The stellar halo in Local Group Hestia simulations

Author:

Khoperskov SergeyORCID,Minchev Ivan,Libeskind Noam,Haywood Misha,Di Matteo Paola,Belokurov Vasily,Steinmetz Matthias,Gomez Facundo A.,Grand Robert J. J.,Hoffman Yehuda,Knebe Alexander,Sorce Jenny G.,Spaare Martin,Tempel Elmo,Vogelsberger Mark

Abstract

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.

Funder

ERDF CoE

ETAg

Israel Science Foundation

ANR

MICINN

ANID FONDECYT

ANID BASAL

Publisher

EDP Sciences

Subject

Space and Planetary Science,Astronomy and Astrophysics

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1. Galactic Archaeology with Gaia;New Astronomy Reviews;2024-12

2. The Three-phase Evolution of the Milky Way;The Astrophysical Journal;2024-08-28

3. Formation of Galactic Disks. II. The Physical Drivers of Disk Spin-up;The Astrophysical Journal;2024-08-26

4. Finding accreted stars in the Milky Way: clues from NIHAO simulations;Monthly Notices of the Royal Astronomical Society;2024-06-21

5. Impact of orbiting satellites on star formation rate evolution and metallicity variations in Milky Way-like discs;Monthly Notices of the Royal Astronomical Society;2023-10-23

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