Reading the CARDs: The Imprint of Accretion History in the Chemical Abundances of the Milky Way's Stellar Halo

Abstract

In the era of large-scale spectroscopic surveys in the Local Group, we can explore using chemical abundances of halo stars to study the star formation and chemical enrichment histories of the dwarf galaxy progenitors of the Milky Way (MW) and M31 stellar halos. In this paper, we investigate using the chemical abundance ratio distributions (CARDs) of seven stellar halos from the Latte suite of FIRE-2 simulations. We attempt to infer galaxies’ assembly histories by modeling the CARDs of the stellar halos of the Latte galaxies as a linear combination of template CARDs from disrupted dwarfs, with different stellar masses M ⋆ and quenching times t 100. We present a method for constructing these templates using present-day dwarf galaxies. For four of the seven Latte halos studied in this work, we recover the mass spectrum of accreted dwarfs to a precision of <10%. For the fraction of mass accreted as a function of t 100, we find the residuals of 20%–30% for five of the seven simulations. We discuss the failure modes of this method, which arise from the diversity of star formation and chemical enrichment histories that dwarf galaxies can take. These failure cases can be robustly identified by the high model residuals. Although the CARDs modeling method does not successfully infer the assembly histories in these cases, the CARDs of these disrupted dwarfs contain signatures of their unusual formation histories. Our results are promising for using CARDs to learn more about the histories of the progenitors of the MW and M31 stellar halos.

The Observable Properties of Galaxy Accretion Events in Milky Way–like Galaxies in the FIRE-2 Cosmological Simulations

“…All The resolution of this suite of simulations enables baryonic subhalos to be well-resolved even near each MW-like galaxy. It also resolves the formation of tidal streams from satellite galaxies down to approximately 10 8 M e in total mass or 10 6 M e in stellar mass (at z = 0), similar to that of the MW's "classical" dwarf spheroidals (e.g., Panithanpaisal et al 2021;Cunningham et al 2022;Shipp et al 2022). Furthermore, dark matter particles in each snapshot are processed with Rockstar (Behroozi et al 2013a) to produce halo catalogs that are connected in time using consistent trees to form a merger tree (Behroozi et al 2013b).…”

“…The first step involves inspecting the density distribution of star particles in each simulation in the age-formation distance plane at z = 0, first introduced in Cunningham et al (2022) and Khoperskov et al (2022a), as shown in Figure 1. Here, star particle ages are shown on the x-axis, and the formation distance of star particles (defined as the distance of the star particle at its formation time from the center of the main MWlike host) is on the y-axis.…”

“…Once a clear merger event was identified in the ageformation distance plane, the next step involved using the halo catalog for each simulation to identify the luminous subhalo ID Here, every "branch"-like feature corresponds to a different subhalo, some of which subsequently merge with the main host (MW-like) halo with time (see Cunningham et al (2022) for an introduction to this diagnostic plane). Highlighted as red/blue/yellow branches are three accretion events identified for this simulation, corresponding to phasemixed/coherent-stream/dwarf classified accretion events, respectively.…”

“…Since the classic papers by and Johnston et al (2008), many works have sought to interpret the resulting phase-space and orbital distribution of stars in the stellar halos of galaxies from a theoretical standpoint (e.g., Cooper et al 2010;Pillepich et al 2014;Amorisco 2017a), and specifically for MW-like galaxies (e.g., Font et al 2011;McCarthy et al 2012;Deason et al 2013Deason et al , 2015Deason et al , 2016Amorisco 2017b;D'Souza & Bell 2018;Monachesi et al 2019;Evans et al 2020;Fattahi et al 2020;Font et al 2020;Grand et al 2020;Santistevan et al 2020;Grand et al 2021;Nikakhtar et al 2021;Panithanpaisal et al 2021;Cunningham et al 2022). The results from such efforts have helped reveal the diversity and complexity of substructure in the stellar halos of galaxies like our own MW, and have helped shed light on the properties of discovered debris in the stellar halo of the Galaxy resulting from hierarchical mass assembly.…”

What is Missing from the Local Stellar Halo?

“…Our clearest view into this hierarchical assimilation comes from the stellar halo of the Milky Way, which is almost entirely comprised of debris from accretion events (e.g., Di Matteo et al 2019;Mackereth & Bovy 2020;Naidu et al 2020). Families of halo stars that arrived as part of the same galaxy retain similar phase space properties (e.g., energies, angular momenta, actions; Brown et al 2005;Gómez et al 2013;Simpson et al 2019) as well as shared chemical abundance patterns (e.g., Lee et al 2015;Cunningham et al 2022). With detailed chemodynamical data, it is challenging (e.g., Jean-Baptiste et al 2017) but possible to determine which populations of stars were originally associated with the same dwarf galaxy merger event.…”