Fraser M. Smith | Searcharxiv

arXiv2026

Ergodicity of FIRE: star formation variations within and between simulated galaxies

We investigate the ergodicity of star formation in simulated galaxies from the FIRE-2 (Feedback In Realistic Environments) project. We restrict ergodicity considerations to being related to deviations from the star-forming main sequence (SFMS), and in turn whether ensemble averages across populations match time-averaged star formation histories (SFHs) based on simulated observable properties. We find that in these high-resolution simulations the deviations of individual galaxies from the SFMS tend to approach ergodic behavior over time, regardless of the SFMS definition adopted and the star formation estimator used. This trend persists when galaxy morphology, as traced by S\'ersic index, is considered despite the spheroid-dominated morphologies showing a smaller range of SFMS deviations than the disk-dominated morphologies. Unsurprisingly, we find more rapid convergence to ergodic behavior for star formation estimators based on shorter time scales ($10^{7}$ years) as opposed to longer ($10^{9}$ years). We caution that these findings should be considered in the context of the current sample and that further studies, particularly of high redshift evolution and the impact of active galactic nuclei should be investigated.

astro-ph.GA↗

arXiv2024

Examining partial ergodicity as a predictor of star formation departures from the galactic main sequence in isolated galaxies

Lacking the ability to follow individual galaxies on cosmological timescales, our understanding of individual galaxy evolution is broadly inferred from population trends and behaviours. In its most prohibitive form, this approach assumes that galactic star formation properties exhibit ergodicity, so that individual galaxy evolution can be statistically inferred via ensemble behaviours. The validity of this assumption is tested through the use of observationally motivated simulations of isolated galaxies. The suite of simulated galaxies is statistically constructed to match observed galaxy properties by using kernel density estimation to create structural parameter distributions, augmented by theoretical relationships where necessary. We also test the impact of different physical processes, such as stellar winds or the presence of halo substructure on the star formation behaviour. We consider the subtleties involved in constraining ergodic properties, such as the distinction between stationarity imposed by stellar wind feedback and truly ergodic behaviour. However, without sufficient variability in star formation properties, individual galaxies are unable to explore the full parameter space. While, as expected, full ergodicity appears to be ruled out, we find reasonable evidence for partial ergodicity, where averaging over mass-selected subsets of galaxies more broadly resembles time averages, where the average largest deviation across physical scenarios is 0.20 dex. As far as we are aware, this the first time partial ergodicity has been considered in an astronomical context, and provides a promising statistical concept. Despite morphological changes introduced by close encounters with dark matter substructure, subhaloes are not found to significantly increase deviations from ergodic assumptions.

astro-ph.GA↗