Search

Stellar chemical abundances and kinematics provide 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/Milky Way (MW) analogues from the HESTIA suite of cosmological hydrodynamics zoom-in simulations of the Local Group. We show that elemental abundances ([Fe/H], [Mg/Fe]) of merger debris in the stellar haloes are chemically distinct from the survived dwarf galaxies, in that they 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 − Lz plane, with the most metal-rich, [α/Fe]-poor stars, which have formed in the inner parts of the disrupted systems before the merger, contributing mainly 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 the parameters, such as the mass and merger time, of MW's most massive merger Gaia-Sausage-Enceladus. The metallicity distribution functions (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 − Lz 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], while retrograde stars typically contribute to the high-[Mg/Fe] sequence only. We find that the most metal-poor stars ([Fe/H] ≲ −1) of the HESTIA galaxies exhibit net rotation up to 80 km s−1, which is consistent with the Aurora population recently identified in the MW. At higher metallicities ([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, and especially stellar age, makes it possible to distinguish between different debris. [ABSTRACT FROM AUTHOR]

Recent progress in understanding the assembly history of the Milky Way (MW) is driven by the tremendous amount of high-quality data delivered by Gaia (ESA), revealing a number of substructures potentially linked to several ancient accretion events. In this work we aim to explore the phase-space structure of accreted stars by analysing six M31/MW analogues from the HESTIA suite of cosmological hydrodynamics zoom-in simulations of the Local Group. We find that all HESTIA galaxies experience a few dozen mergers but only between one and four of those have stellar mass ratios > 0.2, relative to the host at the time of the merger. Depending on the halo definition, the most massive merger contributes from 20% to 70% of the total stellar halo mass. Individual merger remnants show diverse density distributions at z = 0, significantly overlapping with each other and with the in situ stars in the Lz − E, (VR, Vϕ) and (R, vϕ) coordinates. Moreover, merger debris often shifts position in the Lz − E space with cosmic time due to the galactic mass growth and the non-axisymmetry of the potential. In agreement with previous works, we show that even individual merger debris exhibit a number of distinct Lz − E features. In the (VR, Vϕ) plane, all HESTIA galaxies reveal radially hot, non-rotating or weakly counter-rotating, Gaia-Sausage-like features, which are the remnants of the most recent significant mergers. We find an age gradient in Lz − E space for individual debris, where the youngest stars, formed in the inner regions of accreting systems, deposit to the innermost regions of the host galaxies. The bulk of these stars formed during the last stages of accretion, making it possible to use the stellar ages of the remnants to date the merger event. In action space (Jr, Jz, Jϕ), merger debris do not appear as isolated substructures, but are instead scattered over a large parameter area and overlap with the in situ stars. We suggest that accreted stars can be best identified using Jr > 0.2−0.3(104 kpc km s−1)0.5. We also introduce a new, purely kinematic space (Jz/Jr-orbital eccentricity), where different merger debris can be disentangled better from each other and from the in situ stars. Accreted stars have a broad distribution of eccentricities, peaking at ϵ ≈ 0.6 − 0.9, and their mean eccentricity tends to be smaller for systems accreted more recently. [ABSTRACT FROM AUTHOR]

Theory suggests that mergers play an important role in shaping galactic discs and stellar haloes, which was observationally confirmed in the Milky Way (MW) thanks to Gaia data. In this work, aiming to probe the contribution of mergers to the in situ stellar halo formation, we analyse six M 31 and MW analogues from the HESTIA suite of cosmological hydrodynamical zoom-in simulations of the Local Group. We found that all the HESTIA galaxies experience between one to four mergers with stellar mass ratios between 0.2 and 1 relative to the host at the time of the merger. These significant mergers, with a single exception, happened 7 − 11 Gyr ago. The overall impact of the most massive mergers in HESTIA is clearly seen as a sharp increase in the orbital eccentricity (and a corresponding decrease in the rotational velocity Vϕ) of pre-existing disc stars of the main progenitor, thus nicely reproducing the Splash-, Plume-like feature that was discovered in the MW. We do find a correlation between mergers and close pericentric passages of massive satellites and bursts of the star formation in the in situ component. Massive mergers sharply increase the disc velocity dispersion of the in situ stars; however, the latest significant merger often heats up the disc up to the numbers when the contribution of the previous ones is less prominent in the age-velocity dispersion relation. In HESTIA galaxies, the in situ halo is an important component of the inner stellar halo where its fraction is about 30 − 40%, while in the outer parts it typically does not exceed ≈5% beyond 15 kpc from the galactic centre. The simulations suggest that this component of the stellar haloes continues to grow well after mergers conclude; however, the most significant contribution comes from stars that formed recently before the merger. The orbital analysis of the HESTIA galaxies suggests that wedges in Rmax − Zmax (apocentre – maximum height from the mid-plane) space are mainly populated by the stars born in between significant mergers. [ABSTRACT FROM AUTHOR]

Aims. We study the two main constituent galaxies of a constrained simulation of the Local Group as candidates for the Milky Way (MW) and Andromeda (M31). We focus on the formation of the stellar discs and its relation to the formation of the group as a rich system with two massive galaxies, and investigate the effects of mergers and accretion as drivers of morphological transformations. We also assess the effects of varying the assumed feedback model on our results by running two different simulations, a first one where only supernova feedback is included and a second where we also model radiation pressure from stars. Methods. We use a state-of-the-art hydrodynamical code which includes star formation, feedback, and chemical enrichment to carry out our study. We use our two simulations, where we include or neglect the effects of radiation pressure from stars, to investigate the impact of this process on the morphologies and star formation rates of the simulated galaxies. Results. We find that the simulated M31 and MW have different formation histories, even though both inhabit, at z=0, the same environment. These differences directly translate into and explain variations in their star formation rates, in situ fractions, and final morphologies. The simulated M31 candidate has an active merger history, as a result of which its stellar disc is unable to survive unaffected until the present time. In contrast, the MW candidate has a smoother history with no major mergers at late times, and forms a disc that grows steadily; at z=0 the simulated MW has an extended, rotationally-supported disc that is dominant over the bulge. Our two feedback implementations predict similar evolutions of the galaxies and their discs, although some variations are detected, the most important of which is the formation time of the discs: in the model with weaker (stronger) feedback the discs form earlier (later). In summary, by comparing the formation histories of the two simulated galaxies, we conclude that the particular merger and accretion history of a galaxy rather than its environment at the LG-scales is the main driver of the formation and subsequent growth or destruction of galaxy discs. [ABSTRACT FROM AUTHOR]