The Milky Way Tomography with Subaru Hyper Suprime-Cam: Implications for the past orbit of the Large Magellanic Cloud

TL;DR

Deep Subaru imaging reveals a new diffuse stellar substructure in the Milky Way's outer halo, providing insights into the LMC's past orbit and its tidal debris, favoring a second-passage scenario.

Contribution

First detection of this halo substructure using Subaru data, with analysis supporting the LMC's second-passage orbit over the first.

Findings

Substructure extends over at least 100 deg² beyond 30 kpc.

Observed overdensity aligns with second-passage LMC debris predictions.

Chemo-kinematical data needed for further orbit constraints.

Abstract

We report the discovery of diffuse stellar substructure in the Milky Way's outer halo toward Bo\"otes, unveiled by deep imaging data of the Subaru/Hyper Suprime-Cam. This substructure is detected as an excess of faint main-sequence stars, at heliocentric distances beyond 30 kpc, extending over at least 100 $\mathrm{deg^2}$. To infer its origin, we compare the projected spatial distribution of these stars to that of simulated tidal debris from the Large Magellanic Cloud (LMC), under the assumptions that the LMC is on either its first or second passage of the Milky Way. We found that the observed overdensity lies in a region of the halo where debris from the LMC is expected if it is on its initial pericentric phase 7-8 Gyr ago, which is predicted in the second-passage model, while the first-passage model is unable to explain the observed substructure. Chemo-kinematical data are required to further constrain its past orbit and to understand the origin of this new halo substructure, as should be obtained in the near future with deep photometric surveys such as UNIONS and LSST, and wide-field spectroscopy such as possible with PFS and DESI.