Astrobiological Effects of Gamma-Ray Bursts in the Milky Way Galaxy

Abstract

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, which matches major observable properties, such as the inside-out star formation history (SFH), metallicity evolution, and three-dimensional stellar number density distribution. The GRB formation rate is a function of both the SFH 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 SFH. The other model follows the SFH, 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 SFH, 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), which 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 e =8 kpc) is low. We find that at least ∼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0.5, our results suggest that a large fraction of planets would have survived these lethal GRB events.

Multiverse Predictions for Habitability: Fraction of Life That Develops Intelligence

“…For our values, the fraction of the galaxy that can affect our planet is pr : {r disk q 2 " 0.03, leading to a lethal event every few 100 Myr [65]. This is compatible with the expectation that there has been one GRB driven extinction since the emergence of complex life.…”

The Habitability of Our Evolving Galaxy

“…Using the metallicity-dependent GRB rate and moderate GRB fluence lethality threshold values, Piran and Jimenez (2014) find that over the past 1 Gyr leading up to the present day, there is a 60% chance of a planet at the Earth's galactocentric radius to be irradiated by a long GRB. Li and Zhang (2015) find that there is roughly one long GRB at Earth's radius every 500 Myr, and Gowanlock (2016) reports that ∼ 35% of planets at the solar radius are within the beam of a long GRB over the past 1 Gyr.…”

“…The surface density of stars that are not within the beam of a GRB over the past 1 Gyr and 5 Gyr, as a function of galactocentric radius (Model 2 fromGowanlock 2016). Reproduced by permission of the author.…”

Radiation as a Constraint for Life in the Universe

“…These events, then, will not directly track with active star formation. Simulations that track star formation and metallicity have been used to investigate where, as well as when, different regions of our own galaxy may have been habitable, as controlled by SNe and GRBs (see Gowanlock et al, 2011;Morrison and Gowanlock, 2015;Gowanlock, 2016). In general, they find that the inner part of the galaxy is more dangerous.…”