Caltech: Astronomers Discover an Unusual Cosmic Lens

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Caltech: Astronomers Discover an Unusual Cosmic Lens

Post by bystander » Fri Jul 16, 2010 11:09 am

Astronomers Discover an Unusual Cosmic Lens
California Institute of Technology | 16 July 2010
Astronomers at the California Institute of Technology (Caltech) and Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have discovered the first known case of a distant galaxy being magnified by a quasar acting as a gravitational lens. The discovery, based in part on observations done at the W. M. Keck Observatory on Hawaii's Mauna Kea, is being published July 16 in the journal Astronomy & Astrophysics.
Quasars, which are extraordinary luminous objects in the distant universe, are thought to be powered by supermassive black holes in the cores of galaxies. A single quasar could be a thousand times brighter than an entire galaxy of a hundred billion stars, which makes studies of their host galaxies exceedingly difficult. The significance of the discovery, the researchers say, is that it provides a novel way to understand these host galaxies.

"It is a bit like staring into bright car headlights and trying to discern the color of their rims," says Frédéric Courbin of EPFL, the lead author on the paper. Using gravitational lensing, he says, "we now can measure the masses of these quasar host galaxies and overcome this difficulty."

According to Einstein's general theory of relativity, if a large mass (such as a big galaxy or a cluster of galaxies) is placed along the line of sight to a distant galaxy, the part of the light that comes from the galaxy will split. Because of this, an observer on Earth will see two or more close images of the now-magnified background galaxy.

The first such gravitational lens was discovered in 1979, and produced an image of a distant quasar that was magnified and split by a foreground galaxy. Hundreds of cases of gravitationally lensed quasars are now known. But, until the current work, the reverse process-a background galaxy being lensed by the massive host galaxy of a foreground quasar-had never been detected.

Using gravitational lensing to measure the masses of distant galaxies independent of their brightness was suggested in 1936 by Caltech astrophysicist Fritz Zwicky, and the technique has been used effectively for this purpose in recent years. Until now, it had never been applied to measure the masses of quasar hosts themselves.

To find the cosmic lens, the astronomers searched a large database of quasar spectra obtained by the Sloan Digital Sky Survey (SDSS) to select candidates for "reverse" quasar-galaxy gravitational lensing. Follow-up observations of the best candidate-quasar SDSS J0013+1523, located about 1.6 billion light years away-using the W. M. Keck Observatory's 10-meter telescope, confirmed that the quasar was indeed magnifying a distant galaxy, located about 7.5 billion light years away.

"We were delighted to see that this idea actually works," says Georges Meylan, a professor of physics and leader of the EPFL team. "This discovery demonstrates the continued utility of gravitational lensing as an astrophysical tool."

"Quasars are valuable probes of galaxy formation and evolution," says Professor of Astronomy S. George Djorgovski, leader of the Caltech team. Furthermore, he adds, "discoveries of more such systems will help us understand better the relationship between quasars and the galaxies which contain them, and their coevolution."
First case of strong gravitational lensing by a QSO: SDSS J0013+1523 at z = 0.120

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Re: Caltech: Astronomers Discover an Unusual Cosmic Lens

Post by Ann » Fri Jul 16, 2010 2:37 pm

Ummm, fascinating.

And as usual, I always wonder what color these things are. I realize, of course, that the light from both the quasar and the galaxy has to be strongly redshifted, so that we con't see the "true color" of these objectgs. Nevertheless, I wonder what the redshifted colors are like. Is the redshifted color of the quasar really blue-green, as it appears to be here? And is the galaxy a deep red color?

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Re: Caltech: Astronomers Discover an Unusual Cosmic Lens

Post by geckzilla » Sat Jul 17, 2010 11:50 am

It's probably a monotone image with colors applied to make it easier to discern different objects from one another... Looking at the full resolution image it's as easy to tell that the blue is not a part of the original capture as it is to tell that the text is not actually there.
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Re: Caltech: Astronomers Discover an Unusual Cosmic Lens

Post by Ann » Sat Jul 17, 2010 3:42 pm

I think it is quite possible that the quasar would look blue, and certainly the galaxy would look red.

Remember that this is an SDSS picture, a Sloan Digital Sky Survey image. On the SDSS homepage, you can read this about the color of quasars at different redshifts:

http://www.sdss.org/gallery/gal_zqso.html

As you can see, quasars are fairly blue-looking up until a redshift of 3.2, certainly according to the SDSS palette. (SDSS uses green-red-infrared filters, and in the resulting pictures green is shown as blue, red as green and infrared as red.) It is not until we get to a redshift of 4.4 or greater that the SDSS quasars begin to look orange or red.

By contrast, SDSS galaxies begin to look orange at a much lower redshift. Already at redshift 0.5, all the SDSS galaxies in this sample look orange:

http://www.sdss.org/gallery/gal_zgalaxy.html

Given the fact that the blue-looking SDSS quasar is about 1.6 billion light-years away, I think it is quite reasonable that it would look blue in an SDSS image. And since the lensed galaxy is very much further away, it makes perfect sense that it would look very red, since it would probably only be picked up by the infrared channel of SDSS.

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Re: Caltech: Astronomers Discover an Unusual Cosmic Lens

Post by Chris Peterson » Sat Jul 17, 2010 6:34 pm

geckzilla wrote:It's probably a monotone image with colors applied to make it easier to discern different objects from one another... Looking at the full resolution image it's as easy to tell that the blue is not a part of the original capture as it is to tell that the text is not actually there.
Yes, the image was made with the Keck adaptive optics system, which is an IR instrument. This image was made in the K band, which means around 2.1 um. That's well below the visual spectrum. The color was probably added by the PR people just for the press release.
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Re: Caltech: Astronomers Discover an Unusual Cosmic Lens

Post by Chris Peterson » Sat Jul 17, 2010 6:48 pm

Ann wrote:I think it is quite possible that the quasar would look blue, and certainly the galaxy would look red.
The quasar is basically a continuum source with a shallow peak in the UV. At z=0.12, the physical redshift is 79nm. This isn't enough to bring the UV peak into the visible, so the shift will have almost no effect on the perceived color, which should be nearly white. The background galaxy is also a continuum source, approximated by a broadened blackbody at around 5000K, giving a generally yellow appearance without redshift. At z=0.64, the physical shift is 321nm, so the blackbody peak is shifted into the near infrared, leaving the bulk of the visible energy in red. So I'd agree with you that the galaxies would be visually red, although not highly saturated.

The image here is not an SDSS picture. It was made with the Keck AO system. The SDSS data was only used to look for promising targets. Once this quasar was imaged at high resolution with the AO system, its spectrum and that of the companion lensed galaxy were taken with the same telescope, and the redshifts determined.
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Re: Caltech: Astronomers Discover an Unusual Cosmic Lens

Post by Beyond » Wed Jul 21, 2010 4:29 pm

Split from http://asterisk.apod.com/vie ... 47#p127647

As to your link to cosmic lenses -- it shows the Quasar's lens magnifying what is behind and to the upper left of it. Does that mean that the lens is 360' ?

Do the lenses have a limited operating range? has there been a case of more than one object being magnified at the same time? And last but not least, what are the lenses made up of??
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Re: Caltech: Astronomers Discover an Unusual Cosmic Lens

Post by Chris Peterson » Wed Jul 21, 2010 4:41 pm

Split from http://asterisk.apod.com/vie ... 49#p127649
As to your link to cosmic lenses -- it shows the Quasar's lens magnifying what is behind and to the upper left of it. Does that mean that the lens is 360' ?
That isn't what the image shows. What is seen are two distorted images of an object that is actually behind the quasar. The gravitational lens has shifted the apparent position of the distant galaxy, and split it into two images.
Do the lenses have a limited operating range?
They are just lenses. If a ray of light passes through them, that ray will be deviated. The strength of the lens is greatest right at the center of the gravitational field that defines it (in this case, at the quasar), and falls off with distance (because the gravitational field falls off).
has there been a case of more than one object being magnified at the same time?
Not that I know of. But that's what you'd expect, since objects are usually not extremely close. Of course, what is usually seen lensed are galaxies, which are made of billions of stars. So in a sense, we are seeing many objects lensed. ("Magnified" isn't the right word; the lensed images are distorted, but not necessarily magnified.)
And last but not least, what are the lenses made up of??
That depends on how you think about gravity. You could say that the lenses are created by distorted spacetime, or you could simply say that they are caused by the way light moves in a gravitational field. They are not "made" of anything in a physical sense, any more than the magnetic lenses in a CRT or electron microscope are "made" of anything.
Last edited by Chris Peterson on Tue Aug 30, 2011 4:13 am, edited 3 times in total.
Reason: split post
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Re: Caltech: Astronomers Discover an Unusual Cosmic Lens

Post by Beyond » Wed Jul 21, 2010 5:21 pm

So then, what we are seeing being lensed by the Quasar is NOT the red stuff up and to the left of it, but is directly behind it and we can only see it through the lens?
Also, now i have to watch out for the "split" trap that i had just fallen into.
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Re: Caltech: Astronomers Discover an Unusual Cosmic Lens

Post by Chris Peterson » Wed Jul 21, 2010 5:56 pm

beyond wrote:So then, what we are seeing being lensed by the Quasar is NOT the red stuff up and to the left of it, but is directly behind it and we can only see it through the lens?
The red thing up and to the left appears to be a normal galaxy, seen directly. The two small red spots on either side of the quasar are a distant galaxy which is actually directly behind the quasar, but which has been split and shifted in position by the gravitational field of the quasar, so that we now see two images in different places. There is no connection between the galaxy to the upper left and the lensed galaxy (except that the paper suggests it is possible the galaxy may be contributing to the lens effect).
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