Astrobiological Effects of Gamma-Ray Bursts in the Milky Way Galaxy
  A planet having protective ozone within the collimated beam of a Gamma Ray
Burst (GRB) may suffer ozone depletion, potentially causing a mass extinction
event to existing life on a planet's surface and oceans. We model the dangers
of long GRBs to planets in the Milky Way and utilize a static statistical model
of the Galaxy that matches major observable properties, such as the inside-out
star formation history, metallicity evolution, and 3-dimensional stellar number
density distribution. The GRB formation rate is a function of both the star
formation history and metallicity; however, the extent to which chemical
evolution reduces the GRB rate over time in the Milky Way is still an open
question. Therefore, we compare the damaging effects of GRBs to biospheres in
the Milky Way using two models. One model generates GRBs as a function of the
inside-out star formation history. The other model follows the star formation
history, but generates GRB progenitors as a function of metallicity, thereby
favoring metal-poor host regions of the Galaxy over time. If the GRB rate only
follows the star formation history, the majority of the GRBs occur in the inner
Galaxy. However, if GRB progenitors are constrained to low metallicity
environments, then GRBs only form in the metal-poor outskirts at recent epochs.
Interestingly, over the past 1 Gyr, the surface density of stars (and their
corresponding planets) that survive a GRB is still greatest in the inner galaxy
in both models. The present day danger of long GRBs to life at the solar radius
($R_\odot=8$ kpc) is low. We find that at least $\sim$65% of stars survive a
GRB over the past 1 Gyr. Furthermore, when the GRB rate was expected to have
been enhanced at higher redshifts, such as $z\gtrsim0.5$, our results suggest
that a large fraction of planets would have survived these lethal GRB events.
