of things happened in those years, between Einstein, Planck, and deBroglie that really enriched our understanding of the universe. None of them set out to disprove the aether, it's simply that when they finally had a theory which explained every observation, the aether was no longer necessary.

I think dark matter is probably real. We can measure mass via the dynamics of celestial objects, measure mass via gravitational lensing, and come up with similar results: there's more mass there. So it doesn't seem like it's a problem with our theories. That said, there's a lot of observations dark matter doesn't explain, so if it turned out that it doesn't exist after all, I wouldn't be entirely surprised, and it would be very much like the story of the aether. There's nothing wrong with that, and there was nothing wrong with introducing the aether back in those days. That really is what science is about. You make a theory that best fits your observation, and right now dark matter beats any other alternative. If someone comes up with a better alternative, scientists will drop dark matter as fast as they dropped the aether, but until then we need to go with what we have.

Re:Mark my words

[Black Parrot]

Let's get this out of the way first:

And we don't have any way to test for matter whose only property is it brings our mathematical formulae in line with our physical observations.

The, "Gee, that's funny" observation is what drives all science.

Now:

Making observations and theories is part of science. But what sets science apart from superstition is rigorous testing of the theories.

Believe it or not, some scientists do real science.

There was a competing explanation for this family of "Gee, that's funny" observations called MOND - Modification Of Newtonian Dynamics. It was ruled out on the basis of evidence. (There may be a MOND v. 2.0 out there now - not sure.)

One candidate for dark matter is the sterile neutrino, which people - real scientists - are trying to detect right now. A few years ago they were almost ready to dismiss its existence, but more recent results suggest that it may actually exist.

So no, contrary to your majestic disbelief, dark matter is a Real Hypothesis (tm), investigated by Real Scientists (tm), doing Real Science (tm).

If you want to actually learn something about the topic rather than simply using Slashdot as an outlet for you whingeing about the universe not working the way you learned in fifth grade, Wikipedia is an easy place to get started.

Re:SF plot

[vux984]

If there is such a thing as wavefunction collapse, then there is only one universe and hence no branching. Likewise, if the universe branches, then there is no such thing as wavefunction collapse (the different outcomes simply occur in different universes).

To be fair, if universe A branches into A' and A'' then to the inhabitants in each of A' and A'' it looks exactly like the wavefunction collapsed and settled on 'their' universe.