Point a radio telescope at an ordinary corner of the northern sky for long enough and it stops looking empty. Astronomers led by Leiden University spent 200 hours doing just that with the Low Frequency Array (LOFAR). The resulting LOFAR radio image is, by the team’s account, the sharpest and most sensitive view of the sky yet made at low radio frequencies. It holds almost 63,000 radio sources in a patch about 25 times the area of the full Moon.
The target is ELAIS-N1, a northern field that telescopes at other wavelengths have already studied closely. The work, led by Jurjen de Jong of Leiden Observatory and ASTRON, has been accepted for publication in Astronomy & Astrophysics, and a preprint is available. The new image is about 20 times sharper than the most sensitive LOFAR picture of the field before it. It runs to eight billion pixels.
In radio astronomy, sharpness comes from the distance between antennas rather than the size of any single one. LOFAR links 38 stations in the Netherlands and 14 across Europe, the farthest nearly 2,000 km apart. Combining them is the hard part, because the ionosphere, Earth’s electrically charged upper atmosphere, distorts low frequency signals on their way down. The observations produced 400 terabytes of data. Improved calibration cut processing costs by more than a factor of ten, and the whole set was reduced on SURF’s Spider computing platform in a few months.
Two populations dominate the picture. Some sources are jets driven by supermassive black holes; others are galaxies busy forming stars, whose radio glow passes through the dust that hides that activity from optical and infrared telescopes. Fine detail is what lets astronomers tell the two apart and trace individual jets. For scale, an earlier sub-arcsecond map of the same field, built from 32 hours of observing, catalogued about 9,200 sources at its sharpest setting.
There is a catch, and it is distance. A radio image alone cannot say how far away each source lies or how massive it is. Those answers depend on matching the public data with optical, infrared and X-ray surveys of the same field. The cost in telescope time is also steep: 200 hours bought one small patch of sky.
The team argues the result shows that LOFAR2.0, ASTRON’s upgrade now under way, can stay competitive as the Square Kilometre Array takes shape in South Africa and Australia. For now, a field that looks blank to the eye has become a census of tens of thousands of galaxies. Most of them are still waiting for an address.
