The stellar halo in Local Group Hestia simulations III. Chemical abundance relations for accreted and in-situ stars

Abstract

Since the chemical abundances of stars are the fossil records of the physical conditions in galaxies, they provide the key information for recovering the assembly history of galaxies. In this work, we explore the chemo-chrono-kinematics of accreted and in-situ stellar populations, by analyzing six M31/MW analogues from the HESTIA suite of cosmological hydrodynamics zoom-in simulations of the Local Group. We found that the merger debris are chemically distinct from the survived dwarf galaxies. The debris are [α/Fe]enhanced and have lower metallicity in the same stellar mass range. Therefore, mergers debris have abundances expected for stars originating from dwarfs that had their star formation activity quenched at early times. Accreted stellar haloes, including individual debris, reveal [Fe/H] and [Mg/Fe] gradients in the E − L z plane, where the most metal-rich (and [α/Fe]-poor) stars have formed in the inner parts of the disrupted systems before the merger and mainly contribute to the central regions of the host galaxies. This results in negative metallicity gradients in the accreted components of stellar haloes at z = 0, seen also for the individual merger debris. We suggest, therefore, that abundance measurements of halo stars in the inner MW will allow constraining better parameters of building blocks of the MW stellar halo. The MDFs of the individual debris show several peaks and the majority of debris have lower metallicity than the in-situ stars in the prograde part of the E − L z space. At the same time, non-rotating and retrograde accreted populations are very similar to the in-situ stars in terms of [Fe/H] abundance. Prograde accreted stars show a prominent knee in the [Fe/H] -[Mg/Fe] plane, reaching up to solar [Mg/Fe] -abundances, while the retrograde stars typically deposit to a high-[Mg/Fe] sequence only. We found that the most metal-poor stars ([Fe/H] −1) of the HESTIA galaxies exhibit between zero to 80 km s −1 net rotation which is consistent with the Aurora population recently noticed in the MW. At higher metallicities (up to [Fe/H] ≈ −0.5 ± 0.1) we detect a sharp transition (spin-up) from the turbulent phase to a regular disk-like rotation. Different merger debris appear similar in the [Fe/H] -[Mg/Fe] plane, thus making it difficult to identify individual events. However, combining a set of abundances allows to capture chemical patterns corresponding to different debris, which are the most prominent as a function of stellar age.

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

“…Regarding the chemical compositions of disrupted satellites, our findings are also in qualitative agreement with results from N-body models (e.g., Font et al 2006), large-volume cosmological simulations at lower resolution (e.g., Mackereth et al 2019), and zoom-in simulations of similar resolution (e.g., Khoperskov et al 2022b).…”

“…Understanding the [α/Fe] and [Fe/H] abundances of stars associated with a galaxy can help place strong constraints on its formation and evolution. When applied to stars from satellites accreted by an MW-like galaxy, their distribution in this chemical plane is a clear diagnostic of the time at which the accreted satellite ceased star formation (e.g., Robertson et al 2005;Font et al 2006;Mackereth et al 2019;Khoperskov et al 2022b). It also provides an opportunity to study how lower-mass galaxies chemically evolved in the early universe (e.g., Corlies et al 2018).…”

The impact of two massive early accretion events in a Milky Way-like galaxy: repercussions for the buildup of the stellar disc and halo

“…Recently, Khoperskov et al (2022) suggest that the differences between abundance ratios of in-situ and accreted stars could help constrain the accretion history of the MW. These differences are most pronounced between the highest metallicity stars.…”

The Metal-weak Milky Way Stellar Disk Hidden in the Gaia–Sausage–Enceladus Debris: The APOGEE DR17 View

“…The Hestia simulations (Libeskind et al 2020;Khoperskov et al 2022aKhoperskov et al , 2022bKhoperskov et al , 2022c follow three Local Group pairings of galaxies and their respective merger histories. One important aspect of these simulations is that the galactic potential evolves 4 shows the selection criteria used for each of the studies.…”