APOD: Galaxy Cluster Abell 1689 Deflects Light (2013 Sep 17)

[Chris Peterson]

I thought dark matter was distinguished from gravitational matter. IN FACT, I thought that was what defined it-that it does not respond to gravity. If dark matter does respond and produce gravity, then what is the difference between dark matter and the regular kind we know and love? Is it just that we can not see the dark matter and SO do not know what it is. Hence my real question, may be what is it that makes matter "matter".? Obviously matter is defferent from energy but is there a scientific definition that covers regular and dark matter? Really would appreciate an answer from someone who knows the science.

Dark matter has mass, and responds to gravitational forces just like ordinary matter. The reason it is dark is because it doesn't interact with the electromagnetic force. We infer the presence of dark matter by observing its gravitational effects on nearby visible matter, or on light (as with gravitational lenses).

[geckzilla]

wait, wait, wait... I haven’t had a drink of the cool aid...

Isn’t "dark matter" just something we made up to explain something we don’t understand?

You don't usually make fringe statements like this, so I'm going to give you the benefit of the doubt, here... while it's true that it's not yet well understood, it's still the best explanation for the discrepancy between the mass of observable and the mass required for gravitational lensing such as Abell 1689.
https://en.wikipedia.org/wiki/Dark_matter

Try a taste of this. http://www.popsci.com/science/article/2 ... xplanation. You might find it more palatable than the Kool-Aid.

It's probably better not to make references to a clearly delusional cult when discussing the dark matter hypothesis. Spare us the ad hominem next time, you two. MoND isn't exactly free of problems or above critcism, itself.

[ignacio_db]

Are black holes of dark matter possible in theory? If yes, Is there any observational evidence of their existence?

All of these is so intriguing...

best
Ignacio

[Chris Peterson]

Are black holes of dark matter possible in theory? If yes, Is there any observational evidence of their existence?

It isn't obvious what sort of matter we have once it is "inside" a black hole. Similarly, we don't really think of a black hole as being made out of any particular sort of matter. But black holes almost certainly can take in dark matter just like they do ordinary matter. The mechanism that allows matter to be captured by a black hole involves electromagnetic processes (an accretion disc behaves like a viscous fluid). Without that interaction, there isn't anything that allows dark matter orbits to decay significantly, so probably most of the matter that falls into black holes is ordinary. As far as mechanisms very early in the Universe that might have resulted in dark matter collapsing into black holes, I don't think there is much theory describing that.

[neufer]

Are black holes of dark matter possible in theory?
If yes, Is there any observational evidence of their existence?

In theory yes...however:

In actuality dark matter can not possibly condense into small volumes
without first getting rid of an enormous amount of angular momentum.

Regular matter uses electromagnetic forces to get rid of angular momentum
in order to form stars and then black holes; dark matter cannot do so.

There could, I suppose, be dark matter black holes left over from the big bang
but they would be indistinguishable from any other
neutrally charged black holes left over from the big bang

[ignacio_db]

Are black holes of dark matter possible in theory? If yes, Is there any observational evidence of their existence?

It isn't obvious what sort of matter we have once it is "inside" a black hole. Similarly, we don't really think of a black hole as being made out of any particular sort of matter. But black holes almost certainly can take in dark matter just like they do ordinary matter. The mechanism that allows matter to be captured by a black hole involves electromagnetic processes (an accretion disc behaves like a viscous fluid). Without that interaction, there isn't anything that allows dark matter orbits to decay significantly, so probably most of the matter that falls into black holes is ordinary. As far as mechanisms very early in the Universe that might have resulted in dark matter collapsing into black holes, I don't think there is much theory describing that.

Thanks for the response, very interesting. Let me see if I got this. What you are saying is that without the loss of energy that comes with electromagnetic emission, dark matter would not fall completely into a black hole, but stay orbiting it. If so, then there should be a noteworthy accumulation of dark matter around black holes, wouldn't it?

Ignacio

[Chris Peterson]

Thanks for the response, very interesting. Let me see if I got this. What you are saying is that without the loss of energy that comes with electromagnetic emission, dark matter would not fall completely into a black hole, but stay orbiting it.

Not necessarily electromagnetic emission, but electromagnetic interaction, which is integral to the mechanism that makes a fluid interact with itself. But generally, yes. Without a mechanism to remove energy from orbiting particles, their orbits won't decay.

If so, then there should be a noteworthy accumulation of dark matter around black holes, wouldn't it?

Probably not, because there isn't a high density of dark matter to begin with, and statistically, most dark matter particles that encounter a black hole will do so with a relative velocity greater than escape velocity. In terms of orbital dynamics, they will be in hyperbolic orbits with respect to the black hole. The only way they can be captured is by interacting first with another body. So I imagine a black hole sweeping through a sea of dark matter will just create a sort of ripple. The only material that will be captured is that which directly collides with the event horizon, and that is a very small cross section.

[ignacio_db]

Understood. Thanks, Chris and Art.

Ignacio

[Ann]

Chris wrote:
(In this case, the blue display channel consists of data collected through a wideband green filter, 475W, meaning the light is probably on the whitish side visually).

Please explain this to me. I can understand that the "W" means "wide". But what about the "475"? Shouldn't that mean that the wide filter is centered on 475 nm?

In my opinion 475 nm is a blue color, not a green one. According to Joe Hanson, Ph.D. biologist and host/writer of PBS Digital Studios' It's Okay To Be Smart, 475 nm corresponds to the predominant wavelength of the blue sky.

Ann

[Chris Peterson]

Please explain this to me. I can understand that the "W" means "wide". But what about the "475"? Shouldn't that mean that the wide filter is centered on 475 nm?

In my opinion 475 nm is a blue color, not a green one. According to Joe Hanson, Ph.D. biologist and host/writer of PBS Digital Studios' It's Okay To Be Smart, 475 nm corresponds to the predominant wavelength of the blue sky.

I agree, this is a blue filter. In fact, it is the standard Johnson photometric blue filter, which covers deep violet to the edge of green. I can confirm from personal experience that when you look through a Johnson blue filter at white light, you see blue (not sky blue, but the color of a wideband filter is typically very different than the color of its center wavelength).

I just called it green because that's how the NASA site that explains the image describes it. Don't know why that is.

[geckzilla]

Could it be... a mistake?

[Ann]

Well, if we agree that the W475 filter has indeed detected all kinds of short-wave optical light, from deep violet up until the edge of green, and the Hubble image has shown all these short optical wavelengths to the same shade of blue, then I guess my original explanation still holds true. In other words, the lensed and extremely distant galaxies typically contain a lot of star formation and produce a lot of ultraviolet light. The expansion of the universe has lengthened this ultraviolet light to a lot of different, but basically all short, optical wavelengths. These wavelengths are all detected by the W475 filter and are shown as blue in the Hubble picture.

Ann

[Chris Peterson]

Well, if we agree that the W475 filter has indeed detected all kinds of short-wave optical light, from deep violet up until the edge of green, and the Hubble image has shown all these short optical wavelengths to the same shade of blue, then I guess my original explanation still holds true.

Yes, but the actual filter it was collected in doesn't really matter. With redshifts on the order of 7, the only thing we're seeing in all the filter channels is photons originally emitted as UV- extreme UV, in fact. The W475 filter is collecting light emitted at around 50-70 nm, but even the longest wavelength light caught by the F850LP filter was emitted at just 125 nm - still extreme UV. So we're seeing these distant galaxies in an optical regime dominated by very high energy photons, produced not only by thermal processes (hot, young stars)^* but by synchrotron sources and other fairly exotic mechanisms.

^* for those with nanny filters in place, that's high temperature, newly formed stellar bodies.

[Ann]

Well, if we agree that the W475 filter has indeed detected all kinds of short-wave optical light, from deep violet up until the edge of green, and the Hubble image has shown all these short optical wavelengths to the same shade of blue, then I guess my original explanation still holds true.

Yes, but the actual filter it was collected in doesn't really matter. With redshifts on the order of 7, the only thing we're seeing in all the filter channels is photons originally emitted as UV- extreme UV, in fact. The W475 filter is collecting light emitted at around 50-70 nm, but even the longest wavelength light caught by the F850LP filter was emitted at just 125 nm - still extreme UV. So we're seeing these distant galaxies in an optical regime dominated by very high energy photons, produced not only by thermal processes (hot, young stars)^* but by synchrotron sources and other fairly exotic mechanisms.

^* for those with nanny filters in place, that's high temperature, newly formed stellar bodies.

That's really interesting, Chris, but it doesn't really explain why the background galaxies look so blue in the Hubble image. On the face of it, that would mean that these distant galaxies emitted a lot of extreme ultraviolet light at around 50-70 nm (since the W475 filter detected them), but virtually no still-far ultraviolet light at 125 nm, since the F850LP filter did not detect the galaxies.

That is a mystery, indeed.

Ann

[geckzilla]

If they're all like that, maybe something happens to light when it passes through the lens. But are they all like that, or do the redder ones blend in with the foreground easily?

[ErnieM]

Neufer wrote:

K1NS wrote:

If the main mass of galaxies is rotating and moving, I would think that the blue arcs produced by lensing might actually change, in a time scale we might be able to observe. Are there multiple versions of this image, taken at different time, that look slightly different?

Light echoes, relativistic particles and microlensing can be observed to move within our galaxy
but observing any motion outside our galaxy is out of the question.

From Gravitational lensing weighs 70 galaxies and infers
dark matter
BY DR EMILY BALDWIN
ASTRONOMY NOW :
An animation of the gravitational lensing effect can be found at: http://www.ifa.hawaii.edu/info/press-re ... ation.html
Here is the full article http://www.astronomynow.com/080723Gravi ... atter.html.

[ErnieM]

ErnieM wrote:

Is the 1:99 ratio of visible to dark matter true in all known clusters with evidence of background galaxy lensing? If so, then the mass from the visible galaxies is insignificant. I am very much interested in looking at a modified version of these pictures with only the arced background galaxies and the foreground galaxies filtered out.

Oooooooops. NASA Scientists are doing this already!

The large panel shows the Hubble Space Telescope image of one of the SLACS gravitational lenses, with the lensed background galaxy enhanced in blue. The smaller right-hand panels show the components of a model of this image: top, a model for how the more distant background galaxy would appear in the absence of the lensing effect; centre, a smooth model for the brightness of the more nearby massive galaxy; bottom, the appearance of the background galaxy when its image is distorted by the gravity of the nearer galaxy. Image: A. Bolton (UH/IfA) for SLACS and NASA/ESA.

---

So far, gravitational lensing is the only way we "see/infer" location of large mass of dark matter.
Using what we could learn from these modified versions, survey the universe and search for faint spots of gravitational lensing with little or no visible galaxies in the foreground.
There are more dark matter than visible galaxy clusters, so I expect we will find plenty of "lensing" large clusters of mostly or only dark matter.

The images I suspect to see may look like the image at the bottom, arced bluish pictures of distant galaxy lensed by clusters of predominantly dark matter "galaxies".

From Gravitational lensing weighs 70 galaxies and infers
dark matter
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: July 23, 2008
to see the full article http://www.astronomynow.com/080723Gravi ... atter.html

On lighter note this reminds of the story.

A police officer sees a drunken man intently searching the ground near a lamppost and asks him the goal of his quest. The inebriate replies that he is looking for his car keys, and the officer helps for a few minutes without success then he asks whether the man is certain that he dropped the keys near the lamppost.

“No,” is the reply, “I lost the keys somewhere across the street.” “Why look here?” asks the surprised and irritated officer. “The light is much better here,” the intoxicated man responds with aplomb.

[ErnieM]

More on Dark Matter Gravitational Lensing Images
https://www.google.ca/search?q=dark+mat ... B398%3B398