Life in Elliptical Galaxies: Hot Spheroids, Fast Stars, Deadly Comets?
  Elliptical galaxies have dynamically hot ($\sigma_{1D}$ ~ 100 -- 300 km
s$^{-1}$) populations of stars, and presumably, smaller objects like comets.
Because interstellar minor bodies are moving much faster, they hit planets
harder and more often than in the local Galaxy. I estimate the rates for
Chicxulub-scale impacts on an Earth-size planet in elliptical galaxies as a
potential habitability constraint on intelligent life. Around most stars in a
normal elliptical galaxy, these planets receive only ~0.01 -- 0.1 Gyr$^{-1}$,
although hazardous rates may be common in certain compact early-type galaxies
and red nuggets. About 5% of the stellar mass is in a region where the rate is
>10 Gyr$^{-1}$, large enough to dominate the mass extinction rate. This
suggests that elliptical galaxies have an exclusion zone of order one hundred
parsecs in radius around their centers for the evolution of intelligent life.
