2023MNRAS.519.4581G

The Breakthrough Listen Initiative has embarked on a comprehensive Search for Extraterrestrial Intelligence (SETI) survey of nearby stars in the Milky Way that is vastly superior to previous efforts as measured by a wide range of different metrics. SETI surveys traditionally ignore the fact that they are sensitive to many background objects, in addition to the foreground target star. In order to better appreciate and exploit the presence of extragalactic objects in the field of view, the ALADIN sky atlas and NASA/IPAC Extragalactic Database (NED) were employed to make a rudimentary census of extragalactic objects that were serendipitously observed with the 100-m Green Bank Telescope observing at 1.1-1.9 GHz. For 469 target fields [assuming a full width at half-maximum (FWHM) radial field of view of 4.2 arcmin], NED identified a grand total of 143 024 extragalactic objects, including various astrophysical exotica e.g. active galactic nuclei (AGN) of various type, radio galaxies, interacting galaxies, and one confirmed gravitational lens system. Several nearby galaxies, galaxy groups, and galaxy clusters are identified, permitting the parameter space probed by SETI surveys to be significantly extended. Constraints are placed on the luminosity function of potential extraterrestrial transmitters assuming it follows a simple power law and limits on the prevalence of very powerful extraterrestrial transmitters associated with these vast stellar systems are also determined. It is demonstrated that the recent Breakthrough Listen Initiative, and indeed many previous SETI radio surveys, places stronger limits on the prevalence of extraterrestrial intelligence in the distant Universe than is often fully appreciated.