Active Galactic Nuclei: Boon or Bane for Biota?

Abstract

Active galactic nuclei (AGNs) emit substantial fluxes of high-energy electromagnetic radiation, and have therefore attracted some recent attention for their negative impact on galactic habitability. In this paper, we propose that AGNs may also engender the following beneficial effects: (i) prebiotic synthesis of biomolecular building blocks mediated by ultraviolet (UV) radiation, and (ii) powering photosynthesis on certain free-floating planets and moons. We also reassess the harmful biological impact of UV radiation originating from AGNs, and find that their significance could have been overestimated. Our calculations suggest that neither the positive nor negative ramifications stemming from a hypothetical AGN in the Milky Way are likely to affect putative biospheres in most of our Galaxy. On the other hand, we find that a sizable fraction of all planetary systems in galaxies with either disproportionately massive black holes (∼10 9–10 M ⊙ ) or high stellar densities (e.g., compact dwarf galaxies) might be susceptible to both the beneficial and detrimental consequences of AGNs, with the former potentially encompassing a greater spatial extent than the latter.