How does LOFAR Work Parts 8 and 9 (from the KAIRA blog)

Parts 8 and 9 of the series from the KAIRA blog on how LOFAR works, covering how the array is pointed as specific places on the sky:

8: Steering a Phased Array
9: Digital Beam Steering. 

How does LOFAR Work Part 7 (from the KAIRA blog)

Part 7 of the series from the KAIRA blog on how LOFAR works:

Phased Arrays.

What might LOFAR see of the first stars?

One of the main science drivers for LOFAR is the detection of the point in the history of the Universe when the first stars turned on.

We know that the Universe formed in a Big Bang, after which it was filled a hot soup (plasma) of elementary particles (like electrons, protons, neutrons and photons as well as much more exotic things). When the Universe cooled sufficiently, the electrons combined with the protons to make hydrogen atoms. At that point the photons which had previously been stuck bumping around between the particles could finally stream across the universe - and we see from that time the first light in the Universe - now cooled to microwave radiation (what we call the cosmic microwave background, or CMB).

After the CMB not much happened for a long time. There still wasn't a lot of structure in the Universe - no stars or galaxies. There was nothing to create any light - so we call this period the "dark ages". During this time the material in the Universe ever so slowly collects together under its gravity. Eventually parts of the Universe start getting more and more dense, and the first stars turn on in the densest parts. Suddenly there is light in the Universe once more, and all these new photons hit the hydrogen atoms and strip them of their electrons (ionizing them, but since they started before the CMB as separate protons and electrons, we like to call it "reionization").

As I described in a previous blog post "LOFAR in Sky and Telescope", and you can read about in the May 2011 issue of Sky and Telescope, neutral hydrogen emits a characteristic spectral line at 21cm (1.4 GHz), due to hyperfine splitting of it's ground state. The neutral hydrogen that was around in the Universe just before the first stars turned on was emitting this line, which is now redshifted to much lower frequencies (which we think should be in the range that LOFAR can detect). Suddenly at the redshift which the first stars turned on, the amount of this 21cm emission will drop dramatically. It is this phase change in the Universe which LOFAR hopes to detect.

A slice through a simulation of the reionization of the Universe. Credit Illian Illiev, SEPnet/Sussex

Exactly when and how the first stars turn on in the Universe is an important measurement to further our understanding of cosmology. Researchers involved in LOFAR (and LOFAR-UK) are working on models of what LOFAR and other telescopes like it might see. One such researcher is SEPnet Fellow Dr. Illian Illiev at the University of Sussex. It was his images and animations of what reionization might look like which were used to illustrate the Sky and Telescope article.

Reionization Animation.

Here's a description of the image and animation provided by Illian:

This is what a giant radio telescope like LOFAR is expected to see as it looks deeper into the past (going right) - a sea of neutral hydrogen atoms (yellow) gently excited into emission by the cosmic microwave background photons.

As the very first stars and galaxies form in the universe, some of their radiation is energetic enough to kick out the electron out of the hydrogen atom (a process called ionization), which leads to the gas 'disappearing' from point of view of the radion telescope, illustrated by the blue regions above, growing with time (going left) as more stars form over time.

The variation in the intensity of the yellow across the image indicates regions with different density, the higher the density, the more intensive the colour (since there is higher concentration of emitting atoms there). This is a manifestation of the 'Cosmic Web' of structures - a honeycomb-like structure which forms due to gravity.

How does LOFAR Work Part 6 (from the KAIRA blog)

And they've been busy over at KAIRA, here's also Part 6 in the series in how LOFAR works. Seems this is the end of the preamble, the real details will start soon.

Squinted Vision (about the need to steer radio dishes).

How does LOFAR Work Part 5 (from the KAIRA blog)

The next installment in how LOFAR works from KAIRA - a reminder about how traditional radio astronomy dishes work.

The Parabolic Dish.

How Does LOFAR Work Part 4 (from the KAIRA blog)

Part 4 of the KAIRA blog series on how LOFAR works: 


From radio waves to electrical signals

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).