Galactic traversability: a new concept for extragalactic SETI

Abstract

Interstellar travel in the Milky Way is commonly thought to be a long and dangerous enterprise, but are all galaxies so hazardous? I introduce the concept of galactic traversability to address this question. Stellar populations are one factor in traversability, with higher stellar densities and velocity dispersions aiding rapid spread across a galaxy. The interstellar medium (ISM) is another factor, as gas, dust grains and cosmic rays all pose hazards to starfarers. I review the current understanding of these components in different types of galaxies, and conclude that red quiescent galaxies without star formation have favourable traversability. Compact elliptical galaxies and globular clusters could be ‘super-traversable’, because stars are packed tightly together and there are minimal ISM hazards. Overall, if the ISM is the major hindrance to interstellar travel, galactic traversability increases with cosmic time as gas fractions and star formation decline. Traversability is a consideration in extragalactic surveys for the Search for Extraterrestrial Intelligence (SETI).

Artificial Broadcasts as Galactic Populations. I. A Point Process Formalism for Extraterrestrial Intelligences and Their Broadcasts

“…It takes time for intelligence to evolve, thus the cosmic epoch of the galaxy matters; distance also directly affects observables. There are many other factors that could affect the evolution and spread of ETIs: star formation history, metallicity distribution, stellar density, velocity dispersion, nuclear activity, and the nature of the interstellar medium all could be important (e.g., Gonzalez et al 2001;Lineweaver et al 2004;Dayal et al 2015;Di Stefano & Ray 2016;Gowanlock 2016;Balbi & Tombesi 2017;Lingam et al 2019;Lacki 2021).…”

“…Some possible attractor objects trace young stars instead of total stellar mass, like high-mass X-ray binaries (as in Vidal 2011), maybe even concentrating ETIs into a few clumps while leaving the rest of the galaxy empty. Even if they roughly follow stars, the dependence need not be linear; they might be far more prevalent in regions of higher stellar density because interstellar travel is easier, for example (Di Stefano & Ray 2016; Gajjar et al 2021;Lacki 2021). In the absence of secure knowledge of the relevant tradeoffs, I posit that the density of societies in inhabited galaxies follows the stars on a large scale.…”