Artificial Broadcasts as Galactic Populations: III. Constraints on Radio Broadcasts from the Cosmic Population of Inhabited Galaxies

TL;DR

This paper establishes constraints on the population of artificial radio broadcasts in galaxies by analyzing radio source counts and background galaxy limits, including the first SETI constraints at ~250 GHz.

Contribution

It introduces a formalism to set bounds on artificial radio galaxy populations using observational data and compares different methodologies for detection limits.

Findings

Limits on artificial radio broadcasts are derived from radio source counts.

First SETI constraints at approximately 250 GHz are presented.

The abundance of Kardashev Type III broadcast populations is less than one in 10^17 stars.

Abstract

Any population of artificial radio broadcasts in a galaxy contributes to its integrated radio luminosity. If this radio emission is bright enough, inhabited galaxies themselves form a cosmic population of artificial radio galaxies. We can detect these broadcasts individually or set constraints from their collective emission. Using the formalism in Paper I and II, I set bounds on the artificial radio galaxy population using both of these methodologies. Measured radio source counts set limits on radio broadcasts across the radio spectrum, including the first Search for Extraterrestrial Intelligence (SETI) constraints at ~250 GHz. I compare these with commensal limits from background galaxies in the fields of large SETI surveys. The field limits are more powerful, but generally only over a limited luminosity range and for frequencies with dedicated SETI surveys. The limits are weaker when broadcasts clump into discrete hosts that are themselves extremely rare. I find that the abundance of Kardashev Type III radio broadcast populations is less than one in $10^{17}$ stars, about one in a million large galaxies. I also examine limits for a power-law distribution in broadcast luminosity.