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

TL;DR

This study investigates whether the assumption that galaxy star formation properties are ergodic holds true, finding evidence for partial ergodicity in isolated galaxies, which impacts how we interpret galaxy evolution from population data.

Contribution

First to explore partial ergodicity in galaxies, showing it can approximate individual evolution through ensemble averages, with implications for galaxy evolution studies.

Findings

Partial ergodicity is plausible in isolated galaxies.

Subhaloes do not significantly affect ergodic deviations.

Average deviation across scenarios is about 0.20 dex.

Abstract

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.