Anupreeta More, Surhud More and Phil Marshall
Gravitational lensing is a spectacular phenomenon found in the Universe. Predicted by Fritz Zwicky in the 1930’s, galaxies and clusters of galaxies acting as lenses are not just beautiful to look at but they also have plethora of applications, including revealing the whereabouts of the elusive Dark Matter. Gravitational lenses are rare objects since we require the foreground and background galaxies to be aligned on the sky to within a few thousandths of a degree.
Over the coming decade, larger and larger imaging surveys will map out ever wider and deeper regions of the Universe. This means we should be able to find more gravitational lenses, but it also means that we will have increasing amounts of data to inspect in order to find them. As a result, we would like to automate the process of finding gravitational lens systems from these vast treasure troves of data. However, as you know, discovering gravitational lens systems requires some skill, and the lens candidates need to satisfy a varied set of criteria before they can be tagged as promising lens systems. Our brains are more suited to carry out such tasks than are simple computer algorithms, so it makes sense for humans to look at the candidates that the robots flag as interesting. However, so far, astronomers have had difficulties in building robots that are capable of finding all the different kinds of lens systems that are potentially interesting. This is partly because we have not yet discovered very many lenses, nor exhaustively cataloged all the things that look like lenses but are not lenses in reality. To understand how to make the robots work better, we need to jump in to the data alongside them!
Our first project in the Lens Zoo is going to be a slightly unusual one, in that it’s focus will be on beating a lens finding robot, rather than checking through its outputs. We are going to use the optical and infrared data from the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) for this project. With the help of computer algorithms called “ArcFinder” and “RingFinder”, we have found a sample of lens candidates from the CFHTLS – but we know that these algorithms don’t do a very complete job. Opportunity knocks! We would like the citizen scientists of the Lens Zoo to help us search the images of the CFHTLS to discover a variety of lenses which were missed by our robots.
The CFHTLS spans an area of about 170 square degrees of sky. It’s images are both higher resolution than those of the SDSS, with median seeing in the i’ band of around 0.7 arcsec, and deeper (i’ < 24.5 magnitudes) – which means that more gravitational lenses should be visible per square degree. The picture on the left shows a lens from CFHTLS that the ArcFinder did spot, a small galaxy group that is lensing a background star-forming blue galaxy. On the right is the SDSS image of this system, to show you the difference in image quality.
We have two goals for this project. First, we want to find all the gravitational lenses that the aforementioned algorithms missed – perhaps because the sources are quasars, or distant red galaxies, or because the lenses are complex, or confusing. Second, we want to catalog all the objects that look like lenses, but are not: these “false positives” will make an important training set for us to test our improved robots on. This will be the first time that this survey’s images will have been exhaustively inspected – so there are bound to be some surprises!
Zurich: home to Albert Einstein when he first started thinking about light passing through warped spacetime, and so what better place to have our first workshop! The Lens Zoo team and a few Galaxy Zoo forum moderators and Lens Hunters met up at the Institute of Theoretical Physics at the University of Zurich at the weekend, both in person, and remotely via a Google+ video Hangout. Even the team from Chicago who got up at 3am to be projected four feet high onto a screen managed to stay cheerful the whole time! We spent a couple of days thinking through the problems that we’ll face when trying to find thousands of gravitational lenses over the next few years.
So, what did we talk about all weekend? Among other things: how we should display images, and how we can best enable their investigation, how to teach new users about gravitational lensing, which features of the various Zooniverse projects we could make use of, and what tools we have to help advanced Lens Hunters to go the extra mile. For now, you can see the slides that the science team made for some of the sessions in the links below. We’ve got a bunch of problems to solve, but also some good ideas to get started with. The team will be writing their own postcards from Zurich on here soon, and we look forward to hearing your comments as we go. We need and value your input!
PDF files of session slides (watch out, these are quite large files!):
After giving this short talk, I made a video of it for all of you out there on the webs. I wanted to show some of ways we are using gravitational lenses in astrophysics research, and say something about how we might find more lenses to extend these investigations. That’s where the Lens Zoo comes in!
What features should the Lens Zoo website have, to help us find as many lenses as possible? We are planning a workshop in mid-July to discuss the interface and tools for the new Zoo, and to give us something to talk about, we’d love to hear from all you lens-hunters out there. We’ve setup a web form for you to send us any ideas about functionality or tools that you think would be useful in finding lenses. Here’s the address in full:
We’ll go through all your ideas when we meet up in Zurich, and keep you posted!
Stay tuned, and thanks for your help.
Phil, Aprajita & the Lens Zoo team
We’re very pleased to tell you that we’ve been awarded developer time from the Citizen Science Alliance to build a new, exciting Zooniverse project to discover gravitational lenses.
What’s a gravitational lens, you might ask? When a massive galaxy or cluster of galaxies lies right in front of a more distant galaxy, the light from the background source gets deflected and focused towards us. These space-bending massive galaxies allow us to peer into the distant Universe at around 10x magnification, and to make accurate measurements of the total (dark and luminous) mass of galaxies.
As many of you know, there has been a long-running and enthusiastic search for lenses in the “weird and wonderful” part of the forum; although lens-finding was never a goal of the Galaxy Zoo project, this forum has turned up some interesting systems which we are still following up. Up until now, the GZ lens search has been quite informal: it has not been easy keeping track of all the candidates that have been suggested! Nevertheless, the Lens Hunters have done an amazing job, collecting and filtering the suggestions as they come in, and teaching themselves and each other about the astrophysics of lensing.
Impressive stuff: enough to persuade a group of professional astronomers that a specially-designed Zoo for identifying lenses could be a powerful way of analyzing the new wide-field imaging surveys that are coming online. In this Lens Zoo we will be able to provide you with new tools – designed, we hope, with you – to find new lenses more effectively. We have teamed up with astronomers from several big surveys who are eager to harness your citizen science power, and will be providing a lot of new, high quality data to be inspected. Over the next 6-10 months we’ll be working hard with the Zooniverse developers to build the Lens Zoo, and we hope you will join us for the ride: Lens Zoo needs you!
Phil, Aprajita, Anupreeta & the Lens Zoo team.