From metallicity distributions to mutual information: A new perspective on stellar halo assembly

TL;DR

This paper introduces an information-theoretic approach to analyze the spatial and chemical structure of stellar halos, revealing diverse metallicity distributions and correlations that inform galaxy assembly history.

Contribution

It applies mutual information metrics to stellar halo data, providing a novel quantitative framework for understanding chemical and spatial correlations in galaxy formation.

Findings

Diverse metallicity distribution functions observed across halos.

Higher spatial anisotropy in low-metallicity stars, linked to disrupted satellites.

Mutual information increases with radius, indicating residual spatial-chemical coupling.

Abstract

The metallicity structure of stellar halos encodes the fossil record of galaxy assembly, tracing the chemical evolution and dynamical imprint of past mergers. Using five Milky Way-mass halos from the Aquarius simulations, we introduce an information-theoretic framework to quantify spatial-chemical correlations through the mutual information (MI) between angular position and metallicity. We divide stars in each halo into high- and low-metallicity populations based on their median metallicity and examine their metallicity distribution functions (MDFs), spatial anisotropies, and angular-metallicity couplings as a function of galactocentric radius. The MDFs exhibit remarkable diversity, ranging from single-peaked distributions dominated by one or two massive progenitors to broad or bimodal forms shaped by multiple accretion events, revealing the stochastic nature of halo assembly. The low-metallicity stars, primarily contributed by disrupted satellites, display higher spatial anisotropy and stronger angular clustering than their metal-rich counterparts. After removing bound satellites, anisotropy decreases significantly, yet high-metallicity stars remain marginally more anisotropic, reflecting the lingering debris of massive, centrally deposited progenitors. The mutual information between angular position and metallicity increases with radius before saturating in the outskirts, with the difference between the data and randomized controls confined mainly to the inner halo signifying residual spatial-chemical coupling driven by incomplete phase mixing. Our results demonstrate that information-theoretic diagnostics provide a powerful and intuitive way to quantify the chemical complexity of stellar halos and offer a promising route to compare simulations with forthcoming high-dimensional Galactic survey data.