Possible Role of Gamma Ray Bursts on Life Extinction in the Universe

As a copious source of gamma rays, a nearby galactic gamma ray burst (GRB) can be a threat to life. Using recent determinations of the rate of GRBs, their luminosity function, and properties of their host galaxies, we estimate the probability that a life-threatening (lethal) GRB would take place. Amongst the different kinds of GRBs, long ones are most dangerous. There is a very good chance (but no certainty) that at least one lethal GRB took place during the past 5 gigayears close enough to Earth as to significantly damage life. There is a 50% chance that such a lethal GRB took place during the last $500\times 10^6\mathrm{years}$, causing one of the major mass extinction events. Assuming that a similar level of radiation would be lethal to life on other exoplanets hosting life, we explore the potential effects of GRBs to life elsewhere in the Galaxy and the Universe. We find that the probability of a lethal GRB is much larger in the inner Milky Way (95% within a radius of 4 kpc from the galactic center), making it inhospitable to life. Only at the outskirts of the Milky Way, at more than 10 kpc from the galactic center, does this probability drop below 50%. When considering the Universe as a whole, the safest environments for life (similar to the one on Earth) are the lowest density regions in the outskirts of large galaxies, and life can exist in only $\approx 10\%$ of galaxies. Remarkably, a cosmological constant is essential for such systems to exist. Furthermore, because of both the higher GRB rate and galaxies being smaller, life as it exists on Earth could not take place at $z>0.5$. Early life forms must have been much more resilient to radiation.

Figure 1

The percentage of stars as a function of metallicity in the Milky Way disk with ages $1<t/\mathrm{Gyr}<5$ (the solid orange line) and with ages $<1\mathrm{Gyr}$ (the solid black line), as obtained by Casagrande et al. [35]. The distribution of LGRB metallicity, as obtained by Jimenez and Piran [23], by matching the RGB global rate to the global star formation rate of galaxies (the solid green line) and that from direct metallicity determinations of LGRBs (the dashed line) [26] and Cucchiara et al. [28] from DLA systems (solid red line). The overlap between the LGRB and Milky Way star distributions is only at the few % level.

Figure 2

(Left $y$ axis) The mass fraction of the galaxy from which the fluence on a planet will exceed $100\mathrm{kJ}/{\mathrm{m}}^2$ for a given explosion energy (the $x$ axis). The colored curves correspond to different locations of the life-harboring exoplanet (2.15,4,8.5, and 16 kpc from the Galactic center). We have adopted for the MW an exponential disc with a scale length of 2.15 kpc. The right $y$ axis provides (for the gray curves) the number of GRBs per erg in the MW in the past 5 Gyr. For a given energy, the product of the corresponding colored and gray curves gives the number of damaging GRBs to life per energy interval.

Figure 3

The probability distribution function, $p$, of the average number of lethal LGRBs (top panel) and sGRBs (bottom panel) that irradiated Earth in the past Gyr with enough flux to cause severe life extinction ($100\mathrm{kJ}/{\mathrm{m}}^2$). For LGRBs we show the case where we applied a 10% metallicity bias.

Figure 4

The probability, $P(⟨N⟩)$, of having on average more than one lethal GRB in the past Gyr for an exoplanet at a distance $r$ from the center of the Milky Way. The gray line shows the fraction of mass in the Milky encompassed within a radius $r$. The dashed line is for LGRB, assuming no metallicity correction.