Evaluating galactic habitability using high-resolution cosmological simulations of galaxy formation

Abstract

We present the first model that couples high-resolution simulations of the formation of Local Group galaxies with calculations of the galactic habitable zone (GHZ), a region of space which has sufficient metallicity to form terrestrial planets without being subject to hazardous radiation. These simulations allow us to make substantial progress in mapping out the asymmetric three-dimensional GHZ and its time evolution for the Milky Way (MW) and Triangulum (M33) galaxies, as opposed to works that generally assume an azimuthally symmetric GHZ.Applying typical habitability metrics to MW and M33, we find that while a large number of habitable planets exist as close as a few kiloparsecs from the galactic centre, the probability of individual planetary systems being habitable rises as one approaches the edge of the stellar disc. Tidal streams and satellite galaxies also appear to be fertile grounds for habitable planet formation.In short, we find that both galaxies arrive at similar GHZs by different evolutionary paths, as measured by the first and third quartiles of surviving biospheres. For the Milky Way, this interquartile range begins as a narrow band at large radii, expanding to encompass much of the galaxy at intermediate times before settling at a range of 2-13kpc. In the case of M33, the opposite behaviour occurs -the initial and final interquartile ranges are quite similar, showing gradual evolution. This suggests that galaxy assembly history strongly influences the time evolution of the GHZ, which will affect the relative time lag between biospheres in different galactic locations. We end by noting the caveats involved in such studies and demonstrate that high resolution cosmological simulations will play a vital role in understanding habitability on galactic scales, provided that these simulations accurately resolve chemical evolution.

‘Grandeur in this view of life’:N-body simulation models of the Galactic habitable zone

“…In more realistic cases, these effects might be partially balanced by minor mergers, resupplying the central regions with gas and stars, which remains an interesting topic for future studies. The simulations performed here are similar to ones recently reported by Forgan et al (2015), including very similar mass resolution (5 × 10 4 M vs.…”

“…Many caveats systematically presented by Forgan et al apply here as well, notably the impossibility to treat individual sterilization events due to local supernovae with the state-of-the-art resolution. The present model introduces the notion of continuous habitability, which changes its results somewhat in comparison to those presented in particular in Figure 7 of Forgan et al (2015). On the other hand, it is important to note a general similarity between results on the Milky Way habitability obtained in two independent studies, especially when it comes to the seemingly counter-intuitive conclusion that outer edges of the Galactic disk are more habitable than hitherto assumed.…”

“…First and foremost, together with results in Forgan et al (2015), this is the first use of cosmological simulations of galaxy formation for astrobiological purposes, and it makes sense to make it as simple baseline case as possible. It is necessary to get at least some hold on the simplest, unperturbed case and its effective dynamics before proceeding to more complex perturbed histories.…”

“…While the exact reasons will require a separate study to ascertain, we find that one or more of the following played an important part in creating the discrepancy: (i) the model of Gowanlock, Patton & McConnell (2011) applies specifically to the Milky Way, and uses a semi-analytic stellar density and star-formation history models specifically geared toward the local observations. In contrast, the present study like the one of Forgan et al (2015) uses a full numerical treatment of a Milky Way-like giant spiral galaxy. (ii) The metallicity gradient of Naab & Ostriker (2006) used by Gowanlock, Patton & McConnell (2011) is shallower than the effective metallicity gradient obtained in the cosmological simulations, and is shallower than the best-fit global gradient obtained by Rocha-Pinto et al (2000) and Tadross (2003).…”

Galactic habitable zone around M and FGK stars with chemical evolution models that include dust

“…Recently, the GHZ has been also studied in the cosmological context (ΛCDM) by Forgan et al (2015), Gobat & Hong (2016), Zackrisson et al (2016), and Vukotić et al (2016) showing which kind of halos can give rise to galactic structures in which habitable planets could be formed.…”