Abstract
Abstract We explore the contribution of the Gaia Sausage to the stellar halo of the Milky Way by making use of a Gaussian mixture model (GMM) and applying it to halo star samples of Large Sky Area Multi-Object Fiber Spectroscopic Telescope K giants, Sloan Extension for Galactic Understanding and Exploration K giants, and Sloan Digital Sky Survey blue horizontal branch stars. The GMM divides the stellar halo into two parts, of which one represents a more metal-rich and highly radially biased component associated with an ancient, head-on collision referred to as the Gaia Sausage, and the other one is a more metal-poor and isotropic halo. A symmetric bimodal Gaussian is used to describe the distribution of spherical velocity of the Gaia Sausage, and we find that the mean absolute radial velocity of the two lobes decreases with the Galactocentric radius. We find that the Gaia Sausage contributes about 41%–74% of the inner (Galactocentric radius r gc < 30 kpc) stellar halo. The fraction of stars of the Gaia Sausage starts to decline beyond r gc ∼ 25–30 kpc, and the outer halo is found to be significantly less influenced by the Gaia Sausage than the inner halo. After the removal of halo substructures found by integrals of motion, the contribution of the Gaia Sausage falls slightly within r gc ∼ 25 kpc but is still as high as 30%–63%. Finally, we select several possible Sausage-related substructures consisting of stars on highly eccentric orbits. The GMM/Sausage component agrees well with the selected substructure stars in their chemodynamical properties, which increases our confidence in the reliability of the GMM fits.
Highlights
In the Λ CDM cosmological paradigm, Milky Way sized halos are built from the mergers of smaller satellite galaxies (White & Rees 1978)
We explore the contribution of the Gaia-Sausage to the stellar halo of the Milky Way by making use of a Gaussian Mixture model (GMM) and applying it to halo star samples of LAMOST K giants, SEGUE K giants, and SDSS blue horizontal branch stars
The stellar halo within rgc of 30 kpc is defined as the inner halo, and further than rgc of 30 kpc is defined as the outer halo
Summary
In the Λ CDM cosmological paradigm, Milky Way sized halos are built from the mergers of smaller satellite galaxies (White & Rees 1978). According to the stellar mass ratio of the satellite to the host galaxy, mergers can be categorized into major or minor types. These mergers usually happen in the early epoch of galaxy formation, and contribute much to the stellar halo population. The combination of data from the Gaia satellite and large spectroscopic surveys has provided new insights into the assembly history of the Milky Way. One of the most impressive findings is that the local stellar halo is dominated by the stars left from a major merger event named as the Gaia-Sausage (Belokurov et al 2018), or the Gaia-Enceladus (Helmi et al 2018). Recent studies found that the expected head-on collision of the ancient Gaia-Sausage and the Galaxy happened around 8 − 10 Gyr ago (Sahlholdt et al 2019; Bignone et al 2019; Bonaca et al 2020)
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