Thursday, March 24, 2011

LOFAR in Sky and Telescope

One of the feature articles in the May 2011 edition of Sky & Telescope is "The End of the Cosmic Dark Ages" (or "Cosmic Enlightenment - the first stars") which covers how the International LOFAR Telescope plans to try to detect the "phase change" in the early universe when most of the hydrogen changes from neutral to ionized (called "reionization"). This is detectable by low frequency radio telescopes as a sudden decrease in (redshifted) 21cm emission. It works like this.... Neutral hydrogen emits a characteristic spectral line at 1.4 GHz (or 21cm) due to hyperfine splitting in its ground state. Ionized hydrogen does not emit this line. Because of the expansion of the universe, 21cm emission from very early in the universe has been redshifted to much longer wavelengths (detectable by LOFAR hopefully), and the amount of the redshift tells you how far back in the universe you're looking. LOFAR plans to look for a frequency at which redshifted 21cm emission suddenly stops being present (ie. the emission is there at longer wavelengths, but not at shorter ones).  This would provide a time stamp for when the phase change happened which is an important measurement to further our understand of the evolution of the universe.

Anyway check out the article to learn more. It features a series of images (and a link to an animation) from a simulation of the universe being reionized done by LOFAR-UK member Illian Illiev (the SEPnet LOFAR fellow at the University of Sussex) and quotes from LOFAR-UK member Steve Rawlings (from the University of Oxford).

It is a very nice article, and I'm very grateful to Sky and Telescope for sending me a preview version (we helped them locate some nice images of LOFAR stations). There are a couple of small corrections I would like to point out though. The first is that the image shown on Page 28 called "LOFAR First Light", described as being the first image taken by the whole array isn't quite that. That image is described in an earlier blog post "First Images from LOFAR Including Chilbolton" and was taken as part of observations which included the Dutch core and remote stations, as well as Chilbolton, Nancay and Tautenberg (but not the other German stations, or the Swedish one), and as I understand it that image actually only included data from the Dutch stations - it's the zoom in shown in our blog post which included data from the LOFAR-UK station). Also as we have discussed it's not the separation of the array, but the number of antennas and collecting area which make the International LOFAR Telescope the largest telescope in the world (at the moment). Finally Steve Rawlings is not leading LOFAR-UK (although he is very heavilly involved). Rob Fender (Southampton) is our current leader, soon to be replaced by Phil Best (Edinburgh).

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