Artificial Broadcasts as Galactic Populations: II. Comparing Individualist and Collective Bounds on Broadcast Populations in Single Galaxies

TL;DR

This paper compares individualist and collective bounds on extraterrestrial broadcast populations, demonstrating how confusion affects detection and providing constraints on broadcast numbers in various galaxies using observational data.

Contribution

It introduces a framework to evaluate collective bounds on artificial broadcasts, extending previous methods and applying them to multiple galaxies to improve population constraints.

Findings

Confusion blurs narrowband broadcasts in Virgo ellipticals at current observation levels.

The collective bound limits the number of broadcasts in the Milky Way to fewer than ~10^6 per star gigahertz.

Constraints extend to gamma-ray, neutrino, and gravitational-wave broadcasts in nearby galaxies.

Abstract

The search for extraterrestrial intelligence includes efforts to constrain populations of artificial broadcasts in other galaxies. Previous efforts use individualist methods, searching for single broadcasts with high signal-to-noise ratio. These would be detected as observables with extreme values. This approach is limited to very bright broadcasts and also is subject to confusion, where a large number of broadcasts blend together to form a noise continuum. The mean value of the total emission provides an additional collective bound: the luminosity of the transmitters is no higher than the galaxy's observed luminosity. Using the framework developed in Paper I, I evaluate how confusion affects individualist searches. I then compare individualist and collective approaches for radio broadcasts from the Milky Way, M31, and three Virgo Cluster elliptical galaxies. For current observations, confusion blurs narrowband radio broadcasts together in the Virgo ellipticals when there is one broadcast per gigahertz per 1000 stars. The collective bound implies fewer than $\sim 10^6 (\overline{\ell}/10^{13} W)^{-1}$ L-band broadcasts per star gigahertz GHz in the Milky Way and is about 10 and 400 times stronger in M31 and M59, respectively. Applying the collective bound to the far-infrared--radio correlation yields constraints on radio broadcast populations in star-forming galaxies throughout the Universe. The collective bound allows us to rule out large regions of broadcast population parameter space even for distant galaxies. It also imposes constraints on gamma-ray, neutrino, and gravitational-wave broadcasts in the nearest galaxies.