A patch of sky about 25 times the area of the full Moon now holds 62,916 catalogued radio sources. Most of them are faint, and many are distant galaxies that until now showed up as smudges or not at all. The catalogue comes from a new image of the ELAIS-N1 field made with the Low Frequency Array (LOFAR). It is 8 billion pixels across, sharper than one arcsecond, and according to its authors it is the deepest radio image ever made at frequencies below 1.28 GHz.

The Leiden-led international team posted the paper to arXiv on November 4 under the title "Pushing the frontiers of low-frequency radio astronomy: An ultra-deep sub-arcsecond LOFAR image of ELAIS-N1". It has been accepted for publication in Astronomy & Astrophysics. The lead author is J. M. G. H. J. de Jong, and the co-authors include L. K. Morabito, R. J. van Weeren, H. J. A. Röttgering and T. W. Shimwell.

What they actually built

The image combines 200 hours of LOFAR observations in the 115 to 168 MHz band, covering a field 2.5 degrees on a side. The raw data came to about 400 terabytes, according to Astronomie.nl.

Two numbers describe the result. The first is sensitivity. At the centre of the field, the image reaches a point-source noise level of 5.3 microjanskys per beam, at a resolution of 0.33 by 0.39 arcseconds. Averaged over the whole area, the figure is 7.4 microjanskys per beam at 0.36 by 0.45 arcseconds. The paper describes these as the deepest radio images below 1.28 GHz and among the deepest at any radio frequency.

The second is sharpness. The new image is 20 times sharper than the previous most sensitive LOFAR image of the same area. The Astronomie.nl announcement gives a sense of the scale: at this resolution you could pick out a coin from roughly 10 kilometres away.

Resolution matters as much as depth here. In lower-resolution images, distant galaxies and black-hole-driven jets appear as featureless blobs, and it is often hard to tell whether a blob is one galaxy, several galaxies, or a jet. Sub-arcsecond resolution lets astronomers separate those cases and trace the structure inside them.

Getting the pieces to line up

An image this sharp is only useful if it lines up with images taken at other wavelengths. To align the image's positions on the sky, the team used 95 quasars whose positions are measured by Gaia. Quasars are compact and very distant, so they work as stable reference points. According to the paper, the resulting positional offsets for those same sources are tiny: about -3 ± 27 milliarcseconds in right ascension and 2 ± 17 milliarcseconds in declination.

The final catalogue of 62,916 unique sources does not come from the high-resolution image alone. The team combined it with previously released deep LOFAR images at 6 arcseconds resolution, which gives a census across the full 2.5-degree square.

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What's in the picture

Two kinds of object dominate the catalogue, according to the announcement. The first is active galactic nuclei (AGN) that drive radio jets. These are the luminous centres of galaxies, powered by matter spiralling onto a supermassive black hole. The second is star-forming galaxies. Because radio waves pass through the dust that hides star formation from optical and infrared telescopes, radio observations give an unobscured picture of those galaxies.

The image also shows finer structure. Universe Today reports numerous filaments around radio galaxies, especially near galaxy clusters. Being able to trace them in detail gives astronomers more to work with when studying how radio jets interact with their environment and evolve over cosmic time.

The computing problem

Combining 400 TB of interferometer data into one sub-arcsecond image is mostly a computing and calibration problem. The team says improved calibration and data-reduction methods cut the cost of the processing by more than a factor of ten in the past few years.

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Jurjen de Jong, a postdoctoral researcher at Leiden Observatory and ASTRON who led the work, said the entire dataset could be processed on Spider, the high-performance data processing platform hosted at SURF in the Netherlands, in just a few months. That timescale is part of the point. A method that takes years of computing per field is a one-off showpiece. One that takes months can become a standard tool.

Why It Matters

The obvious headline is the record: the deepest radio image below 1.28 GHz. The more lasting value is probably elsewhere, in three places.

The data is public. The team has made the data publicly available for the astronomical community. Much of the scientific value will come from combining it with images of the same patch of sky from other telescopes, in optical, infrared and X-ray light, from which astronomers can derive properties such as the distance and mass of each radio source. A catalogue of nearly 63,000 radio sources with precise positions gives other researchers a large, ready-made sample.

The method scales. A more than tenfold drop in processing cost, and a turnaround of a few months on a shared national computing platform, suggest that deep, high-resolution LOFAR imaging of large fields is now computationally feasible. The paper makes the same point, saying that imaging many observations over a wide field at sub-arcsecond resolution is now computationally feasible.

It points to what comes next. According to the researchers, the work shows the potential of the upgraded LOFAR2.0, which ASTRON is carrying out together with its European partners. De Jong says it shows that LOFAR2.0 will remain competitive in the era of the Square Kilometre Array, which Universe Today describes as combining the MeerKAT array in South Africa and the Murchison Radio-astronomy Observatory in Australia. A field imaged this deeply, with methods that can be repeated, gives a benchmark for comparing future instruments.

For now, the result is a 2.5-degree square of sky in which tens of thousands of galaxies, jets and filaments are resolved in more detail at these frequencies than ever before, and the data is available to the astronomical community.

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