Galactic Traversability: A New Concept for Extragalactic SETI
  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 (CRs) 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 favorable 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).
