Radio Astronomers Measure a Brighter Sky Than They Expected

Astronomers have been underestimating the brightness of the low-frequency radio sky. Precision measurements by Australian and Italian radio astronomers reveal that the faint-sky background at low radio frequencies is up to 50% brighter than earlier estimates.

The low-frequency radio sky background comes primarily from synchrotron radiation within the Milky Way. These faint radio waves are produced as high-energy electrons spiral around the magnetic field lines that permeate our galaxy. Unresolved sources of radio waves in the distant universe also contribute to the background.

In order to measure the brightness of individual radio sources, such as radio-bright galaxies, researchers need to precisely characterize background radiation. In the same way, astronomers have to subtract airglow and light pollution when measuring the true brightness of nebulae and galaxies.

“At higher radio frequencies, calibration is much easier,” says Michiel Brentjens (ASTRON, the Netherlands Institute for Radio Astronomy). “You can just aim your telescope at the Moon or a planet, as the absolute high-frequency radio brightness of these objects can be derived from their known surface temperatures.”

However, with the advent of sensitive low-frequency radio observatories such as ASTRON’s Low-Frequency Array (LOFAR) and the future SKA-Low (the low-frequency part of the Square Kilometre Array, under construction in Western Australia), the need for accurate calibration at radio frequencies below 500 megahertz (MHz) has become ever more important.

Using a specially designed type of SKA-Low antenna, a team led by Luke McKay (CSIRO, Australia) and including eminent Australian radio astronomer Ron Ekers, has now obtained high-precision measurements of the low-frequency sky background between 60 and 350 MHz.

The observations, spanning about eight hours, were carried out on October 23, 2024, at the radio-quiet Inyarrimanha Ilgari Bundara Observatory in Murchison, Western Australia, which is also home to the Australian SKA Pathfinder (ASKAP) array.

In a paper published in Nature Astronomy, the team compares their results with a 10-year-old model (based on 20th-century observations) that has served as the sky background standard up until now. They found that the background is 20% brighter at radio frequencies between 60 and 200 MHz, and 50% brighter at 350 MHz.

“They did a great job,” says Brentjens, “and I’m impressed by the result. This has only been possible thanks to recent developments in electronics and increasing computer power, enabling fast data analysis.”

According to the researchers, improved sky models based on the new measurements will be important to interpret future measurements of the epoch of reionization in the very early universe, when the first stars and gas-guzzling black holes “turned on,” ionizing intergalactic neutral hydrogen around them.

Astronomers expect to see  a “swiss cheese”-like signature in the 21-centimeter (1.4 gigahertz) radio emission from neutral hydrogen, as stars and black holes blast out ionized cavities within the neutral-hydrogen surroundings. But the 21-cm emission is redshifted to very low frequencies by cosmic expansion as it makes the billions-of-years-long journey to Earth. So radio astronomers must use low frequencies to map out the swiss-cheese hydrogen in the early universe.

The new result also suggests that astronomers may have underestimated the number of high-energy electrons speeding around the Milky Way Galaxy. Also, there might me more unresolved extragalactic sources.

The team even suggests that some of the excess brightness at low radio frequencies might come from the decay of dark matter particles. But according to Brentjens, the evidence for that is “very thin.”

“The most important result is that they’ve determined the absolute background brightness at low radio frequencies for a very large part of the sky,” Brentjens says.