Habitability of galaxies and the application of merger trees in astrobiology

Abstract

Galaxies represent the main form of organization of matter in our universe. Therefore, they are of obvious interest for the new multidisciplinary field of astrobiology. In particular, to study habitability of galaxies represents one of the main emerging challenges of theoretical and numerical astrobiology. Its theoretical underpinnings are, however, often confused and vague. Here we present a systematic attempt to list and categorize major causal factors playing a role in emergent habitability of galaxies. Furthermore, we argue that the methodology of cosmological merger trees is particularly useful in delineating what are systematic and lawful astrobiological properties of galaxies at present epoch vs. those which are product of historical contingency and, in particular, interaction with wider extragalactic environment. Employing merger trees extracted from cosmological N-body simulations as a new and promising research method for astrobiology has been pioneered by Stanway et al. (2018). We analyse the general issue of applicability of merger trees and present preliminary results on a set of trees extracted from the Illustris Project. In a sense, this approach is directly complementary to using large-scale cosmological simulations to study habitable zones of individual galaxies with high mass/spatial resolution; taken together they usher a new era of synergy and synthesis between cosmology and astrobiology.

Dynamical aspects of Galactic habitability in N-body simulations

“…All these different results suggest that habitability estimates are far too sensitive to initial assumptions, fine-tuning of the models, experiment design, and habitability determining factors explored (for a review of different factors influencing galactic habitability, see, e.g. Stojković, Vukotić, & Ćirković, 2019a). Considering the effect of stellar migrations on galactic habitability, this work has demonstrated that, in contrast to traditional GHZ studies, the boundaries of such an annulus are not likely to be firmly constrained.…”

“…However, it is certainly not impossible to discuss habitability in multiple ways. Since the dynamical issues have been mentioned but mostly skirted around, ever since the emergence of the GHZ concept, it makes sense to use the well-developed apparatus of N-body simulations to clearly separate dynamical parameters from the rest of the parameters of galactic habitability (for extensive discussion of those see, Stojković, Vukotić, & Ćirković, 2019a).…”

“…Bojnordi Arbab & Rahvar (2021) argue that the rate of disruptive stellar encounters is the most dependent on the stellar number density. This should be intuitively understandable, as discussed by Stojković, Vukotić, & Ćirković (2019a), just by examining Equation 2: cross-section should be treated as a constant, leaving us with two variables, one of which, relative stellar velocity v , does not vary as much. Naturally, this leads to conclusion that the rate of close encounters very well may be irrelevant, or at least redundant, on larger scales (i.e., for constraining the GHZ) while still applicable on smaller scales (e.g., when considering substructures and local over-densities).…”

“…While we still lack sufficient computing power to resolve mass at the level of individual stars, numerical simulations can still prove useful for studying continuous habitability conditions in galaxies. They already made their way into the galactic habitability studies (Vukotić et al, 2016;Forgan et al, 2017;Vukotić, 2018;Stanway et al, 2018;Stojković et al, 2019b;Stojković, Vukotić, & Ćirković, 2019a). It became evident that galactic habitability is far more subtle to understand than under the pioneering annular zone concept, which is still extensively used in circumstellar habitability.…”