The Milky Way's Missing In-Situ Halo
Abstract
The Milky Way's stellar halo preserves a record of its mass assembly history, encoding accretion events in its structural properties. Among the most prominent of these features is a strong break in the halo density profile at ≈20-30 kpc, long attributed to the apocenter pile-up of stars from the Gaia-Enceladus/Sausage merger. However, whether this interpretation is consistent with state-of-the-art cosmological simulations remains unclear. In this work, we compare the Milky Way's measured stellar halo density profile between galactocentric radii of 1unicodex2013100 kpc to those of Milky Way-mass galaxies from the FIRE-2 and Auriga cosmological zoom-in simulation suites, spanning a total of 24 simulated galaxies. We find that simulated halo profiles are significantly steeper than Milky Way measurements within ≈15 kpc, and that profile breaks are rare in simulations and never as strong as the Milky Way's. Of the three galaxies with statistically significant breaks, only one exhibits a break attributable to apocenter pile-up of an accreted merger remnant. Decomposing the simulated profiles, we find that the accreted halo profiles are broadly consistent with Milky Way measurements, while the in-situ halo dominates within ≈20 kpc and drives the discrepancy. These results raise a compelling question: where is the Milky Way's in-situ halo? The origin of this tension may reflect systematic biases in current halo measurements, an overproduction of early spheroidal star formation in cosmological simulations, or more fundamental differences in the underlying physics (e.g., dark matter) governing the assembly and structure of the inner Galaxy.
Related
- ARRAKIHS: The observability of tidally stripped satellites in a Milky Way-like halo
- Tracing the accretion history of the Milky Way with halo stars
- Cluster Ages to Reconstruct the Milky Way Assembly (CARMA). VI. The age-metallicity relation of in situ clusters and the age of the Universe
- The high-velocity dark matter halo of the Milky Way in light of the LZ 248 keV event
- Implications of a contracted dark matter halo for the Milky Way’s inferred virial mass
BibTeX
@misc{lucey2026milky,
title = {{The Milky Way's Missing In-Situ Halo}},
author = {Madeline Lucey and Robyn E. Sanderson and Alexander H. Riley and Andreia Carrillo and Jenna Samuel and Emma Tasso},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2606.16277},
eprint = {2606.16277},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.16277},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The Milky Way's Missing In-Situ Halo
AU - Madeline Lucey
AU - Robyn E. Sanderson
AU - Alexander H. Riley
AU - Andreia Carrillo
AU - Jenna Samuel
AU - Emma Tasso
PY - 2026
DA - 2026/11/02
PB - arXiv
DO - 10.48550/arxiv.2606.16277
UR - https://arxiv.org/abs/2606.16277
AB - The Milky Way's stellar halo preserves a record of its mass assembly history, encoding accretion events in its structural properties. Among the most prominent of these features is a strong break in the halo density profile at ≈20-30 kpc, long attributed to the apocenter pile-up of stars from the Gaia-Enceladus/Sausage merger. However, whether this interpretation is consistent with state-of-the-art cosmological simulations remains unclear. In this work, we compare the Milky Way's measured stellar halo density profile between galactocentric radii of 1unicodex2013100 kpc to those of Milky Way-mass galaxies from the FIRE-2 and Auriga cosmological zoom-in simulation suites, spanning a total of 24 simulated galaxies. We find that simulated halo profiles are significantly steeper than Milky Way measurements within ≈15 kpc, and that profile breaks are rare in simulations and never as strong as the Milky Way's. Of the three galaxies with statistically significant breaks, only one exhibits a break attributable to apocenter pile-up of an accreted merger remnant. Decomposing the simulated profiles, we find that the accreted halo profiles are broadly consistent with Milky Way measurements, while the in-situ halo dominates within ≈20 kpc and drives the discrepancy. These results raise a compelling question: where is the Milky Way's in-situ halo? The origin of this tension may reflect systematic biases in current halo measurements, an overproduction of early spheroidal star formation in cosmological simulations, or more fundamental differences in the underlying physics (e.g., dark matter) governing the assembly and structure of the inner Galaxy.
ER -