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arXiv · 2610.34001

Observational evidence of the Gaia-Enceladus-Sausage merger's influence on the opposite side of the Galactic Center

Yuxiao Chen·Gaochao Liu·Yang Huang·Huawei Zhang·Timothy C. Beers·Dejian Liu·Sarah A. Bird·Peng Zhang·Yiping Zhang
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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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Yuxiao Chen, Gaochao Liu, Yang Huang, Huawei Zhang, Timothy C. Beers, Dejian Liu, Sarah A. Bird, Peng Zhang, Yiping Zhang. 2026-10-25. Observational evidence of the Gaia-Enceladus-Sausage merger's influence on the opposite side of the Galactic Center. https://arxiv.org/abs/2610.34001

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