Observational evidence of the Gaia-Enceladus-Sausage merger's influence on the opposite side of the Galactic Center
Abstract
The Gaia-Enceladus-Sausage (GES) merger, which occurred approximately 8-11 Gyr ago, is a key event that shaped the Galactic inner halo. Its remnants have been studied extensively and are known to dominate the halo on the Near-side of the Galactic Center (GC), i.e. the side facing the Sun, but systematic evidence for whether the Far-side (the side beyond the GC as seen from the Sun) is similarly affected has been lacking. Using 14,056 red giant stars selected from SMSS DR4 and Gaia DR3 through stringent quality cuts, we compare the metallicity distributions, velocity-space distributions, velocity dispersions, anisotropy parameters, and action-space properties of the Near- and Far-sides. We find clear observational evidence for the influence of GES on the Far-side: it hosts the same metal-rich component peaked at [Fe/H] ~ -1.4, and its metal-rich population (-1.7 < [Fe/H] <= -1.3) shows GES-typical features consistent with the Near-side, namely a "sausage"-shaped velocity distribution and a velocity anisotropy as high as beta ~ 0.8-0.9. Action-space analysis shows that 10.86% +/- 0.23% of Far-side stars carry the GES dynamical imprint, consistent with 11.56% +/- 0.16% on the Near-side. The Far-side shows a residual prograde rotation that persists under increasingly aggressive disk-removal cuts, indicating a genuine kinematic signal rather than disk contamination, and a battery of robustness tests confirms that the symmetry is not an artifact of measurement uncertainties, residual disk contamination, or incomplete Sagittarius-stream removal. These results demonstrate that GES shaped the entire inner halo (r < 28 kpc) globally and symmetrically, with its imprint clearly present on the Far-side as well as the Near-side.
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BibTeX
@misc{chen2026observational,
title = {{Observational evidence of the Gaia-Enceladus-Sausage merger's influence on the opposite side of the Galactic Center}},
author = {Yuxiao Chen and Gaochao Liu and Yang Huang and Huawei Zhang and Timothy C. Beers and Dejian Liu and Sarah A. Bird and Peng Zhang and Yiping Zhang},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.34001},
eprint = {2610.34001},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.34001},
}
RIS
TY - UNPB
TI - Observational evidence of the Gaia-Enceladus-Sausage merger's influence on the opposite side of the Galactic Center
AU - Yuxiao Chen
AU - Gaochao Liu
AU - Yang Huang
AU - Huawei Zhang
AU - Timothy C. Beers
AU - Dejian Liu
AU - Sarah A. Bird
AU - Peng Zhang
AU - Yiping Zhang
PY - 2026
DA - 2026/10/25
PB - arXiv
DO - 10.48550/arxiv.2610.34001
UR - https://arxiv.org/abs/2610.34001
AB - The Gaia-Enceladus-Sausage (GES) merger, which occurred approximately 8-11 Gyr ago, is a key event that shaped the Galactic inner halo. Its remnants have been studied extensively and are known to dominate the halo on the Near-side of the Galactic Center (GC), i.e. the side facing the Sun, but systematic evidence for whether the Far-side (the side beyond the GC as seen from the Sun) is similarly affected has been lacking. Using 14,056 red giant stars selected from SMSS DR4 and Gaia DR3 through stringent quality cuts, we compare the metallicity distributions, velocity-space distributions, velocity dispersions, anisotropy parameters, and action-space properties of the Near- and Far-sides. We find clear observational evidence for the influence of GES on the Far-side: it hosts the same metal-rich component peaked at [Fe/H] ~ -1.4, and its metal-rich population (-1.7 < [Fe/H] <= -1.3) shows GES-typical features consistent with the Near-side, namely a "sausage"-shaped velocity distribution and a velocity anisotropy as high as beta ~ 0.8-0.9. Action-space analysis shows that 10.86% +/- 0.23% of Far-side stars carry the GES dynamical imprint, consistent with 11.56% +/- 0.16% on the Near-side. The Far-side shows a residual prograde rotation that persists under increasingly aggressive disk-removal cuts, indicating a genuine kinematic signal rather than disk contamination, and a battery of robustness tests confirms that the symmetry is not an artifact of measurement uncertainties, residual disk contamination, or incomplete Sagittarius-stream removal. These results demonstrate that GES shaped the entire inner halo (r < 28 kpc) globally and symmetrically, with its imprint clearly present on the Far-side as well as the Near-side.
ER -