Reanalysis of Breakthrough Listen data places new constraints on powerful extragalactic technosignatures

At the 2022 International Astronautical Congress in Paris, France The University of Manchester and Breakthrough Listen (the initiative to find signs of intelligent life in the universe) announced a reanalysis of existing data that extends the search for extraterrestrial intelligence (SETI) into a new realm of parameter space and places stringent limits on the existence of extragalactic technosignatures.

Recognising that radio surveys targeting nearby stars are also sensitive to background cosmic objects, in particular galaxies, galaxy groups and galaxy clusters, Prof. Michael Garrett at The University of Manchester, collaborating with Berkeley SETI Director Dr Andrew Siemion (who is also a Visiting Professor at Manchester) have been able to place new limits on the prevalence of very powerful transmitters in galaxies and other cosmic objects located outside of our own Milky Way. 

They focused on previous observations made by the Green Bank Telescope (GBT) looking at 469 Breakthrough Listen target fields that were located away from the obscuring gas and dust in the plane of our own Milky Way. In these fields they identify more than 140000 extragalactic systems, including various astrophysical exotica: interacting galaxies, various types of active galactic nuclei, radio galaxies, and several gravitational lens systems.

Most of these sources are located at cosmological distances, but the inventory also includes several nearby galaxies, galaxy groups and galaxy clusters. Although these systems are located many millions of light years away, if the strength of technosignatures follow an approximate power-law distribution (as transmitters here on Earth do), there might be a few rare but very bright signals that are detectable.

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