Radio Survey Maps Hundreds of Thousands of New Galaxies

The LOFAR survey, based in The Netherlands, has released a bonanza of new sources. And with only 2% of the sky covered so far, this is only the beginning.

A unique sky survey at low radio frequencies has turned up more than 300,000 new sources, many of which are radio galaxies and quasars in the distant universe. And there’s much more to come: The new discoveries, published in a special issue of Astronomy & Astrophysics, are based on just 2% of the eventual survey volume.

“For the first time, we now have high-quality images of the radio sky at these low frequencies,” says Huub Röttgering (Leiden University, The Netherlands), principal investigator of the LOFAR Surveys Key Science Project. “That’s the real breakthrough.”

The Low Frequency Array (LOFAR), operated by ASTRON, Netherlands Institute for Radio Astronomy, is a novel instrument, consisting of some 100,000 simple antennas, grouped in 50 stations and connected to a central supercomputer through fiber optics. Most of the LOFAR stations are located in the Netherlands, but 12 of them are spread all over Europe, from Ireland to Poland and from Sweden to France.

Eventually, the Low-frequency Two-meter Sky Survey (LoTSS) will map the whole northern sky, but the first data release covers just 424 square degrees, centered on the handle of the Big Dipper. A total of 58 “radio pointings,” each 8 hours long, have been carried out at frequencies between 120 and 168 megahertz, yielding enough data to fill 10 million DVDs.

According to Röttgering, the biggest problem has been removing the effects of ionospheric turbulence, the radio equivalent of stars twinkling at visible wavelengths. Through clever mathematical tricks, scientists have sharpened the data to see details 6 arcseconds across.

The new catalog has 325,694 radio sources, accompanied by 58 high-resolution mosaic radio images. For about 70% of the sources, visible-light counterparts were found in existing data from the Sloan and Pan-STARRS surveys, giving astronomers rough distance estimates to the objects.

The radio waves from the galaxies that LOFAR has found are primarily generated by electrons spiraling along magnetic field lines that thread jets coming from the galaxies’ central black holes. Once these electrons have been spiraling for a while, they slow down and emit lower-frequency radio waves, so LOFAR is seeing jet activity on much longer timescales than existing higher-frequency radio surveys.

One intriguing result is that all relatively massive galaxies exhibit jet activity close to their cores, which suggests that their central black holes are feeding more or less continuously. The new observations should shed light on the evolution of supermassive black holes over cosmic time. “Eventually, we hope to find the very first supermassive black holes in the history of the universe,” says Röttgering.

The 26 papers in Astronomy & Astrophysics, authored by more than 200 scientists from 18 countries, also cover LOFAR data on galaxy clusters. According to Annalisa Bonafede (University of Bologna, Italy), one surprising result is that even isolated clusters that aren’t interacting with their neighbors still produce radio emission from charged particles speeding among the gas between galaxies, though the emissions are at a very low level.

The future Square Kilometre Array (SKA) has a low-frequency segment based in Australia, which will be even more sensitive than LOFAR, but it won’t see details as sharply. According to Röttgering, LOFAR’s high spatial resolution (it may discern details up to 0.5 arcseconds across under ideal circumstances) will be unsurpassed for a long time to come.