First members of the ATLAS of lenses

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It’s been a good week for the Herschel-ATLAS survey that I work on – last Friday we released our first set of data to the public and this Friday we’re publishing some exciting results, led by Mattia Negrello from the Open University, on a new way to find cosmic lenses.

Gravitational lensing occurs when light from a distant galaxy is bent by a massive nearby object, like a large galaxy or galaxy cluster. This distorts and magnifies the image of the distant galaxy, meaning that galaxies that would normally be too faint to be detected can be seen easily. As Rita has blogged previously, this is like a free pass for observing things in the early Universe, using a giant cosmic telescope! The drawback to this is that the lensing will only happen if the two objects happen to be aligned in the right way. This is a rare occurrence so in the past people have had to sift through large amounts of data to find a few lenses. For example, the Cosmic Lens All Sky Survey, which used radio maps, only found 22 confirmed detections from 16,000 candidates!

In gravitational lensing light from a distant galaxy (red dot) is distorted and magnified by a foreground object, causing it to appear brighter and thus easier to detect. Herschel sees lensed galaxies as bright orange dots (top two panels), but doesn't detect the foreground galaxy. To see that you need optical telescopes like Keck in which they appear blue as seen in the two panels underneath. The distortion in the background galaxies is shown in pink in these panels - the data in this case come from the Submillimetre Array (SMA). Image credit: ESA/NASA/JPL-Caltech/Keck/SMA

Clearly what’s needed is a more efficient way to identify these interesting objects and, luckily, that’s where the Herschel-ATLAS survey comes in. It turns out that if you look at the galaxies detected by the Herschel Telescope in its longest, reddest, wavelength band (500 microns) the brightest ones are a mixture of lensed objects, low redshift spiral galaxies and AGN. The latter two are easy to identify and remove as they’ll already have been detected in previous surveys, leaving a sample consisting of essentially only lensed galaxies. The graph below explains it all a bit better, though feel free to skip ahead and trust me!

This graph from Negrello et al 2010 plots the number of sources per square degree of different brightnesses (labelled 'flux density') seen in the Herschel-ATLAS survey (points and solid line) alongside the breakdown of the number of sources predicted for different populations (i.e. adding up the contributions from the coloured lines gives the total number shown in the solid black line). The yellow rectangle illustrates the region where lensed galaxies (red dashed line) are easy to find, if you pick sources bright enough to lie here, and exclude the contaminating AGN and nearby galaxies (green and blue lines respectively).

When this technique was applied to the initial 16 square degree map from the ATLAS survey, 5 lens candidates popped out immediately. Subsequent follow-up observations with other telescopes confirmed them. The picture below is a false-colour image of the ATLAS map with the 5 lenses highlighted. Pretty much every one of the thousands of other dots you can see is a distant galaxy (a fact that I am now intimately familiar with given that my job is to help with the cataloguing of every single one of these things)! This is only about a thirtieth of the total area which will eventually be covered, which means we’ll eventually have a sample of hopefully several hundred lenses which can then be used to help us understand how galaxies in the early Universe formed.

Image credit: ESA/SPIRE/Herschel-ATLAS/SJ Maddox

For more details on these results and the Herschel-ATLAS survey check out the official survey website or the UK Herschel Telescope site. At the former you can even download our initial dataset if you want to dig around in it yourself!

ResearchBlogging.orgNegrello, M., Hopwood, R., De Zotti, G., Cooray, A., Verma, A., Bock, J., Frayer, D., Gurwell, M., Omont, A., Neri, R., Dannerbauer, H., Leeuw, L., Barton, E., Cooke, J., Kim, S., da Cunha, E., Rodighiero, G., Cox, P., Bonfield, D., Jarvis, M., Serjeant, S., Ivison, R., Dye, S., Aretxaga, I., Hughes, D., Ibar, E., Bertoldi, F., Valtchanov, I., Eales, S., Dunne, L., Driver, S., Auld, R., Buttiglione, S., Cava, A., Grady, C., Clements, D., Dariush, A., Fritz, J., Hill, D., Hornbeck, J., Kelvin, L., Lagache, G., Lopez-Caniego, M., Gonzalez-Nuevo, J., Maddox, S., Pascale, E., Pohlen, M., Rigby, E., Robotham, A., Simpson, C., Smith, D., Temi, P., Thompson, M., Woodgate, B., York, D., Aguirre, J., Beelen, A., Blain, A., Baker, A., Birkinshaw, M., Blundell, R., Bradford, C., Burgarella, D., Danese, L., Dunlop, J., Fleuren, S., Glenn, J., Harris, A., Kamenetzky, J., Lupu, R., Maddalena, R., Madore, B., Maloney, P., Matsuhara, H., Michaowski, M., Murphy, E., Naylor, B., Nguyen, H., Popescu, C., Rawlings, S., Rigopoulou, D., Scott, D., Scott, K., Seibert, M., Smail, I., Tuffs, R., Vieira, J., van der Werf, P., & Zmuidzinas, J. (2010). The Detection of a Population of Submillimeter-Bright, Strongly Lensed Galaxies Science, 330 (6005), 800-804 DOI: 10.1126/science.1193420