See A Quasar Gravitationally Lens a Galaxy (for the First Time!)

“If you only look at a person through one lens, or only believe what you’re told, you can often miss the truth that is staring you in the face.” –Kevin Spacey

One of the most powerful ideas from Einstein’s theory of Gravity — General Relativity — is that any massive object in the Universe not only causes a gravitational force on other masses, but also bends light!

Image credit: Dave Jarvis of

(Link to David Jarvis’ gallery.) This was confirmed in 1919 by observing the positions of stars during a total solar eclipse; the stars closest to the Sun had their apparent position shift due to the gravitational bending of the light rays!

How does this happen? The mass acts just like a lens does, bending the light rays! Only, instead of being a glass, plastic or acrylic lens, it’s a gravitational lens.

In 1936, Fritz Zwicky, the same guy who theorized the existence of dark matter, realized that distant galaxies could act as gravitational lenses also! After all, they have mass, and if there are other objects emitting light behind them, that light could get bent towards us!

Although it wasn’t discovered observationally until 1979, this phenomenon, known as strong gravitational lensing, has given us some of the most remarkable images in the Universe.

Those weird arcs in the above image — of cluster Abell 370 — are due to gravitational lensing. Gravitational lensing is great for distorting the light from background objects, and therefore for distorting the shapes of lensed galaxies.

Any other neat effects of gravitational lenses?

Multiple images! Do you see what looks like five bright blue stars (with crosshair-style rays) in the image above? Those are actually five multiple images of the same quasar! In many of the images, you can actually see the host galaxy that the quasar resides in!

In fact, just two years ago, we discovered an incredible alignment of three objects nearly all in a perfect row from our vantage point. What does that give us?

Two almost perfect, concentric rings!

It turns out that there’s only one thing that determines how much (and what type of) bending we get, given your galaxies in a certain place. All that matters is the mass of the thing acting as a gravitational lens! So if I put down a distant galaxy in the background and then a lens in the foreground, by observing what the background light does, I can easily figure out how much mass is in the lens. In fact, there are software packages out there that will even do it for you.

This is true whether it’s a galaxy, a cluster of galaxies, or an individual star doing the lensing. But that’s usually not such a big deal. After all, by observing a galaxy, a cluster of galaxies, or an individual star, we can usually learn a lot about its mass from other means.

But you know what would be a huge advance?

If we could find something of unknown mass acting as a gravitational lens!

Well, there’s a new paper out (with some great new images) by two joint teams from Caltech and Ecole Polytechnique Federale de Lausanne, respectively, led by George Djorgovski and Georges Meylan. And they found exactly that, for the first time.

Instead of a star, galaxy, or cluster of galaxies, the object acting as a lens is a quasar (shown below), where the galaxy its found in is completely obscured by the blazing core!

Under normal circumstances, we’d never be able to know the mass of the galaxy housing this quasar. But because of the gravitational lens, we can figure it out!

Want to know something that makes this even more amazing? This image was taken from the ground, with all of the Earth’s atmosphere to contend with! It is amazing how good adaptive optics have gotten!

And thanks to these observations, this is the first time we’ve ever seen a quasar act as a strong gravitational lens!

And what they found is that there’s a mass of 22 billion Suns within the innermost kiloparsec (about 3,000 light years) of this quasar and its host galaxy!

This is the very first time this technique has been used to measure the mass of a quasar’s host galaxy, and the very first time we’ve seen a quasar act as a gravitational lens! You can bet it won’t be the last. So, welcome to the birth of a new way to do astronomy, and enjoy the images and analysis from this first discovery!