Cosmological insights into the assembly of the radial and compact stellar halo of the Milky Way

Abstract

Recent studies using Gaia DR2 have identified a massive merger in the history of the Milky Way (MW) whose debris is markedly radial and counterrotating. This event, known as the Gaia-Enceladus/Gaia-Sausage (GE/GS), is also hypothesized to have built the majority of the inner stellar halo. We use the cosmological hydrodynamic simulation Illustris to place this merger in the context of galaxy assembly within $\Lambda$CDM. From $\sim$150 MW analogs, $\sim 80\%$ have experienced at least one merger of similar mass and infall time as GE/GS. Within this sample, 37 have debris as radial as that of the GE/GS, which we dub the Ancient Radial Mergers (ARMs). Counterrotation is not rare among ARMs, with $43\%$ having $>40\%$ of their debris in counterrotating orbits. However, the compactness inferred for the GE/GS debris, given its large $\beta$ and its substantial contribution to the stellar halo, is more difficult to reproduce. The median radius of ARM debris is r${}_{\ast ,deb}\simeq 45$kpc, while GE/GS is thought to be mostly contained within $r\sim 30$ kpc. For most MW analogs, a few mergers are required to build the inner stellar halo, and ARM debris only accounts for $\sim 12\%$ of inner accreted stars. Encouragingly, we find one ARM that is both compact and dominates the inner halo of its central, making it our best GE/GS analog. Interestingly, this merger deposits a significant number of stars (M${}_{\ast }\simeq 1.5\times 10^9{M}_{\odot }$) in the outer halo, suggesting that an undiscovered section of GE/GS may await detection.

References in corpus (29)

• Introducing the Illustris Project: Simulating the coevolution of dark and visible matter in the Universe
• Properties of galaxies reproduced by a hydrodynamic simulation
• Introducing the Illustris Project: the evolution of galaxy populations across cosmic time
• The Illustris simulation: the evolving population of black holes across cosmic time
• Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo
• The accretion origin of the Milky Way's stellar halo
• The Arepo public code release
• Streams, substructures and the early history of the Milky Way
• The Biggest Splash
• The stellar halo of the Galaxy
• Multiple retrograde substructures in the Galactic halo: A shattered view of Galactic history
• Feedback and the Structure of Simulated Galaxies at redshift z=2
• Halo Mass and Assembly History Exposed in the Faint Outskirts: the Stellar and Dark Matter Haloes of Illustris Galaxies
• The total stellar halo mass of the Milky Way
• The Fall of a Giant. Chemical evolution of Enceladus, alias the Gaia Sausage
• The Dragonfly Nearby Galaxies Survey. I. Substantial variation in the diffuse stellar halos around spiral galaxies
• Diverse Stellar Haloes in Nearby Milky Way-Mass Disc Galaxies
• Sausage & Mash: The dual origin of the Galactic thick disc and halo from the gas-rich Gaia-Enceladus-Sausage merger
• Resolving the Metallicity Distribution of the Stellar Halo with the H3 Survey
• Weighing the stellar constituents of the Galactic halo with APOGEE red giant stars
• Origin of the excess of high-energy retrograde stars in the Galactic halo
• Touching The Void: A Striking Drop in Stellar Halo Density Beyond 50 kpc
• The colors of satellite galaxies in the Illustris Simulation
• Galaxies grow their bulges and black holes in diverse ways
• A Gaia-Enceladus Analog in the EAGLE Simulation: Insights into the Early Evolution of the Milky Way
• The little things matter: relating the abundance of ultrafaint satellites to the hosts' assembly history
• The central spheroids of Milky Way mass-sized galaxies
• The morphology and kinematics of neutral hydrogen in the vicinity of z=0 galaxies with Milky Way masses -- a study with the Illustris simulation
• Galaxies with Shells in the Illustris Simulation: Metallicity Signatures

Cited by in corpus (22)

• Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure
• Cygnus X-1 contains a 21-solar mass black hole -- implications for massive star winds
• Reconstructing the Last Major Merger of the Milky Way with the H3 Survey
• Chemo-kinematics of the $Gaia$ RR Lyrae: the halo and the disc
• Reconstructing the Disrupted Dwarf Galaxy $Gaia$-Sausage/Enceladus Using its Stars and Globular Clusters
• A tale of two populations: surviving and destroyed dwarf galaxies and the build up of the Milky Way's stellar halo
• Galactic Archaeology with Gaia
• GASTRO library I: the simulated chemodynamical properties of several GSE-like stellar halos
• Merger-induced galaxy transformations in the ARTEMIS simulations
• Constraints on the assembly history of the Milky Way's smooth, diffuse stellar halo from the metallicity-dependent, radially-dominated velocity anisotropy profiles probed with K giants and BHB stars using LAMOST, SDSS/SEGUE, and Gaia
• The impact of early massive mergers on the chemical evolution of Milky Way-like galaxies: insights from NIHAO-UHD simulations
• How Unusual is the Milky Way's Assembly History?
• Distant Echoes of the Milky Way's Last Major Merger
• Are the Milky Way and Andromeda unusual? A comparison with Milky Way and Andromeda Analogs
• VINTERGATAN-GM: The cosmological imprints of early mergers on Milky-Way-mass galaxies
• Age Determination of Galaxy Merger Remnant Stars using Asteroseismology
• Contribution of Gaia Sausage to the Galactic Stellar Halo Revealed by K Giants and Blue Horizontal Branch Stars from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, Sloan Digital Sky Survey, and Gaia
• Evidence For Multiple Accretion Events in the $Gaia$-Sausage/Enceladus Structures
• Cosmological Simulations of Stellar Halos with Gaia Sausage-Enceladus Analogues: Two Sausages, One Bun?
• Observing EAGLE galaxies with JWST: predictions for Milky Way progenitors and their building blocks
• Tidal mass loss in the Fornax dwarf spheroidal galaxy through N-body simulations with Gaia DR3-based orbits
• Stellar halos of bright central galaxies II: Scaling relations, colors and metallicity evolution with redshift