A discovery of faint radio sources by astronomers using the LOFAR (LOw Frequency ARray) radio telescope has important implications in understanding a mysterious period in the early Universe.

A team of astronomers at the Netherlands Institute for Radio Astronomy, known as ASTRON, along with the University of Groningen, have used LOFAR to obtain deeper wide-field images of the little-explored 150 mega-Hertz region than has previously been possible. The image shows a number of radio sources with the faintest only being a few milli Janskys (a unit of flux density). In contrast, 3C 273, one of the brightest quasars in the sky, has a flux density at least 1,000 times greater in the radio region.

“We have not yet investigated the nature of the newly discovered sources,” says Michiel Brentjens of ASTRON. “They are, however, most likely radio galaxies with active galactic nuclei (AGN) at redshifts of about 1 to 2, with here and there possibly a few starburst galaxies.”

LOFAR operates at low frequencies: from 10 to 240 mega-Hertz. Observing at low frequencies is difficult as the Earth's ionosphere, a layer between 400 and 1,000 kilometres above the Earth, interferes with incoming radio waves. Brentjens describes this effect as akin to “observing them through the hot air above a barbeque; the sources wobble about, and focus and defocus on timescales of tens of seconds.” LOFAR's high sensitivity, along with image post processing similar to adaptive optics, are what allow radio observations to be made in this difficult region of the spectrum.

One of the main goals of LOFAR is to create a map of the Universe between 400 and 800 million years after the big bang. Prior to this era the gas in the Universe was solely in a neutral state. During the 400 million years that followed, the first stars and galaxies began to ionise the neutral hydrogen in the Universe, a time period known as the epoch of reionization (EoR), and much of it remains a mystery. This is because the only way to observe neutral hydrogen is via its emission line at 21-centimetres. However, as the EoR occurred such a long time ago, the expansion of the Universe has shifted the observed position of this emission line to around 2.1 metres. This wavelength happens to coincide with a radio frequency of around 140 mega-Hertz, which is the region that LOFAR is primed for. It is hoped that observing the 21-centimetre line will help to create a map of the EoR, and thus further our understanding of what exactly happened during this era.

According to Brentjens the observation of the new faint radio sources is a step in the right direction towards observing the EoR. “The fact that we can now see these faint objects, shows that we are close (within a factor of 10) to the imaging capabilities and data quality that are required to meaningfully record EoR data,” he says.