Cosmological predictions for minor axis stellar density profiles in the inner regions of Milky Way-mass galaxies

TL;DR

This study uses cosmological simulations to analyze the inner stellar density profiles of Milky Way-mass galaxies, revealing a consistent exponential disk and power-law component that correlate with dark matter profiles rather than accretion history.

Contribution

It provides the first detailed predictions for minor axis stellar density profiles in the inner regions of Milky Way-mass galaxies based on cosmological simulations.

Findings

Inner stellar density profiles are exponential with a scale height <0.3 kpc.

Power law component with slope approximately -4 describes the outer profile.

Stellar profiles correlate strongly with dark matter profiles, not accretion history.

Abstract

$\Lambda$CDM cosmology predicts the hierarchical formation of galaxies which build up mass by merger events and accreting smaller systems. The stellar halo of the Milky Way has proven to be useful a tool for tracing this accretion history. However, most of this work has focused on the outer halo where dynamical times are large and the dynamical properties of accreted systems are preserved. In this work, we investigate the inner galaxy regime, where dynamical times are relatively small and systems are generally completely phase-mixed. Using the FIRE-2 and Auriga cosmological zoom-in simulation suites of Milky Way-mass galaxies, we find the stellar density profiles along the minor axis (perpendicular to the galactic disk) within the NFW scale radii (R$\approx$15 kpc) are best described as an exponential disk with scale height <0.3 kpc and a power law component with slope $\alpha\approx$-4. The stellar density amplitude and slope for the power law component is not significantly correlated with metrics of the galaxy's accretion history. Instead, we find the stellar profiles strongly correlate with the dark matter profile. Across simulation suites, the galaxies studied in this work have a stellar to dark matter mass ratio that decreases as $1/r^2$ along the minor axis.