Reconciling observed and simulated stellar halo masses

TL;DR

This study uses cosmological simulations to evaluate methods for estimating stellar halo masses from images, revealing systematic underestimations and biases, and suggests metallicity or color data could improve accuracy.

Contribution

It demonstrates that common observational methods underestimate accreted stellar mass and introduces biases, proposing metallicity or color information as a potential solution.

Findings

Observational methods underestimate accreted stellar mass by up to a factor of ten.

Estimated accreted stellar mass depends on galaxy size and mass, causing biases.

Metallicity or color data can help distinguish accreted stars in observations.

Abstract

We use cosmological hydrodynamical simulations of Milky-Way-mass galaxies from the FIRE project to evaluate various strategies for estimating the mass of a galaxy's stellar halo from deep, integrated-light images. We find good agreement with integrated-light observations if we mimic observational methods to measure the mass of the stellar halo by selecting regions of an image via projected radius relative to the disk scale length or by their surface density in stellar mass . However, these observational methods systematically underestimate the accreted stellar component, defined in our (and most) simulations as the mass of stars formed outside of the host galaxy, by up to a factor of ten, since the accreted component is centrally concentrated and therefore substantially obscured by the galactic disk. Furthermore, these observational methods introduce spurious dependencies of the estimated accreted stellar component on the stellar mass and size of galaxies that can obscure the trends in accreted stellar mass predicted by cosmological simulations, since we find that in our simulations the size and shape of the central galaxy is not strongly correlated with the assembly history of the accreted stellar halo. This effect persists whether galaxies are viewed edge-on or face-on. We show that metallicity or color information may provide a way to more cleanly delineate in observations the regions dominated by accreted stars. Absent additional data, we caution that estimates of the mass of the accreted stellar component from single-band images alone should be taken as lower limits.