Astronomers compile largest MeerKAT radio source catalog to date

Using MeerKAT data, an international team including astronomers from MPIfR (Bonn, Germany) has compiled the largest catalogue of radio sources in the entire MeerKAT survey to date. With this catalogue, they were able to measure the cosmic radio dipole, a cosmological effect that results from the Earth’s motion through the universe, and provides an important test of our theories of cosmology at the largest scales.

The new catalog and accompanying scientific results are described in a paper published on the preprint server arXivand have been accepted for publication in Astronomy and astrophysics.

This new measurement demonstrates the value of MeerKAT’s sensitive data and shows that this deep data provides extremely valuable information on the origin of the cosmic dipole effect.

When we look at the sky at radio wavelengths, we mostly see extremely distant galaxies rather than stars. Observing the radio sky provides a clear window into the evolution of galaxies, black holes, and gas in the universe, and can also show what the universe looks like on the largest scale.

The MeerKAT Absorption Line Survey (MALS), powered by the exceptional imaging sensitivity and fidelity of the MeerKAT radio observatory in South Africa, has produced extremely sensitive images detecting nearly a million radio sources from 391 telescope pointings.

This is the largest catalogue produced by a MeerKAT survey to date, one of the few radio catalogues containing a million or more sources. Because of the emphasis on depth rather than sky coverage, many sources were detected for the first time.

“The depth and breadth of this continuum catalog occupies a unique position among modern radio continuum studies. The public release will allow the community to address a wide range of questions related to the evolution of galaxies and the universe,” says Neeraj Gupta, an astronomer at the Interuniversity Center for Astronomy and Astrophysics (IUCAA) and principal investigator of the MALS project.

To obtain these detailed images from the large amounts of raw data produced by MeerKAT, a sophisticated processing pipeline and data storage facility are maintained at IUCAA in India. The images and catalogues were further analysed and prepared for public release at the Max Planck Institute for Radio Astronomy (MPIfR) in Germany by Jonah Wagenveld, lead author of the paper presented here.

This vast catalog allowed the MALS team to measure the cosmic dipole, a subtle effect caused by the movement of the solar system through the universe. This effect makes sources appear more numerous in the direction of this movement and fewer in the opposite direction.

The direction and amplitude of the Earth’s motion in the universe have so far been established by measurements of the cosmic microwave background. However, numerous measurements have shown that the amplitude of the cosmic dipole effect, which should be directly related to the speed of this motion, is much higher than expected.

This suggests that the dipole might not be caused simply by the speed of motion, but by a real difference in the density of sources in different directions of the sky, something that should not happen according to cosmological models.

Somewhat surprisingly, the new MALS measurement is consistent with predictions. Although it is not yet clear why this is so, it may be related to the design of the survey, which covers small areas of the sky at a very deep level rather than the broader but shallower sky coverage of other radio surveys. As a result, many faint “normal galaxies” are present in the deep catalogue, which undoubtedly influences the dipole measurement.

“Measuring the dipole is an extremely important test of cosmology and can tell us whether our fundamental assumptions about the structure of the universe are correct,” says Jonah Wagenveld, an astronomer at MPIfR and lead author of the paper.

The mystery is far from solved, however, and future larger catalogues, either from MALS using MeerKAT’s low-frequency UHF band or from future observatories, will have to unpack these results and resolve the tension.

“The consistent and automated processing was essential to properly control subtle effects in the data that could hamper the accuracy of our measurements. This new study is the stepping stone for future large-scale radio studies, with the Square Kilometer Array and the Deep Synoptic Array,” notes Hans-Rainer Klöckner, a researcher at MPIfR who conceptualized the use of MALS for dipole measurements.

This is the second in a series of radio continuum and spectral line data from MALS. The release of these data is a team effort. The MALS catalogues and images are publicly available at: The MALS team is an international collaboration of researchers from around the world. The project is led by Neeray Gupta of IUCAA, India.