Record Resolution With LoFar’s 70,000 Antennas (Astronomy)

An international team of astronomers, including researchers from INAF and the University of Bologna, has published the most detailed radio band images ever seen of distant galaxies, revealing their inner workings in unprecedented detail. Images with an angular resolution below the second of arc, obtained thanks to data collected by the Low Frequency Array (LoFar), a network of over 70 thousand small antennas scattered in nine European countries

What you see in the image opposite are jets of radio waves emitted by the supermassive black hole at the center of Hercules A, a galaxy with an active core located in the constellation of Hercules at a distance of two billion light years from us. It’s not an artistic representation: those jets are just “the real thing” – the image that emerges from the data. Not an artistic representation, we said, but not even a traditional photograph. Being invisible to optical telescopes, radio antennas are needed to “see” these jets. Special radio antennas, however, tuned to very normal frequencies: more or less around the traditional 88-108 MHz range used by FM stations here on Earth. But there’s more: to produce such detailed images of galaxies billions of light years away, a single radio antenna is not enough. A technique is needed – saidinterferometry – which requires dozens of antennas. Sometimes hundreds or thousands. And they must be far from each other. Well, to reconstruct the image at the beginning – and the others you see in the animation below – over 70 thousand antennas spread over eight European countries were used: the antennas of the LoFar network . The result, presented in a series of ten articles put online today and which will be published starting tomorrow in Astronomy & Astrophysics , is a record: a resolution of 300 milliarcoseconds, the highest ever reached at these frequencies.

“These are the first low-frequency radio images ever obtained with a quality and angular resolution comparable to those produced by the Hubble telescope in the optical band. To do this “, the first author of one of the ten articles, the astrophysicist Etienne Bonnassieux , postdoc researcher at the University of Bologna and associate Inaf , explains to Media Inaf ,” we had to understand how to correct the effects of the ionosphere, which cause the correlation of radio signals received by international LoFar stations: it is a disturbance similar to seeing for ground-based telescopes, but our stations are far from each other even many hundreds of km ».

In this animation, some radio galaxies taken from the article by Morabito et al. (2021). The gif illustrates the improvement obtained by moving from standard resolution to high resolution, showing the details that we can see using the new techniques (click to enlarge). Credits: LK Morabito; LoFra Surveys Ksp

And what do these images tell us? Thanks to the fact that radio waves – unlike normal visible light – can easily pass through the clouds of gas and dust that envelop many astronomical objects, LoFar’s antennas allow astronomers to see what is happening in star-forming regions. for instance. Or in active galactic nuclei, in fact, where supermassive black holes lodge. Black holes that devour the falling matter towards their event horizon, thus triggering the emission of the very powerful radio jets picked up by the LoFar antennas, allowing us to observe in detail the internal structure and to reveal aspects unknown until now. The resolution obtained is also a record for LoFar itself: these images are 20 times sharper than those that the European radio telescope had previously managed to produce. This is thanks to the innovative way the team of scientists used the array– that is to say, the set of antennas. Usually, to produce standard resolution images, while acquiring data with all antennas, only the signals from those located in the Netherlands are correlated, which allows to obtain with interferometry a “virtual telescope” with a collecting mirror from 120 km in diameter – such is in fact the maximum distance between the Dutch antennas. Instead, using the signals of all 70 thousand European antennas, as was done in this case, it is as if the diameter of the virtual mirror had increased up to almost 2000 km, thus increasing the resolution by about twenty times.

Comparison of resolutions (see panels above): in optical band, in low resolution radio band and with high resolution LoFar (click to enlarge). Credits: LK Morabito / WL Williams; Desi Legacy Imaging Surveys (optical)

However, the high number of antennas and the large extension of the area in which they are arranged are only two of the requirements necessary to obtain such sharp images. It is also necessary that the 70 thousand antennas work as if they were a single antenna, therefore it is essential to correlate the signal acquired by each of them. Unlike arraysconventional, which to produce images correlate the signals from the various antennas in real time, the signals collected by each LoFar antenna are first digitized, then sent to the central processor and finally combined to create an image. Each LoFar image is therefore the result of the combination of signals from over 70,000 antennas, which implies enormous computing powers: we are talking, for a single image, of over 13 terabits of raw data per second – the equivalent of three hundred DVDs – from to elaborate.

And we are only at the beginning. “What we want to do now”, in fact, another of the authors of the articles published on A&A , the astrophysicist of INAF from Bologna, Gianfranco Brunetti, tells Media Inaf, Italian coordinator of the LoFar collaboration, «is to map large areas of the sky with high angular resolution. In this case a difficulty is represented by the great computational demands: it will in fact be necessary to be able to effectively distribute the calculations of the complex algorithms for data analysis on supercomputers. This is the new challenge. Think that today to map a single LoFar pointing at 140 MHz at high angular resolution – an area of ​​the sky of 2.5 × 2.5 degrees, therefore – it takes about seven thousand hours of computation on a node of a latest generation cluster ».

Speaking of calculation, it should finally be emphasized that, in addition to the scientific results, the LoFar team has also made public its algorithms – and in particular the so-called data analysis pipeline , described in detail in one of the articles published on A&A , so to allow anyone to produce high-resolution images with relative ease. “Our goal”, concludes another scientist on the LoFar team, radio astronomer Leah Morabito of Durham University, “is to enable the scientific community to use the entire European network of LoFar telescopes to produce science. without being forced to take years to become experts ”.

Featured image: Hercules A is powered by a supermassive black hole, located at its center, which feeds on the surrounding gas and channels part of this gas in extremely fast jets. The new high-resolution observations made with LoFar (click to enlarge) have shown how the intensity of this jet has a modulation – from stronger to weaker, and vice versa – over a period of a few hundred thousand years. This variability is at the origin of the beautiful structures that we can admire in the giant lobes, each of which is about the size of the entire Milky Way. Credits: R. Timmerman; LoFar & Hubble Space Telescope

Provided by INAF

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