Space Warps - HSC

Gravitational Lensing

Einstein's theory of gravity, General Relativity, made a remarkable prediction. Massive objects, such as stars, would bend the space around them such that passing light rays follow curved paths. Evidence for this revolutionary theory was first obtained by Arthur Eddington in 1919, when during a solar eclipse he observed that stars near the edge of the Sun appeared to be slightly out of position. The Sun was behaving like the lens in a magnifying glass and bending the light from the background stars!

In 1937, Fritz Zwicky realized that massive galaxies (which can contain anywhere from ten million to a hundred trillion stars) or clusters of galaxies could be used to magnify distant galaxies that conventional telescopes couldn't detect. As you can see, not unlike a conventional magnifying glass, these gravitational lenses not only magnify and focus the light of the distant background galaxies but they can, and mostly do, distort them as well.

When one of these gravitational lenses happens to sit right in front of a background galaxy, the magnification factor can be up to x10 or even more, giving us a zoomed-in view of the distant universe, just at that particular point. Lenses can help us investigate young galaxies more than halfway across the universe, as they formed stars and started to take on the familiar shapes we see nearby.

Observations of the distorted background galaxy can also give us useful information about the object that is behaving as a gravitational lens. The separation and distortion of the lensed images can tell astronomers how much mass there is in the object, and how it is arranged. It is one of the few ways we have of mapping out where the dark matter in the universe is, how clumpy it is and how dense it is near the centers of galaxies. Knowing this can provide crucial information about how galaxies evolve.

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See the following lensing animations showcasing the lensed image configurations that can usually form.

Left: A distant background galaxy moves across a massive elliptical galaxy in the foreground
Center: A distant background quasar moves across a massive elliptical galaxy in the foreground
Right: A distant background galaxy moves across across a galaxy group in the foreground. The configuration of the lensed images can be very different based on how the multiple galaxies are arranged in the galaxy group.

The speeds of the background galaxies/quasars are too slow for us to see the changing image configurations in our lifetime. In reality, we only see a single static configuration.

Needles in a Haystack

There is a lot of interesting science to be done with gravitational lenses, from precisely weighing galaxies to measuring the expansion rate of the Universe. The problem is that they are very rare. Only about one in a thousand massive galaxies is aligned with a background object well enough to cause it to appear multiply-imaged. We currently know of about 700 objects that are behaving as gravitational lenses, largely because we have become very good at observing the night sky! Modern optical surveys cover thousands of square degrees, with images sharp and deep enough to resolve about 1 lens per square degree. There should be thousands of lenses that we can detect, but we will need to look at millions of galaxy images to find them!

The ideal solution would be to get a computer to look through all of the images, but unfortunately this is not a straightforward solution. Teaching a computer to recognize the effects of gravitational lensing is not too difficult, but they can be easily confused by galaxies that look very similar to a distorted background galaxy. Also, in order for the computer to run fast enough to analyse lots of images quickly, they have to cut a lot of corners, and this makes them less effective.

Human beings have a remarkable ability to recognise patterns and detect the unusual with only minimal training. With a basic understanding of what the distorted images of galaxies that have passed through a gravitational lens look like, participants in the SpaceWarps project can help discover new examples of this amazing phenomenon, and enable our survey scientists to carry out new investigations of stars and dark matter in the Universe.

A mostly untapped source of lenses: The Hyper Suprime-Cam Survey

The Hyper Suprime-Cam (HSC), a camera mounted on the Subaru Telescope on Mauna Kea, is currently surveying a large area of the sky with excellent image quality and great depth, ideal conditions for detecting gravitational lenses. Only a fraction of the available data from HSC has been searched for lenses, using far from perfect automatic algorithms. We estimate that the images of hundreds of lenses are lying in the data, still waiting to be found!

We will be showing images centered around galaxies that are massive enough to potentially act as gravitational lenses. The task will then be to assess whether or not they actually are!

This is a tricky mission: nature is very creative, and there are galaxies of all shapes and colors out there, many of which can mimic the features of a genuine gravitational lens. You can learn more about them under Education.
The challenge is to come up with the most plausible explanation for what is going on, in collaboration with the rest of the SpaceWarps community. Do you think you can spot outer space being warped?