Reconciling Observed and Simulated Stellar Halo Masses

Andrew Wetzel,Robyn E. Sanderson,Tsang Keung Chan,Philip F. Hopkins,Claude-André Faucher-Giguère,Xiangcheng Ma,Ivanna Escala,Shea Garrison-Kimmel,Dušan Kereš

doi:10.3847/1538-4357/aaeb33

Abstract

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 10, 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 are 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.

Highlights

Recent advances in observational astronomy have begun to reveal the faint stellar halos surrounding Milky-Way-mass galaxies in integrated light (Martınez-Delgado et al 2010; Bakos & Trujillo 2012; D’Souza et al 2014; Merritt et al 2016)
In this paper we compared the stellar halos of highresolution simulated galaxies from the FIRE-2 suite (§§2–3) with recent measurements by Merritt et al (2016; M16) of stellar halo mass fractions for eight Milky-Way-like galaxies
Inspired by M16 and other recent works, we considered both spatial selection based on modeling and subtracting the disk and bulge regions (§5.1) and a selection based on surface mass density as proposed in Cooper et al (2010) (§5.3)

Summary

Introduction

Recent advances in observational astronomy have begun to reveal the faint stellar halos surrounding Milky-Way-mass galaxies in integrated light (Martınez-Delgado et al 2010; Bakos & Trujillo 2012; D’Souza et al 2014; Merritt et al 2016). In the CDM picture, the process of hierarchical accretion ties the variation in the mass fraction of accreted stars to the accretion history of the host galaxy (Bullock & Johnston 2005; Tissera et al 2012, 2013; RodriguezGomez et al 2016; Amorisco 2017). In principle, this component holds some of the few memories of the accretion process. Identifying regions of galaxies that are primarily made up of accreted material is the first step toward testing these predictions

Objectives

Methods

Findings

Conclusion