The Galactic Bulge exploration VI.: Gaia Enceladus/Sausage RR Lyrae stars in the inner-central stellar halo of the Milky Way

Andrea Kunder; Zdenek Prudil; Antonela Monachesi; Samuel J. Morris; Kathryn Devine; Joanne Hughes; Kevin Covey; R. Michael Rich; Elisa A. Tau

arXiv · 2507.11741

The Galactic Bulge exploration VI.: Gaia Enceladus/Sausage RR Lyrae stars in the inner-central stellar halo of the Milky Way

Andrea Kunder·Zdenek Prudil·Antonela Monachesi·Samuel J. Morris·Kathryn Devine·Joanne Hughes·Kevin Covey·R. Michael Rich·Elisa A. Tau

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Abstract

We present a view of the stellar halo in the inner-central regions of the Milky Way (R <~ 10 kpc) mapped by RR Lyrae stars. The combined BRAVA-RR/APOGEE RR Lyrae catalog is used to obtain a sample of 281 RR Lyrae stars located in the bulge region of the Galaxy, but with orbits indicating they belong to the inner-central halo. The RR Lyrae stars in the halo are more metal-poor than the bulge RR Lyrae stars and have pulsation properties more consistent with an accreted population. We use the Milky Way-like zoom-in cosmological simulation Auriga to compare the properties of the RR Lyrae stars to those expected from the "Gaia-Enceladus-Sausage" (GES) merger. The integrals of motions and eccentricities of the RR Lyrae stars are consistent with a small fraction of 6-9 +- 2 % of the inner-central halo RR Lyrae population having originated from GES. This fraction, lower than what is seen in the solar neighborhood, is consistent with trends seen in the Auriga simulation, where a GES-like merger would have a decreasing fraction of GES stars at small Galactocentric radii compared to other accreted populations. Very few of the Auriga inner Galaxy GES-18 particles have properties consistent with belonging to a bulge population with (z_max < 1.1 kpc), indicating that no (or very few) RR Lyrae stars with bulge orbits should have originated from GES.

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Andrea Kunder, Zdenek Prudil, Antonela Monachesi, Samuel J. Morris, Kathryn Devine, Joanne Hughes, Kevin Covey, R. Michael Rich, Elisa A. Tau. 2025-07-15. The Galactic Bulge exploration VI.: Gaia Enceladus/Sausage RR Lyrae stars in the inner-central stellar halo of the Milky Way. https://arxiv.org/abs/2507.11741

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