To verify the predictions of CDM, there have been recent attempts to compare the amount of stellar mass observed in the outskirts of galaxies with the mass fraction in accreted stars predicted by simulations (e.g., Font et al. 2008; Pillepich et al. 2014; Merritt et al. 2016; D’Souza & Bell 2018; Elias et al. 2018; Huang et al. 2018). For the purposes of this paper, we will use the term “stellar halo” in an observational sense, referring to the faint structure in the outskirts of galaxies beyond the central concentration of stellar mass. In practice, there are many observational definitions of this term that can include radial profile, surface brightness, or metallicity characteristics. Most recent observational attempts to characterize the stellar halos of galaxies, including the MW (e.g., Carollo et al. 2010) and the Andromeda galaxy (M31; e.g., Courteau et al. 2011), have used analysis of resolved stellar populations to identify the accreted component, usually by searching for an old, metal-poor population extending far from the central galaxy (e.g., Seth et al. 2007; Cockcroft et al. 2013). However, this method requires extremely deep images, mainly obtained using the Hubble Space Telescope (with the exception of Greggio et al. 2014), and has therefore been limited to a small handful of galaxies so far. Observing stellar halos in integrated light is, in principle, more easily scalable to the sample sizes needed to explore the wide variation in the accreted component that is predicted by simulations (e.g., Bakos & Trujillo 2012; D’Souza et al. 2014; Duc et al. 2015; Merritt et al. 2016; Huang et al. 2018), presuming that it is possible to account for the contribution of scattered light (de Jong 2008; Slater et al. 2009; Sandin 2014). However, the lack of resolved stellar population information makes it far more challenging to identify the regions of an image dominated by accreted material. So far, no work has attempted to account for how the method used to select the stellar halo from a galaxy observed in integrated light may bias the comparison to simulations, where the provenance of material is perfectly known and a variety of definitions of “stellar halo” are imposed. Despite efforts such as that in Rodriguez-Gomez et al. (2016) to understand whether the mass in the stellar halo comes primarily from accreted material or from stars formed in the central galaxy (which in this work we call “formed in situ”) and expelled to the halo, and its dependence with separation from the central galaxy, it is not straightforward to apply these results to the spatial selections in projection that are commonly used in integrated-light images. In fact, most prior work has focused on comparisons between observed galaxies and predictions for the stellar halo based on dark matter–only (DM-only) simulations tagged with stars, where the stellar halo is by definition 100% accreted. However, in simulations that include baryonic physics, both of these distinct channels are observed to contribute to the stellar halos of galaxies (Font et al. 2011; Cooper et al. 2013; Tissera et al. 2013; Pillepich et al. 2015; Anglés-Alcázar et al. 2017; Gómez et al. 2017a), and both are interesting for what they tell us about the process of galaxy formation, as well as the cosmology in which galaxies are formed.
Citation: Duc et al. 2015
Citation function: Motivation