The assembly histories of nearby galaxies
Galaxy formation and evolution is a collection of complex physical processes. Different processes can affect galaxies in vastly different ways, and so the final state of a galaxy depends on the delicate interplay between them. While evolution is a continuous process over many billions of years, galaxies are only observed at a single instance in time. From single-epoch data of external galaxies, star-formation histories provide constraints on when the stars formed, but not their actual history - namely where they came from and when they entered their present-day host (if formed externally). These histories are instead encoded in the orbits which the stars inhabit within their host. For resolved galaxies such as the Milky Way and other Local-Group members, orbital, chemical, and chronological information can be measured for individual stars, and such studies have clearly shown that this combination of properties reveals distinct events in a galaxy’s history. Due to the inherent limitations of projected data, however, this has not yet been achieved for unresolved galaxies.
In this work we develop a new methodology in order to access intrinsic (de-projected) properties from the projected measurements of external galaxies. We build upon Schwarzschild orbit-superposition dynamical models, which allow for general shapes of the velocity ellipsoid and matter distribution. Using numerically-integrated orbits within a self-consistent model for the gravitational potential, we are able to reproduce all available stellar properties in projection, which we show is able to recover the true underlying distributions. In doing so, we are able to investigate the chemistry (metallicity), ages, and kinematics of the stars simultaneously, analogous to resolved studies. We develop diagnostics that enable the derivation of unique assembly histories constrained by the observational data. Intrinsic chemo-dynamical correlations are measured from these models, showing that stars which formed in the early Universe exhibit less ordered rotation compared to those which formed later. We find that for the massive field galaxy NGC 3115, the accretion of external material is the dominant driver of its evolution.
These models are applied to a small sample of galaxies in a nearby (z = 0) cluster with high-quality spatially-resolved spectroscopy. We find evidence that the dense environment suppresses the accretion of external material, while also inducing mild kinematic perturbations. Using the increased sample size, we uncover additional intrinsic chemo-dynamical correlations which were previously only accessible to Local-Group galaxies, indicating that stars with higher heavy-element abundances exhibit stronger rotational support, irrespective of their age.
This methodology is finally extended to include new spatially-resolved measurements of the stellar Initial Mass Function (IMF). These models provide further evidence of significant variation of the IMF between galaxies. They also provide tentative evidence that it is approximately consistent for all the stars formed within a given galaxy, but that those which were brought in externally were formed with distinct IMF. This result, together with the other correlations studied by this work, show that the local conditions of star-formation across cosmic time can be analysed even with single-epoch, projected data of external galaxies.
