Finding Gravitational Lenses, the Herschel Way…
It’s nice to have the chance to blog for once about some exciting astrophysics rather than doom and gloom about budget cuts. Tomorrow (5th November) sees the publication of a long-awaited article (by Negrello et al.) in the journal Science (abstract here) that presents evidence of discovery of a number of new gravitational lens systems using the Herschel Space Observatory.
There is a press release accompanying this paper on the Cardiff University website, and a longer article on the Herschel Outreach website, from which I nicked the following nice graphic (click on it for a bigger version).
This shows rather nicely how a gravitational lens works: it’s basically a concentration of matter (in this case a galaxy) along the line of sight from the observer to a background source (in this case another galaxy). Light from the background object gets bent by the foreground object, forming multiple images which are usually both magnified and distorted. Gravitational lensing itself is not a new discovery but what is especially interesting about the new results are that they suggest a much more efficient way of finding lensed systems than we have previously had.
In the past they have usually been found by laboriously scouring optical (or sometimes radio) images of very faint galaxies. A candidate lens (perhaps a close-set group of images with similar colours), then this candidate is followed up with detailed spectroscopy to establish whether the images are actually all at the same redshift, which they should be if they are part of a lens system. Unfortunately, only about one-in-ten of candidate lens systems found this way turn out to be actual lenses, so this isn’t a very efficient way of finding them. Even multiple needles are hard to find in a haystack.
The new results have emerged from a large survey, called H-ATLAS, of galaxies detected in the far-infrared/submillimetre part of the spectrum. Even the preliminary stages of this survey covered a sufficiently large part of the sky – and sufficiently many galaxies within the region studied – to suggest the presence of a significant population of galaxies that bear all the hallmarks of being lensed.
The new Science article discusses five surprisingly bright objects found early on during the course of the H-ATLAS survey. The galaxies found with optical telescopes in the directions of these sources would not normally be expected to be bright at the far-infrared wavelengths observed by Herschel. This suggested that the galaxies seen in visible light might be gravitational lenses magnifying much more distant background galaxies seen by Herschel. With the relatively poor resolution that comes from working at long wavelengths, Herschel can’t resolve the individual images produced by the lens, but does collect more photons from a lensed galaxy than an unlensed one, so it appears much brighter in the detectors.
Detailed spectroscopic follow-up using ground-based radio and sub-millimetre telescopes confirmed these ideas : the galaxies seen by the optical telescopes are much closer, each ideally positioned to create gravitational lenses.
These results demonstrate that gravitational lensing is probably at work in all the distant and bright galaxies seen by Herschel. This in turn, suggests that in the full H-ATLAS survey might provide huge numbers of gravitational lens systems, enough to perform a number of powerful statistical tests of theories of galaxy formation and evolution. It’s a bit of a cliché to say so, but it looks like Herschel will indeed open up a new window on the distant Universe.
P.S. For the record, although I’m technically a member of the H-ATLAS consortium, I was not directly involved in this work and am not among the authors.
P.P.S. This announcement also gives me the opportunity to pass on the information that all the data arising from the H-ATLAS science demonstration phase is now available online for you to play with!