Zooite Guide to Gravitational Lenses
This guide has many examples from the SDSS database, used by Galaxy Zoo 2.
The images are in three colours: blue, green and red.
The new Hubble Zoo is using data from the Hubble Space Telescope, and those images have better resolution.
The images are in two colours: light blue and orange.
Part 1 Overview of gravitational lenses
Part 2 What to look for using confirmed strong lenses in SDSS
Part 3 Double quasars in SDSS,... and triple, and quadruple.
Part 4 These are not lenses
Part 5 Strong Gravitational lenses in Hubble Space Telescope photos
Part 6 Lenses from Hubble Zoo
Part 7 Index to the Possible Strong Gravitational Lenses forum posts What is a lens?
Here is model of a gravitational lens. We are looking at it from the side.
A gravitational lens consists of a massive lensing galaxy, and lensed galaxies that are behind it.
Here is a diagram of a gravitational lens system, looking at it from the side.
Earth... a massive lensing galaxy with strong gravity.... a distant spiral galaxy behind it
Light starts to travel from distant spiral galaxy towards Earth (straight white lines).
We can't see the distant spiral galaxy directly because the massive lensing galaxy is in the way. The spiral galaxy would have been too dim to see anyway.
The light that passes near a massive galaxy, is pulled towards that galaxy by gravity (the bending path of the light is shown by the curving white lines). A great deal of light that should have been heading in other directions is now heading towards Earth.
The light reaches telescopes on Earth, and a picture is taken. (The photograph of the galaxies as seen from Earth is superimposed in middle of the diagram. ) We see the massive lensing galaxy in the middle of the photograph, with the light from the distant spiral lensed galaxy as arcs around it. (The apparent path
of the light is shown by yellow lines.)
|Here is what a lens looks like in a photograph taken by the SDSS telescope. |
We are looking at it, of course, from the Earth.
If the distant source is directly behind the lensing galaxy, the lensed galaxy is a ring.
587742572151374147 The "Cosmic Horseshoe"
The orange galaxy is the massive lensing galaxy.
The distant galaxy is directly behind the massive lensing galaxy, so we can't see it directly.
Much light from the distant galaxy has been bent towards us by the massive lensing galaxy.
It is concentrated into a ring around the massive lensing galaxy.
|If the distant galaxy is not directly behind the massive lensing galaxy, arcs are formed instead.|
The orange galaxy is the massive lensing galaxy.
The lensed galaxy is the white arc. It is much further away than the massive lensing galaxy.
|Why look for lenses on SDSS when Hubble Space Telescope pictures can see more detail?|
The (Sloan Digital Sky Survey) telescope covers a lot of sky quickly, while other telescopes can look at only a small piece of sky. Computers help set up the SDSS data base. Computers can sort galaxies by size, location and proximity to other galaxies, colours and redshift, if that is available. You can learn to use the SDSS search. But computers cannot recognize shapes.
Galaxy Zoo 2 gets its pictures from SDSS, and help sort galaxies by shape.
People can classify galaxies according to shape, recognize unusual shapes and interactions, and find new objects.
After classifying their galaxy, Zooites can post unusual discoveries on the Forum, and discuss the galaxy with other Zooites. For gravitational lenses, the thread is Possible strong gravitational lenses in Weird and Wonderful.
A scientist may look at the post and think that it is a possible lens, and then the galaxy can be put on the waiting list for analysis by other telescopes. The galaxy could be confirmed as a lens, and Galaxy Zoo could be credited with a find.
Why are gravitational lenses important?
1. To refine the numbers that we need understand our universe, how and when it started, how it is growing, what will happen in the future. See wikipedia Gravitational lens : Geometry of the Universe.
|2. To weigh the amount of dark matter around a galaxy. |
The amount of bending of lensed light around a galaxy shows how much the galaxy weighs. If it seems the galaxy weighs more than the mass of stars and gas in it, the rest of the mass must be due to dark matter, and we can learn more about what our universe is made of.
This composite image shows the hot gas around the galaxy cluster 1E 0657-56, also known as the "bullet cluster." Normal matter between the galaxies is shown in pink: dark matter is shown in blue.
photo: Hubble Space Telescope
|3. To find galaxies that are normally too far away to be seen.|
The concentration of light of distant objects means that we can see galaxies that are very far away that are ordinarily too dim to see. With the help of gravitational lenses we will be able to see galaxies 13 billion light years away. The edge of our visible universe is about about 13.7 billion years away. So it adds to our understanding of the early universe.
The long bright blue streaky things are the lensed galaxies.
SDSS can only see the biggest and brightest of these.
There are three types of gravitational lens
See Wikipedia Gravitational Lens Strong lensing
forms rings, arcs and multiple images. This is what we are looking for in the Forum.Weak lensing
shows only stretching of images.Micro-lensing
shows no change in the lensed object's shape, just in brightness.
I would like to thank Gumbosea for his inspiration and knowledge on the Possible Strong Gravitational Lens thread, and for looking over this guide. Thanks also to and Pluk, blt and Aprajita for the use of their illustrations. Thanks also the the major contributors to the thread who are getting expert in analyzing pictures for lenses and sharing their knowledge with newbies like me.