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 traversabil- ity, 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 galax- ies 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 hin- drance 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)
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