Bad Astronomy

Hubble spots 67 gravitational lenses

Gravitational lensing is all the rage these days! The fact that massive objects have gravity that bends lights was a prediction by Einstein, but it has come into its own over the past few years as a way of learning more about the Universe. We’ve used it to map dark matter, and to find planets orbiting other stars. The more lenses we find, the better we can map out the distant Universe.

And now Hubble has found new ones. 67 of them, in fact!

Gravity bends the path light takes in a similar manner in which a car has to follow a dip in the road. Gravity warps space, and a beam of light follows that warp. Galaxies have lot of mass, and therefore a lot of gravity. Imagine a distant galaxy, billions of light years away. Between us and it is another galaxy, much closer to us. The distant galaxy is sending out light in all directions. Light that happens to be aimed at us will eventually reach our telescopes.

But now imagine a beam of light that just misses us. It’s not quite aimed right at us. However, the galaxy in the middle bends the path of that beam, angling it just enough to hit us. Light from the distant galaxy that would otherwise miss us can hit us due to the lensing action of the galaxy in the middle! That makes the distant galaxy brighter to us: we’re getting more light from it.

But it also distorts the shape we see for that distant galaxy. It can be stretched out into arcs, for example, if the intervening galaxy is off to the side a bit. Like this:

Back when I worked on Hubble, one of our Early Release Observations (cool pictures taken right after a new camera is checked out) was of a cluster of galaxies – think of it as a condominium of galaxies – surrounded by four separate arcs. When I put the images together into a mosaic, I just stared at it. I had seen lenses before, in books and such, but never on data that I had personally been working on. That was very nifty.

But it gets better. If the lensing galaxy is smack dab between us and the farther object, the light can be focused into a perfect circle, called an Einstein Ring. The first image above is one of those. They’re pretty. They’re also terrifically cool.

We can learn a lot from such lensing. For one, we can see distant galaxies that might otherwise be invisible. Sure, they’re all twisted and weird, but we can still investigate them: spectra can tell us if stars are forming, for example. Also, the amount of distortion can yield the mass of the lensing galaxy (since the amount of lensing depends on the lensing galaxy’s gravity, which itself depends on the galaxy’s mass) and that can even be used to map out dark matter.

So this is pretty interesting news, and worth poking over the images. Certainly they are scientifically useful, but images of lensing events are also pretty weird. Looking at them is a reminder that the Universe is a pretty strange place.