Austrian Early Career Conference 2024

Contribution:
Talk

Authors:
Christine Ackerl

Affiliations:
Universität Wien - Institut für Astrophysik

Title:
Galaxy archaeology - The quest for ancient mergers

Abstract:
The LCDM cosmological model requires that gravitational attraction continuously leads to mergers of dark matter halos. The galaxies and their halos we observe in the local universe, must have formed hierarchically over time, with smaller systems being accreted by larger ones. State-of-the-art cosmological simulations do reproduce the observed large-scale hierarchical clustering of galaxies. However, challenges concerning the assembly of galaxies and the interplay of baryonic processes and dark matter remain.
To bridge the gap between simulations and observations it is crucial to understand how the structure and observed properties of galaxy stellar halos are impacted by mergers. To achieve this we need detailed *observational* constraints on the hierarchical assembly of galaxies. Because the dominant epoch of galaxy assembly lies in the early universe, this is an extremely challenging task. Typically the stellar systems of the most significant mergers are well mixed by redshift ~0.
The aim of galaxy archaeology is to disentangle these accreted stellar populations from the stellar halo light of nearby galaxies using techniques such as stellar population fitting and dynamical modelling. With the advent of integral field spectroscopy, this research field has seen significant progress. However, the required mapping from spectra of the stellar halo light to full age and metallicity distributions is methodologically challenging. Interpreting merger signatures in the derived age and metallicity distributions adds an additional level of uncertainty. This cumulative build-up of systematic uncertainties makes it challenging to uncover ancient mergers with such a "top-down" and step-by-step approach.
In this talk I present a novel "bottom-up" approach, where we forward model the assembly of galaxy halos. In this manner we can condense the full assembly history into multiple model outputs suitable for comparison to raw/simpler observational data of galaxy halo light. Additionally we can, for the first time, produce simultaneous predictions for multiple complementary tracers of the assembly of a galaxy's halo - globular cluster populations and chemical/age distributions of stars.
Our approach is as follows: we model accretion times, mass ratios and tidal stripping for each dark matter subhalo. We use agnostic/stochastic star formation histories, which are based on observations of galaxy population star formation rates at different redshifts to set the stellar mass growth for each galaxy - including chemistry and quenching. Additionally, we model the chemical properties of each galaxy’s star clusters self-consistently with the star formation and chemical evolution histories of their host galaxies.
The resulting library of models contains millions of self-consistent halo + baryon assembly histories for hundreds of different total stellar ex-situ fractions and host galaxy stellar masses. For each assembly history we have the flexibility to predict observables of:
- Composite (in-situ, mixed with ex-situ) spectra and spectral energy distributions of the galaxy stellar halo light
- Age-metallicity distributions with local ex-situ fraction predictions in different age bins (to assess merger times)
- Ages, merger times and chemical properties of all surviving ex-situ and in-situ globular clusters.
With our approach we can assess self-consistently whether two different tracers of merger histories (e.g., globular cluster colours, and age-metallicity distributions from stellar spectra of the host galaxy), are both consistent with observations. Furthermore, the method prevents the build-up of systematic errors known from existing step-by-step approaches. Additionally it is well suited to recover the ex-situ fraction and merger history in major merger cases, where the stellar population properties between the primary and secondary system may be very similar.
In this talk I will showcase the first results from applications of our method to stellar halo spectra and globular cluster populations of nearby galaxies.