Building Blocks of the Milky Way's Accreted Spheroid

TL;DR

This study investigates the assembly history of the Milky Way's stellar spheroid using simulations, revealing differences in star formation and chemical enrichment between building blocks and satellites, which impact observed element ratios.

Contribution

It combines semi-analytical models with high-resolution simulations to analyze the properties and formation history of spheroid building blocks in Milky Way-like galaxies, highlighting their distinct star formation and chemical enrichment patterns.

Findings

Building blocks have higher star formation rates than satellites.

Early building blocks dominate star formation, leading to higher alpha-element enhancement.

Variations in star formation timescales suggest differences in chemical abundances among galaxies.

Abstract

In the $\Lambda$CDM model of structure formation, a stellar spheroid grows by the assembly of smaller galaxies, the so-called building blocks. Combining the Munich-Groningen semi-analytical model of galaxy formation with the high resolution Aquarius simulations of dark matter haloes, we study the assembly history of the stellar spheroids of six Milky Way-mass galaxies, focussing on building block properties such as mass, age and metallicity. These properties are compared to those of the surviving satellites in the same models. We find that the building blocks have higher star formation rates on average, and this is especially the case for the more massive objects. At high redshift these dominate in star formation over the satellites, whose star formation timescales are longer on average. These differences ought to result in a larger $\alpha$-element enhancement from Type II supernovae in the building blocks (compared to the satellites) by the time Type Ia supernovae would start to enrich them in iron, explaining the observational trends. Interestingly, there are some variations in the star formation timescales of the building blocks amongst the simulated haloes, indicating that [$\alpha$/Fe] abundances in spheroids of other galaxies might differ from those in our own Milky Way.