Date: January 21, 2009
Title: Observing Quasars with Nature’s Telescope
Podcaster: Robert Knop
Organization: The Meta Institute of Computational Astronomy
Description: Gravity affects the motion of objects– apples fall, planets orbit. Even light, when passing a massive object, will have its path deflected. A massive galaxy in front of another more distant massive galaxy can bend the light from the more distant galaxy, acting like a lens and brightening the light of the more distant galaxy. Astronomers have used this to make observations about the environment immediately around supermassive black hole at the core of a very distant quasar.
Bio: Rob Knop obtained his PhD from Caltech in 1997, doing infrared observations of Seyfert Galaxies. He then worked with the Supernova Cosmology Project, and was a co-recepient of the Gruber Prize in Cosmology for the discovery that the expansion of the Universe is accelerating. From 2001 to 2007, he was an assistant professor at Vanderbilt University. Today, he works for Linden Lab, the creators of Second Life. He continues his astronomy avocation with MICA, the organization of professional astronomers and other interested people that is based entirely in virtual worlds, including Second Life. His occasional blog is at http://www.pobox.com/~rknop/blog
Today’s Sponsor: Astronomy Cast
Transcript:
This is Rob Knop. I am a member of MICA, the Meta-Institute of Computational Astronomy. You can find us on the web at mica-vw.org, and in Second Life in the StellaNova region.
People who aren’t astronomers, either amateur or professional, are often surprised to find out that magnification is not what’s most important about an astronomical telescope. Yes, when you use a pair of binoculars, a telephoto lens, or even a telescope to watch birds, or to look at something else distant on the Earth, it’s the magnification you’re interested in. However, with astronomical telescopes, magnification takes a back seat to light gathering power.
When your eyes are dark adapted, the pupils of your eyes are apertures five to seven millimeters wide. If you look up at the sky, imagine the light coming down at you from the stars; they are coming in waves, in some ways like waves you see on the ocean. Imagine a big ocean wave, and a small tube that is five millimeters wide facing into the wave. That tube will assuredly collect some of the water coming in the wave, but not very much of it. Now, instead, imagine putting a huge funnel in front of the tube– say a funnel half a meter wide. A whole lot more water will be making its way into the tube now. This is exactly what telescopes do. The aperture of the telescope collects much more light, which can then be funnelled down into your eye (if you’re looking directly), or on to an astronomical detector similar to a digital camera.
This is also true for the use I’m talking about today of “nature’s telescope”, gravitational lensing. But first, I want to talk a little bit more about gravitational lensing itself, for you may not be aware that gravity can act as a lens for light.
There are a lot of ways to think about gravity, but today I want you to think about the gravity of a large object as something that changes the path of smaller objects moving near it. An apple in the air over the surface of the Earth would normally just stay there– it’s path is to remain in one place, motionless. However, the gravity of the Earth modifies that path, pulling the apple down to the Earth. Similarly, as the Earth itself moves through space, it would naturally just continue on in the direction it’s going; however, the gravity of the Sun bends that path into a circle, holding the Earth in orbit around the Sun.
Gravity also modifies the path of light; light rays passing a massive body, like the Sun, a galaxy, or a galaxy cluster, are bent slightly as a result of that object’s gravity. Measurements of this in the year 1919 were the first confirmation of our modern theory of gravity, Einstein’s General Relativity. Today, astronomers regularly observe gravitational lenses, where the gravity of one distant galaxy bends and focuses the light of a much more distant galaxy. Often we can see the more distant galaxy, or the lensed galaxy, stretched and distorted as a result of the lensing effect. This is similar to the distortions you’ll see when looking at somebody through a glass of water.
Sometimes, the lensing effect will lead to multiple images of the same background object. An example of this is Einstein’s Cross, which is a central galaxy surrounded by four quasars. It turns out that the four quasars are actually four images of the same quasar, which is much more distant than the central galaxy. What’s more, these images of the quasar are brighter than they would have been if we were observing the quasar directly. This is a case of nature’s telescope working the way our own astronomical telescopes work; additional light rays are focused in our direction, allowing more light from this quasar to reach Earth than otherwise would have. Astronomers have used this effect to study this distant quasar, combining the light gathering power of large telescopes on Earth with nature’s telescope to make more detailed observations about an object. Without the gravitational lens effect, the quasar would have been too dim for these observations. Once again, it’s not the magnification of the gravitational lensing effect that the astronomers were harnessing, but the light gathering power.
A quasar is a galaxy that harbors a black hole at its center that may be hundreds of millions of times as massive as our Sun. Our model of quasars is that the black hole has around it an accretion disk, which swirls around the black hole at a tremendous rate, and is heated to temperatures of millions of degrees. This accretion disk is extremely small, however, on astronomical scales. Any observation of it will be mixed up with the large number of stars at the center of the galaxy that we can’t resolve from the accretion disk. Most of the conclusions we’ve made about accretion disks in quasars are based on secondary evidence– the other things that have been lit up by the quasar.
However, another gravitational lensing effect has allowed astronomers to directly probe the accretion disk itself of the quasar in Einstein’s cross. While the lensing galaxy magnifies the whole quasar, there can also be “microlensing” events due to individual stars in the foreground galaxy. Stars moving in that galaxy may pass exactly in front of the background quasar, temporarily magnifying that quasar. The size of the region magnified by the individual stars in the foreground galaxy is much smaller than the size of the region magnified by the whole galaxy. That is, while the galaxy as a whole increases the brightness of the whole quasar, the microlensing events temporarily increase the brightness of just the accretion disk. By subtracting out the light observed not during the microlensing event, astronomers can then get measurements of the accretion disk, relatively uncontaminated by other light. These observations been able to show that the light we see from the accretion disk does match what is expected for a thin, hot disk swirling around a supermassive black hole.
This is a powerful confirmation that our understanding of these extremely distant objects is on the right track.
The distant quasar in Einstein’s Cross is so far away that the light took nearly 10 billion years to reach us; we are looking at a quasar as it was when the Universe was less than a third of its current age! Only by combining the power of our own large telescopes with the power of nature’s telescope are astronomers able to make such detailed observations of such a small region of such a distant object.
365 Days of Astronomy
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