Cosmic Lenses with Simon Dye
20th May 2013
Astronomer and cosmic optician Dr. Simon Dye joins us in the studio to talk about about cosmic lenses – the magnified images of distant galaxies that we wouldn’t normally be able to study. Simon studies these rare distant objects and attempts to work out what they look like. By doing this, Simon can study how galaxies formed and evolved in detail not available in unmagnified systems.
To begin our journey into cosmic lenses, we must remind ourselves about that ubiquitous force that is responsible for many a dropped phone – gravity. You can listen to our previous show all about gravity below, or skip to the next paragraph for the condensed version especially for those on the go.
The Science Show Talks Gravity
This week we’re talking about Gravity! Travelling from the basics with Newton and his apples onto more complicated stuff with Einstein and black holes, gravitational lensing and even how you can measure the acceleration due to gravity in Student Science! Check out the show’s blog.
Don’t forget to check out our famous Hammer & Feather experiment from our Student Science video series!
Gravity doesn’t just affect planets and stars, it can also change the path of light. Einstein first realised this back in the early 20th Century. However, it wasn’t confirmed until an expedition to the island of Principe off the west coast of Africa in 1919. Sir Arthur Eddington made the trip armed with his camera to observe a solar eclipse which would allow stars near the Sun on the sky to be observed.
What they found supported Einstein’s general theory of relativity – the stars seemed to shift positions on the sky as the Sun moved by. The Sun’s gravitational field bends the path of light travelling close to it, altering the observed positions of the stars on the sky. Obviously, the stars themselves didn’t move.
Now imagine this effect scaled up to objects like galaxies or clusters of galaxies! These massive systems can have a huge effect on the light emitted from objects behind them, as the diagram below demonstrates.
On these scales there are actually 2 types of gravitational lensing that we observe. The first is weak lensing which is the small distortions in the shapes of background galaxies. For example, a perfectly circular disk galaxy could appear as an ellipse due to this effect. Sometimes this is called shear as well. The other type, and the type that Simon is interested in, is strong lensing which produces the beautiful distorted arcs we see below. These are indicitive of massive lenses and are often found around the most massive clusters of galaxies.
On these massive scales, background sources can be magnified by a foreground lens galaxy or cluster up to around a factor of 30, and so are easily studied. Simon finds these lensed objects in images taken by the recently retired ESA Herschel Space Observatory. This telescope looked at the sub-mm part of the spectrum.
The horseshoe lens is the largest diameter Einstein ring system currently known that’s produced by an isolated lens. Simon has written a paper on this system using low resolution ground based data and is now working on modelling this with high resolution data. You can see just by looking at the ring that the source is irregularly shaped – not like most of the galaxies we see.
The underlying math behind gravitational lensing is pretty simple! For a source, like a galaxy, perfectly aligned behind a lens, like a cluster of galaxies, the radius of the lensed image is given by
where M is the mass of the lens and DLS, DL and DS being the distances between the lens and source, distance to the lens and distance to the source respectively. This is an Einstein Ring and the mass of the lens is dominated by dark matter.
More on Dark Matter
You can find out more about dark matter in our two previous shows on the elusive substance. The first, with astronomer Dr. Meghan Gray, discussed dark matter and its role in gravitational lensing in more detail.
The second, with particle theorist Dr. Anne Green, discussed the origins of dark matter and our (so far) fruitless attempts to detect it.
Once Simon finds these lensed objects, he acquires higher resolution images from other telescopes, such as the Hubble Space Telescope. With this image, the distorted lensed object can be reconstructed. Using software he created, the source is reconstructed by finding a model that most accurately represents the lens we observed. Below is an example of this process using a real lens Simon worked with.
In the above image, a lensed object was found and the lens (middle) removed so only the lensed image remains. From this, a source is constructed and then artificially lensed. This is then compared to the observation. Simon found that this source contained two distinct objects! A cloud of cold gas (top of third from left) falling into an irregularly shaped galaxy (bottom). This was confirmed by observations of Carbon Monoxide (CO) in its spectrum.
What makes reconstructing the source galaxy hard is that areas of the galaxy can be lensed multiple times, creating extremely distorted images, or arcs, on the sky. Simply put, Simon’s software chooses models of the source and lens that produces the best match to the observations!
You can get involved and find your own gravitational lenses! Head on over to the SpaceWarps website and help scientists all around the world find and analyse gravitational lenses like Simon uses. You could even be mentioned in a scientific paper. Let us know how you get on…
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