Archive by Author | Aprajita

Your work in print!: Space Warps CFHTLS papers accepted for publication

We’re really pleased to announce that our papers on the first Space Warps gravitational lens search on the Canada France Hawaii Telescope Legacy Survey (CFHTLS) have been accepted by the Monthly Notices of the Royal Astronomical Society, and will appear online and in print next month. In the meantime, you can find the accepted papers at Paper I & Paper II.

We’ve also resurrected the Space Warps site and are eagerly awaiting your classifications! You’ll be able to look at some CFHTLS images that have been seen before but we think are likely to contain interesting objects that didn’t quite make it into our final sample according to the analysis pipeline we ran while the project was live. Subsequently, Chris Davis, a research student who works with Phil at SLAC, put together an “offline” analysis that considers your classifications slightly differently. This threw up some new candidates that we would like your second opinion on. These may be false positives (i.e. objects that look like lenses but aren’t), difficult candidates (where you all didn’t agree), or genuinely missed candidates (not enough people viewed them). There’s an example of a missed simulated lens below. Check-out the Spotter’s Guide to refresh your memory on real lenses, false positives and artifacts. There are far fewer images this time round so you may find yourself out of subjects quickly.

We’ve also prepared a few articles from our home institutes that will come out today (Thursday 24th September) to mark the acceptance – a press release from Anu’s home institute in Tokyo, Kavli IPMU, a Symmetry article (online this afternoon) from SLAC, Phil’s home institute, and an Oxford Science Blog post from my home institute – so watch out for those!

We’re hoping that we may get to meet a few new lens spotters as a result of the articles, so please do help out any newbies you spot on Talk if you get the chance! Some of the images you see may already have been discussed on Talk as potential lenses as well, so please do add to those discussions too.

Happy classifying!

Aprajita.

Example false negative

The image on the left shows a training image used in Space Warps. We inserted a simulated lens (top right) and this was presented as part of the classification stream. The right panel shows how it was classified by each viewer, the number of views increases from top to bottom. Each kink in the blue line indicates each new classification. Moves to the left indicate those who thought there was no lens here, and moves to the right show those who did. Overall the source stayed roughly 50:50, so the community classification for this object was undecided. A community classified lens candidate would lie to the right of this plot (trajectory crossing the blue dotted line on the right), and an image containing no lens candidate would cross the red dotted line on the left. Similarly, real (non-training/non-simulated) lenses may have been undecided in the CFHTLS search. We are now asking you to view some more images that could contain potential lens candidates that didn’t quite meet our detection threshold in the initial search.

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Space Warps: New Candidate Gravitational Lenses in the CFHTLS!

It’s been a while, but we are very pleased to be able to write to you today with the results of the first Space Warps project launched in May 2013. We asked for your help with finding lenses which may have been missed by robotic searches in the Canada France Hawaii Telescope Legacy Survey (CFHTLS) imaging. We have now combined and analysed all your classifications, and carefully sifted through the results. It’s good news:  in addition to finding 80 previously published candidate gravitational lenses, you helped discover 29 new candidate gravitational lenses (and another 30 objects that might turn out be lenses). Nice work, people!

fig

We just posted two research papers on the “arxiv” pre-print server (where astronomers put their work for their colleagues to read), showing our results from our first project. You can check out the two papers here and here. The first paper is about how well “the system” (that’s you!) performed, in terms of spotting the sims and rejecting the duds. The second paper is about the new lens candidates that you found – and how they compare with the “known lenses” that two robots had previously found.

We’ll be submitting these papers to an astronomy journal (Monthly Notices of the Royal Astronomical Society) in a couple of weeks, so if you have comments or questions about either paper, you can post them as “issues” on the Space Warps GitHub repository, and we’ll work them into the text in the meantime. Thanks!

So, what do we mean by “candidates” that “might turn out to be lenses”? A few of the objects you found are clearly gravitational lenses – we can tell just by looking at them. For those that are less obvious, making a model that reproduces the image configurations seen can give us more confidence. While we were working on putting the sample together, Rafael Kueng and Prasenjit Saha wrote up a test of their web-based lens modeling software, which some of you took part in – you can read their paper on the arxiv too!

Here is an example of what astronomers can do with the candidate lenses that you discover – Since you found this amazing red ring (called 9io9 based on your popular choice) in the VICS82 project last January, Jim Geach (co-PI of VICS82) and his team have been busy making more observations with a variety of telescopes. They recently finished analyzing the data. Jim, Aprajita and others helped confirm that 9io9 is indeed a lens with spectroscopy, and Anupreeta and Neal Jackson (University of Manchester) made a well-constrained mass model of the system to understand how magnified is the red galaxy in the background. This red lensed galaxy turns out to be pretty interesting – check out the paper to find out more.

We’ll write more on all this good stuff in the coming weeks. If you like, ask us a question in the comments below, and we’ll try and answer them as we go 🙂

Thanks for all your classifications, these are very exciting results!

Phil, Aprajita, and Anupreeta

X marks the spot: new gravitational lens candidates from Space Warps

One of the objectives of the Space Warps project was to find gravitational lens candidates that have been missed by previous computer algorithm searches of the CFHT-LS. We are very excited to be able to show you a few of the new objects that have been found by the Space Warps community, and are currently under discussion in Talk. They are what we would consider to be highly probable lens candidates, and have not been found by the automated searches in the CFHT-LS. These are examples of exactly what we were hoping the Space Warps project would find with citizen scientists – so well done everyone!!!

The first of the candidates is ASW0004dv8.

ASW0004dv8_zoom

Here, the collective mass of the foreground group of yellow galaxies (primarily the two bright galaxies) is large enough to bend the light of a blue galaxy lying behind forming a beautiful blue arc. There are some fainter and smaller yellow galaxies spread across the field that are also likely to be members of the galaxy group. Such galaxy groups (and the more massive clusters of galaxies) mark the most massive and dense regions of dark matter in the Universe. In contrast, the background blue galaxy that is being lensed into an arc is of much lower mass. It’s blue because it has many younger stars, and only just forming the bulk of its stars.

In contrast to the large blue arc in ASW0004dv8, there are also easy-to-miss, compact arcs like ASW0003wsu. On a quick glance, the nearby bright star might have grabbed your attention sooner than the potential lensed arc. Just to the left of the star in the image below, there’s a nice clear arc next to the yellow lensing galaxy. Other similarly coloured yellow galaxies in this image that may form a galaxy group.

Another interesting system is at the bottom of ASW00047ae.

On initial inspection, this looks to be like the examples above, a blue arc around a yellow galaxy. However, if you look closely at the arc, it’s not as smooth as the blue arcs in the examples above. The lensed blue feature actually comprises three blue emission peaks that are aligned in a “cusp”-like configuration around the primary lensing galaxy. Also there is a neighbouring small galaxy to the right of the primary lens that may also contribute to the gravitational lens. To verify this we would need to make a lens model to try and explain the image features we see. Using custom software, we program in the position, extent and mass of a lens (a single galaxy or group of galaxies), then place a virtual distant blue galaxy behind it, and then try to reproduce the shape and light of the lensed blue arc or features. (We’ll be blogging more about lens modelling in the coming weeks – it’s an important part of the discovery process!)

Like ASW00047ae, the Space Warps Community have found several more multiply imaged examples including  very compact ones that are tricky-to-spot . Some good examples of these are

ASW0005mp6

ASW0005mp6_zoom

ASW0004q9e

ASW0001yqb

These are very different lenses to the galaxy group lenses above with distinct blue arcs. Rather, in these examples a blue, compact background galaxy or quasar is being lensed by a single foreground galaxy into multiple discrete images. In these cases, the lensing galaxy is the red galaxy at the centre of the zoomed in images above. There are actually 2 red galaxies seen in ASW0004q9e but the galaxy with the blue images around it is the primary lens. As with ASW00047ae, we would need to model the system to see if the second red galaxy had an impact on the lensed images. There are two images of the background blue quasars or compact galaxies in ASW0005mp6 and ASW0004q9e. In ASW0001yqb there are three distinct images of the blue background galaxy, arranged in a circular fashion around the lens. In ASW0001yqb and ASW0004q9e, the blue images of the lensed background galaxy are so bright that they almost overshadow the red light of the lens.

Note the colour of this lensing galaxy is much redder than the yellow colour of the lensing galaxies in the examples of blue arcs shown above. These red lenses must lie at a higher redshift than the yellow galaxy groups, i.e. the strongest emission from this type of galaxy, seen in yellow in ASW0004dv8, moves into the redder bands at higher redshift making the galaxy appear red rather than yellow.

Among all of the candidates shown here, ASW0001yqb is probably the hardest to identify. It is a very compact system and it is not easy to differentiate the three blue images of the background galaxy from the faint, red, central galaxy. In fact, ASW0001yqb is very similar to a lens found by the RingFinder computer algorithm of Raphael Gavazzi et al. seen in ASW0000x1lASW0000x1l has been confirmed to be a gravitational lens by the SL2S team but just hasn’t been published yet! The similar ASW0001yqb was not found by RingFinder, this demonstrates how Citizen Scientists have a vital role to play in identifying potential lenses missed by the algorithms and therefore providing key information on how to optimise lens finding algorithms. Raphael is going to help us do a complete cross-check between the RingFinder and Space Warps samples, when we have them.

How did the Space Warps Community collectively classify these lens candidates?

The lens candidates shown here were selected from good lens candidates that are being discussed in Talk.In our last blog post, we described what happens after you have made your clicks, the way we analyse your marks to remove images that don’t contain lenses and select potential lens candidates from the stream.

The following image is the same trajectory plot explained in the “What happens to your markers” blog post. The probability of being a lens is shown increasing from left to right, and the number of classifications increases from top to bottom. Lens candidates, as voted by you when you mark images, appear on the right hand side of the plot as with each mark they aggregate a higher probability of being a lens.

The two magenta lines shown are the tracks for ASW0004dv8 and ASW00047ae that we discussed above. You see both track move to the right hand edge of the plot meaning that you collectively voted that these systems are very good lens candidates!

traj_ASW00047ae_crop

ASW0004dv8 was classified as a highly probable lens candidates after 9 classifications (the upper magenta track) and ASW00047ae, after 8 views (lower magenta track). That’s excellent, it means that our collective classifications are producing very probable lens candidates after only ~10 views.

What’s Next

We’ll be blogging about more highly probable candidates from the early dataset releases as they come up – and we are putting together a sample of candidates from the collective classification for us all to analyse further.

These first examples demonstrate how well the Space Warps community is doing. Thanks for all your hard work and we’re looking forward to some exciting times ahead!

It’s good to Talk! Why we need to hear from you to find Space Warps

Some of you may be wondering what happens to an image after you hit “Next” and why “Talk”ing about your lens candidates is important, so here’s a brief explanation!

WHAT HAPPENS TO THE IMAGES YOU DON’T MARK?
Each night, we retire images from the pool based on your collective classifications. If the community together says no (i.e. by enough people not placing a marker on the image), we throw out the image so that we can focus your classifications on fresh data and images that might contain gravitational lenses. After only five weeks, you guys have made an astonishing 5.2 million classifications. This means we’ve been able to already reject about 60% of the total CFHT Legacy Survey as not containing gravitational lenses!

WHAT HAPPENS TO THE IMAGES YOU DO MARK?
When you mark an image two things happen. First, we record your mark in our database so we can compare it with what other people thought. Second, that image is automatically saved into your Talk profile under a collection called “My Candidates”. Talk allows you to discuss your interesting candidates with the rest of the Space Warps community. It’s great to see so many discussions happening there already, so please keep talking! Talking in Space Warps is an essential part of refining the list of plausible candidates, which is explained next.

HERE’S HOW YOU CAN HELP
As we work our way through the images, it looks as though we are going to end up with a sample of a few thousand lens candidates from your markings. That’s great – it means Space Warps is a very effective filter! But a few thousand is still several times more than the number of actual lenses we expect – so we’ll need to investigate the images of the candidates further before presenting them to the rest of the astronomy community. This is where you, and Talk, can really help us out!

BECOME CURATORS OF YOUR LENS COLLECTIONS
If you see a lens candidate, either when browsing Talk, or while you are marking, that you would like to see investigated further, make a “collection” called ‘Probable Lens Candidates’ and add this object to it! Remember, you can also add images you think are the most likely lens candidates from your automatically filled ‘My Candidates’ collection. Then, later on, you might do some further investigation of the images in your collection – or someone else in the collaboration might do it, after browsing your collections. Either way, collecting the candidates is the first step.  You can start a discussion about any candidate or collection any time, and ask the Space Warps community to share their thoughts.

WHAT HAVE YOU FOUND SO FAR?
We’ve just started looking at the most commonly marked images, and there are some promising candidates already being discussed in Talk. Some of these are previously known lens candidates: as you may know, the CFHT Legacy Survey has been searched using automated computer algorithms. We’ve started to label the candidates from those searches in Talk, you’ll see the label “Known Lens Candidate” at the bottom right of the image in the individual object page of Talk. As well as the labels, Budgieye has done a phenomenal job in collecting known CFHT-LS lens candidates from the research literature in a dedicated discussion board. Much like the tricky simulations, some of these known candidates may be difficult to spot.

Most excitingly, some of you have started discussing a few lens candidates that we think have been missed by the algorithms – watch this space for a special post about these potential new lens candidates next week!!!

HOW TO GET STARTED IN TALK
If you want some top tips on using Talk, please visit the discussion board (thanks Budgieye!)

Thanks again for your phenomenal work – and let’s get Talking!!!

Space Warps Update

Wow, what a week! Space Warps has had a phenomenal response since it launched on May 8th. You have made a staggering 1.7 million classifications so far – that’s over 11,000 images swiped per hour! A big thank you to all of you, our new collaborators, in making this first week so successful.

We’ve just begun to analyse the first batch of data. Your classifications have already allowed us to retire about 80% of the images in the first dataset (or D1 as we call it) that we are pretty sure don’t contain lenses. New images are now up on this site awaiting your classifications, this is the second of ten datasets in total.

Don’t forget that you can discuss any interesting potential lenses you find using Talk. As you mark images, the system automatically collects both the potential lens candidates and simulations that you have marked. You can find these in “My Collections”, found on the “Profile” section of Talk. Many discussions are already happening on a variety of images and topics. Please do start discussing your favourites with our community, or join existing discussions. For example, you might like to visit the  “Where do I go to see the good stuff?” discussion board to get started.

With your help, we can make dataset 2 (or D2) as successful as D1. You can read more about the progress of the project on our blog, and we’ll email you from time to time.

It’s great to see you enjoying Space Warps. Thank you very much for your contributions!

Aprajita, Phil, Anupreeta and the Space Warps Team