New Year, New Data: Hunting for lenses in the infrared with VICS82

Happy New Year everyone! We’re starting 2014 off with a bang, with a brand new dataset, and hopefully a whole new army of spotters who’ll have heard about Space Warps from the BBC Stargazing Live programmes. Welcome to Space Warps, you guys ūüôā

So how about this new data then? Here’s an example gravitational lens from the VICS82 infrared survey – and here’s PI Jim Geach of the University of Hertfordshire to explain the survey.


Jim says: VICS82 stands for “VISTA-CFHT Stripe 82”, and is the largest near-infrared¬†imaging survey of its kind, mapping nearly 200 square degrees of the Sloan¬†Digital Sky Survey ‘Stripe 82’ – a narrow strip of sky that is the deepest¬†part of the SDSS. VICS82 is using two 4-m class telescopes fitted with¬†large-format near-infrared cameras: the Canada-France-Hawaii Telescope (atop¬†Mauna Kea in Hawaii) and the VISTA survey telescope in the Chilean Atacama.

Over the last few years, Stripe 82 has received much attention from a wide¬†range of different telescopes, covering the millimetre and radio bands,¬†through the optical and infrared and (soon) high-energy x-rays. Its location¬†along the celestial equator makes the Stripe a great target for facilities in¬†both the northern and southern hemispheres – in fact, it’s¬†shaping up into the first of a new generation of very large and deep¬†extragalactic survey fields. Previously there has been a compromise between¬†survey depth and sky area that has limited the size of the fields we can¬†observe if we want to study the distant Universe (you can go very deep and¬†therefore see very far, but only over very small patches of sky – like the¬†Hubble Deep Field). But with ever-improving sensitivity and mapping¬†capabilities in instrumentation right across the electromagnetic spectrum¬†we’re now able to map much larger areas to much deeper depths than every¬†before. While certainly not as deep as the HDF, VICS82 is the deepest¬†near-infrared survey that exists for the size of the sky it has imaged, and can¬†see normal galaxies out to a redshift of about 1 or so, when the Universe was¬†roughly half its present age. It can see quasars out to much higher redshifts¬†– these objects shine like beacons across the Cosmos.

So, one of the main goals of VICS82 is to survey a huge volume of the Universe to¬†detect a mixture of massive and passive (ie, not star-forming) galaxies, and also dusty and actively star-forming¬†galaxies and quasars. While VICS82 uses wavebands that complement the existing¬†SDSS imaging (improving photometric redshifts for example), some of the¬†objects detected by VICS82 are expected to be very faint or even invisible in¬†the current optical imaging of the Stripe, showing up only at the longer¬†wavelengths (1-2 microns) probed by VICS82. We call these systems ‘red’, and¬†they are very important to consider in our census of galaxies if we are to¬†properly piece-together the story of galaxy evolution.

With SpaceWarps we hope to identify examples of rare ‘red arcs’ which might¬†be very distant, highly reddened galaxies lensed by a foreground mass like a¬†group or cluster. If these galaxies contain lots of dust, their visible light¬†might be completely extinguished internally, and so would not be detectable at¬†shorter wavelengths, but the infrared photons can more easily escape. If we¬†can find even a few of these systems, then the possibilities for detailed¬†follow-up work are tremendous, since – when armed with a model of the lensing¬†mass – we can really dissect the galaxy, exploring its inner workings in a way¬†that is simply impossible without the benefit of lensing.

What we’ve done is select about 40,000 images from the survey, that each contain either a possible lens (ie a massive galaxy or group of galaxies), or a possible quasar source. The images are a little fuzzy, because the night sky is so bright in the infrared – this makes it quite difficult for computers to detect the faint lensed features. Sounds like a job for Space Warps! Good hunting, and thanks for all your contributions – see you on Talk!


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