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 in the United Kingdom, 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 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.
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. Nearby galaxies, galaxy groups and galaxy clusters are a great place to look for these rare powerful signals, as these systems contain hundreds of billions of stars and many of these will host potentially habitable planets.
Since the original Breakthrough Listen surveys did not detect any technosignatures, Garrett & Siemion were able to place constraints on the luminosity function of potential extraterrestrial transmitters and limits on the prevalence of very powerful transmitters associated with the billions of stars comprising these systems have also been determined.
The paper, "Constraints on extragalactic transmitters via Breakthrough Listen observations of background sources", has been accepted for publication in Monthly Notices of the Royal Astronomical Society.
An optical colour image of the stellar field centred on a nearby star targeted by BL (HIP 71181, about 43 light years distant) showing the extent of the Green Bank Telescope beam circled in red. Members of a galaxy cluster 640 million lightyears distant are identified with the white circles. Yellow circles indicate cluster members outside the nominal sensitivity area of the Green Bank Telescope. Other indicated extragalactic objects include two quasi-stellar objects (QSOs) and an active galactic nuclei (AGN).
A plot of the transmission rate vs. pseudo-luminosity for this and other recent SETI results. The transmission rate is essentially a measure of how common radio technosignatures are, and the pseudo-luminosity is a measure of a given technosignature’s power. The gray line is defined by the two most constraining values - this work and that of Wlodarczyk-Sroka et al. (2020). Below the line, "terra incognita" is area of parameter space yet to be explored.