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.

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).

Hydrogen 21 cm transition

A hydrogen atom consists of a proton and an electron, and both of these particles have a quantum property called spin, which is related to their angular momentum. It is a similar concept to rotation around the axis, for example of the Earth. The proton and electron can have a configuration where their spins are pointing in the same direction (parallel) or pointing in opposite directions (anti-parallel). The first configuration has a slightly higher energy than the second configuration.

When a hydrogen atom makes a transition from the parallel to the anti-parallel state, it will emit some electromagnetic radiation with a wavelength of 21 cm (1420 MHz). Alternatively, a hydrogen atom in the anti-parallel state can change to the parallel state by absorbing electromagnetic radiation with a wavelength of 21 cm. This transition is extremely weak, but the masses of hydrogen in galaxies are so large that it can be detected in nearby galaxies. Due to the fact that it occurs at a very specific frequency (or wavelength) this type of emission is known as line emission in contrast to continuum emission, such as free-free and synchrotron radiation.

LOFAR will use this physical process to study the ‘Epoch of Reionisation’. In the early Universe, the vast majority of hydrogen in the Universe was in the form of neutral atoms unlike nowadays, when it is mostly ionised plasma so the protons and electrons are separated. As the first stars switched on, they produced ionising radiation that began to separate neutral hydrogen atoms into electrons and protons (also referred to as ions). We use the prefix ‘Re-‘ in Reionisation because shortly after the Big Bang the Universe was ionised, and it later cooled down and electrons and protons joined to form neutral Hydrogen atoms. Hence the epoch of Reionisation is the second epoch when the Universe was ionised.

The epoch of Reionisation occurred when the scale of the Universe was significantly smaller, approximately one seventh of the present scale or smaller. The radiation at 21 cm has scaled up with the Universe, and reaches us with a wavelength of about 1.5 metres or longer (corresponding to a frequency of 200 MHz or less) and it is therefore observable with LOFAR. Studies of the 21 cm line can yield information on the density of neutral hydrogen and its distribution in the early Universe.

The figure below shows the signal of the 21 cm line from the epoch of reionization which LOFAR is expected to measure. The colour scale shows the difference in the observed intensity caused by regions with a high or low content of neutral hydrogen: light regions have the most neutral hydrogen compared to the average, dark have the least (remember the average changes with epoch, so we are looking at the contrast). The vertical axis shows the physical extent of the regions, the units are megaparsecs, Mpc, which correspond to approximately three million light years. The horizontal axis corresponds to observed frequency (in MHz), with lower frequencies looking at earlier epochs when the Universe was younger. This image was produced by Garrelt Mellema using a simulation of the young Universe by Ilian Iliev.

Against a strong source of radiation, for example synchrotron emission from a background source, the neutral hydrogen will leave an imprint of absorption due to the transition at 21 cm. Below is a simulation of the spectrum of such a strong source. The horizontal axis is frequency, in MHz, the vertical axis is flux density (another measure of intensity). The source is at redshift 10, and at this redshift the 21 cm line appears at 129 MHz. The absorption from neutral hydrogen can be seen as strong dips in the spectrum to the right of 129 MHz. The solid line serves as a guide to the eye, it shows what the spectrum would have looked like without any absorption. Only neutral hydrogen between us and the source can cause absorption, and hence the spectrum is only affected at frequencies higher than 129 MHz. Image credit: C.L. Carilli, N. Gnedin, S. Furlanetto, F. Owen.