LOFAR Captures the Deepest Low-Frequency Radio Image of the Sky Yet, Revealing 63,000 Sources

A patch of northern sky called ELAIS-N1, chosen because it holds very little galactic dust, gas, or stars, has now been captured in the sharpest low-frequency radio image ever made. The picture comes from the Low Frequency Array, or LOFAR, and from an international team led by researchers at Leiden University. It contains 63,000 radio sources.

ELAIS-N1, short for European Large-Area ISO Survey-North 1, is a field astronomers favor for deep studies across multiple wavelengths, because so little in the foreground gets in the way of the distant Universe. Astronomers at LOFAR have been watching it for years to map its low-frequency radio sources. The new image has twenty times the resolution of the previous LOFAR image of the field, a gain made possible by advances in interferometry, the technique of combining signals from many telescopes into one clearer picture.

The scale of the work is considerable. The image rests on 200 hours of observation, amounts to 400 terabytes of data, and is rendered in 8 billion pixels. A few years ago, processing radio data at this resolution was not feasible. The change came from the Spider data-processing platform, developed by researchers at the Netherlands Institute for Radio Astronomy (ASTRON) and their collaborators, along with improvements in calibration and data reduction. Together they cut processing times by a factor of ten. Jurjen de Jong, a postdoctoral researcher at Leiden Observatory and ASTRON who led the research, said the whole dataset could be processed on Spider, which is hosted at SURF in the Netherlands, in just a few months.

Much of what shows up in the image is active galactic nuclei, also called quasars: the bright centers of galaxies powered by supermassive black holes and their accretion disks. These nuclei are known to launch relativistic jets of superheated gas and dust from the black holes' polar regions, and the jets radiate considerably themselves. LOFAR is especially well suited to seeing such structures because it is sensitive to extended low-frequency radio emission. The high sensitivity also revealed numerous filaments around the radio galaxies, particularly near galaxy clusters, along with details of their shape and spectral character.

The resolution changes what can be done with these objects. In earlier, lower-resolution surveys, distant galaxies and their jets looked like featureless blobs, and telling individual galaxies apart from clusters was difficult. The new data let scientists pick out individual radio sources and trace massive jets in detail, opening the way to studying how jets interact with their surroundings and evolve over cosmic time.

The second major population is small, young galaxies still forming stars at high rates. Radio waves pass through the gas and dust that hide star formation from optical and infrared light, so radio observations give a clear view inside these galaxies. That helps astronomers trace how galaxies of the early Universe grew into the older, larger spiral and elliptical galaxies seen nearby.

The research team says the work shows the promise of LOFAR2.0, the upgraded telescope ASTRON is developing with European partners. The upgrade is meant to deliver wider, higher-resolution images and keep LOFAR competitive with next-generation instruments such as the Square Kilometre Array, which will combine the MeerKAT array in South Africa with the Murchison Radio-astronomy Observatory in Australia.

The next step is to lay this image beside observations of the same sky at other wavelengths. Optical, infrared, and X-ray data would allow astronomers to work out properties such as the distance and mass of each radio source.