APOD: A Horseshoe Einstein Ring from Hubble (2011 Dec 21)

[APOD Robot]

A Horseshoe Einstein Ring from Hubble

Explanation: What's large and blue and can wrap itself around an entire galaxy? A gravitational lens mirage. Pictured above, the gravity of a luminous red galaxy (LRG) has gravitationally distorted the light from a much more distant blue galaxy. More typically, such light bending results in two discernible images of the distant galaxy, but here the lens alignment is so precise that the background galaxy is distorted into a horseshoe -- a nearly complete ring. Since such a lensing effect was generally predicted in some detail by Albert Einstein over 70 years ago, rings like this are now known as Einstein Rings. Although LRG 3-757 was discovered in 2007 in data from the Sloan Digital Sky Survey (SDSS), the image shown above is a follow-up observation taken with the Hubble Space Telescope's Wide Field Camera 3. Strong gravitational lenses like LRG 3-757 are more than oddities -- their multiple properties allow astronomers to determine the mass and dark matter content of the foreground galaxy lenses.

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[Ron]

Fascinating pic.

Remind us again how it is that a large amount of gravity can bend light out around a galaxy ?
Why isn't the light attracted / deflected IN TO the galaxy, and blocked/absorbed from our view ?
Does gravity repel light ?

And if a large amount of gravity can bend light a lot, does that mean that a smaller amount of gravity will bend light a little ? Does that mean that nothing in the Universe is really where we see it ?

Could it be that our Universe is really a cube, but deflected light is making it appear less cube-like ? One for the cubists out there....

[Chris Peterson]

Remind us again how it is that a large amount of gravity can bend light out around a galaxy ?

Although it is usually expressed that way, gravity isn't actually bending the light at all. What it's doing is changing the geometry of space, so that what is a straight line to a photon appears curved from our viewpoint. Light always travels in a geodesic- a straight line or the shortest possible path- in its own frame.

Why isn't the light attracted / deflected IN TO the galaxy, and blocked/absorbed from our view ?

From our perspective, it appears to be bent towards the galaxy. Since the source galaxy is behind the deflecting galaxy, some photons do pass through (or are absorbed within) the deflector. The deflector doesn't have enough mass to actually capture photons... only a black hole could do that, and even it would only capture photons on a path that intersected it.

Does gravity repel light ?

No. Gravity doesn't affect photons at all- just the spacetime they travel through.

And if a large amount of gravity can bend light a lot, does that mean that a smaller amount of gravity will bend light a little ?

Yes.

Does that mean that nothing in the Universe is really where we see it ?

Yes.

Could it be that our Universe is really a cube, but deflected light is making it appear less cube-like ?

There is no reason to think so.

[Ann]

Ah, Chris, I love those super-cool answers. There not an ounce of unnecessary verbal flab on those answers of yours!

Ann

[bystander]

http://asterisk.apod.com/viewtopic.php?f=29&t=26230

[nstahl]

If I might take a stab at answering Ron's question: Ron the horseshoe is light that, had the galaxy not been between us, would have missed us by a lot. But the galaxy's presence caused the light's path to bend toward us. It went around the galaxy in between but in doing so its path was bent in enough that it hit us right in the eye. Well, telescope.

[Oldfart]

Two questions: (1) Do we, or do our computers, have the capability to produce an accurate image of the galaxy that was lensed? And (2) Does this sort of lensing let us look further away with our telescopes than would otherwise be the case with no lensing?

[biddie67]

Fascinating alignment! I have such a difficult problem understanding the bending of the spacetime network. Is gravity the only force that affects it?

The galaxy behind the front one must be either quite close to it or awfully huge ??

[500pesos]

That galaxy in front must be really enormous, and it doesn't seem to have any dust and looks quite reddish. It looks as if there is no new star formation going on and all the stars are super old.
What do you say is to happen to such a big galaxy populated by Betelgeuse-like stars? How is it going to be when all these millions of old stars swell up and start going supernovas (supernovae?)? will they trigger some new star formation from the debris and shock waves and breathe some new life into the old galaxy or it all's just going to go mega-boom in the sky?

[Buzz]

Remember that even though the path of the light photons are being altered, in the grand sceme of things, the deviation in their path is very minute. Consider the distance between those two galaxies, and the distance to us from them. I would wager that the angle of change is very small, so small that it would probably be expressed as much, much less than one percent.

[Byork]

gravitational lensing is only one aspect of such alignment between an LRG and eclipsed distant galaxy. the distant eclipsed galaxy appears blue because the frequency of light waves from eclipsed galaxy have been increased as a result of gravitational attraction. eclipsed galaxy is probably a normal galactic object with white and yellow clouds of stars and dust. gravitational lensing has imparted energy to light waves traveling from the distant object thereby causing the object to appear blue. einstein's general law of relativity does not cover this aspect of celestial mechanics.

menu for the day: aunt jemima's pancake with pure maple syrup
apple cider
yogurt

[Ann]

That galaxy in front must be really enormous, and it doesn't seem to have any dust and looks quite reddish. It looks as if there is no new star formation going on and all the stars are super old.
What do you say is to happen to such a big galaxy populated by Betelgeuse-like stars? How is it going to be when all these millions of old stars swell up and start going supernovas (supernovae?)? will they trigger some new star formation from the debris and shock waves and breathe some new life into the old galaxy or it all's just going to go mega-boom in the sky?

There are probably no Betelgeuse-like stars in this galaxy at all, assuming there has been no recent star formation there. Stars like Betelgeuse, massive stars that have turned into enormous supergiants, are all young, and they are not found in elliptical galaxies like the one in today's APOD. Therefore, the supernovae that are likely to explode in this galaxy will not come from supergiant stars like Betelgeuse, but from compact white dwarfs smaller than the Earth.

So if there are no Betelgeuse-like stars in this galaxy, then why is it red? Well, frankly, it isn't red, but yellow. There are undoubtedly trillions of small "red" stars in that galaxy, stars much smaller and cooler than the Sun, and yet they are no redder than the light from an ordinary light bulb. So these "red" stars are really yellow. (What about the color of our own Sun, then? Isn't it yellow, too? No, our Sun is much whiter than the light from an ordinary light bulb, so it should be described as white.)

(Admittedly, the red galaxy in today's APOD may contain trillions of "brown" (but really red) dwarfs, but the "brown" dwarfs are so faint that they basically don't contribute to the light of this galaxy at all.)

But while the "red" galaxy contains no "Betelgeuses" it does contain a respectable number of relatively modest "red" (but really yellow) giants like Arcturus and Aldebaran. All these yellow stars give the galaxy a yellow color.

However, the galaxy is sufficiently far away that the expansion of the universe "stretches" the light that is emitted from this galaxy, so that it actually looks orange or red. But the intrinsic color of this galaxy is yellow.

The "red" galaxy is indeed huge and massive, because otherwise its mass wouldn't have made such an impressive "indentation" in the "fabric" of spacetime. And if that hadn't happened, the light from background galaxy wouldn't have been forced to change its path due to the altered "topography" of spacetime. And if that hadn't happened, the galaxy wouldn't have looked like a ring to us, and it would have looked very much fainter to us than it does now.

Ann

[Ann]

gravitational lensing is only one aspect of such alignment between an LRG and eclipsed distant galaxy. the distant eclipsed galaxy appears blue because the frequency of light waves from eclipsed galaxy have been increased as a result of gravitational attraction. eclipsed galaxy is probably a normal galactic object with white and yellow clouds of stars and dust. gravitational lensing has imparted energy to light waves traveling from the distant object thereby causing the object to appear blue. einstein's general law of relativity does not cover this aspect of celestial mechanics.

menu for the day: aunt jemima's pancake with pure maple syrup
apple cider
yogurt

The background galaxy is probably intrinsically relatively blue. It is obvious that the blue "horseshoe" contains "clumps", which are a typical sign of of an uneven light distribution due to the presence of clusters of brilliant and mostly blue young stars. Therefore, the blue color of the galaxy is not a product of gravitational "compression" by the foreground galaxy, but an intrinsic quality of the background galaxy itself.

However, the background galaxy is not as intrinsically blue as it appears to be in this image. The filters that have been used to make this picture are probably orange and infrared. The infrared filter detects the light from the elliptical galaxy, and the orange filter detects the light from the blue galaxy. The dominant color of this galaxy is therefore orange. However, the background galaxy is highly redshifted, and what we see here is probably ultraviolet light from that galaxy that has been redshifted all the way into the orange part of the spectrum. So the background galaxy is really intrinsically bluish, but not as intensely blue as it appears to be here.

Ann

[All4vols]

guys, help me out here. My understanding of the lensing effect requires great distance between us and the lensed object. If that understanding is correct, why isn't the horseshoe significantly red-shifted?

[orin stepanek]

I liked today's APOD! I can see there are other lensed galaxies in the Photo!

[Boomer12k]

Now THAT, is curved space-time.

:---[===] *

[Czerno1]

Ann, I ever enjoy to read your entusiastic, informative comments!

Pardon me if I pin point a detail :

Well, frankly, it isn't red, but yellow. There are undoubtedly trillions of small "red" stars in that galaxy, stars much smaller and cooler than the Sun (....)

Trillions (underlined by me in quote) ? You're sure your enthusiasm isn't carrying you a bit far ? AFAIR the count of stars in our own Galaxy is an estimated tenth of that trillion of yours, maybe two tenths (star population counts enjoy inflation, just like money) - out of these, I have no idea how many are "small red" stars, I bet rather a small proportion - Chris will know (wild guess : 1%? would be in the billions, not trillions!).

Unless the distant, miraged, galaxy is huge, and even then, it is difficult to swallow your trillions absent a detailed, convincing argument.

Sorry for the nit, and - to you and all APOD fans alike - happy and merry Xmas. Joyeux Noël !

[Boomer12k]

At first glance, I thought I was looking at a Bubble Nebula....

:----[====] *

[rstevenson]

... Trillions (underlined by me in quote) ? You're sure your enthusiasm isn't carrying you a bit far ? AFAIR the count of stars in our own Galaxy is an estimated tenth of that trillion of yours, maybe two tenths (star population counts enjoy inflation, just like money) - out of these, I have no idea how many are "small red" stars, I bet rather a small proportion - Chris will know (wild guess : 1%? would be in the billions, not trillions!).

Unless the distant, miraged, galaxy is huge, and even then, it is difficult to swallow your trillions absent a detailed, convincing argument.

If I may ...

From the original description (not the APOD description) we read...

The galaxy — one of a group of galaxies called Luminous Red Galaxies — has an unusually large mass, containing about ten times the mass of the Milky Way.

Our Milky Way galaxy has about 200 to 400 billion stars, and assuming (perhaps not wisely) the same average mass per star, then that yellow galaxy in the middle of the ring could have on the order of 2 to 4 trillion stars in it. There's no strong estimate of how many red or brown dwarf stars there are in the "average" galaxy, let alone in such an old giant, so Ann was guessing, but yes, I'd agree with you that "trillions of small 'red' stars" seems a bit over the top. But multiple billions would surely be in the ballpark.

Rob

[Czerno-1]

Oldfart asked earlier and I am bumping his question, as I too would be very interested in an informed answer by one of our local, friendly experts :

(1) Do we, or do our computers, have the capability to produce an accurate image of the galaxy that was lensed?

I suspect the requirement of nan accurate[ image is a bit too much, but from image of mirages such as today's APOD can we reconstruct something of the original, making a few hypotheses & applying appropriate mathematical transforms, (de)convolution and what-have-you... ?

[Chris Peterson]

Two questions: (1) Do we, or do our computers, have the capability to produce an accurate image of the galaxy that was lensed?

Somewhat. The problem is, there is more than one possible distribution of mass that can produce the same pattern. If you know nothing about either the shape of the background galaxy, or the mass distribution in the foreground galaxy, there are an infinite number of solutions to modeling either. But in practice, the background galaxies in these cases are so far away they can be treated as point sources- like stars. Once you know the background is a point, you can find plausible models for the mass distribution of the foreground galaxy. That's what the people who study these things are doing.

And (2) Does this sort of lensing let us look further away with our telescopes than would otherwise be the case with no lensing?

Definitely. But not typically to figure out how a distant object is shaped (for the reason given above), but because the gravitational lens directs more photons our way than we would otherwise get, allowing us to see objects farther away. I think the current redshift record for a galaxies is a lensed galaxy.

[Chris Peterson]

Fascinating alignment! I have such a difficult problem understanding the bending of the spacetime network. Is gravity the only force that affects it?

Gravity is the end result of how mass and spacetime affect each other. In the terms of general relativity, gravity isn't really a force at all. The mass of the foreground galaxy changes the geometry of the surrounding spacetime, and the photons from the background galaxy which pass through that region therefore change direction with respect to the the surrounds.

The galaxy behind the front one must be either quite close to it or awfully huge ??

The background galaxy may be any size. Probably, it is typically sized (to the extent that such a thing exists). It is very distant, otherwise there would be no detectable lensing. If the foreground galaxy disappeared, we'd see the background one as nothing more than a point, like a star.

[Chris Peterson]

gravitational lensing is only one aspect of such alignment between an LRG and eclipsed distant galaxy. the distant eclipsed galaxy appears blue because the frequency of light waves from eclipsed galaxy have been increased as a result of gravitational attraction. eclipsed galaxy is probably a normal galactic object with white and yellow clouds of stars and dust. gravitational lensing has imparted energy to light waves traveling from the distant object thereby causing the object to appear blue. einstein's general law of relativity does not cover this aspect of celestial mechanics.

We don't see gravitational red or blue shifting in lensed galaxies. Whatever energy the distant photons gain as they approach the foreground galaxy, they lose again on the trip away from it. The photons reach us with very close to the same energy they left with, except of course what they have lost due to the expansion of space during their travel- ordinary cosmological redshift.

Where we can observe gravitational redshift, it is with photons emitted deep in a gravity well, as in a star or near a black hole, not photons passing by a gravity well.

[TNT]

Are we sure that this isn't a planetary nebula?

Just a quick question here: does the lensing effect affect how light travels through space-time? If so, how?

[Chris Peterson]

guys, help me out here. My understanding of the lensing effect requires great distance between us and the lensed object. If that understanding is correct, why isn't the horseshoe significantly red-shifted?

The background galaxy is highly redshifted (z = 2.379; object is 18.8 billion ly away, light was emitted 10.9 billion years ago). Just because something is redshifted, that doesn't mean it will necessarily appear red. Keep in mind that while blue light is shifted towards red, red light disappears into the IR, and ultraviolet is shifted into the blue. So the overall color can shift in unexpected ways. In addition, this is not a "true color" image. It was collected through three wideband photometric filters (B, centered on blue, V centered on green, and I, centered in the near-IR). These filters are mapped to the blue, green, and red of your monitor, but certainly don't produce colors as you'd see them with your eye.