Characterizing the SHARDS of Disrupted Milky Way Satellites with LAMOST

TL;DR

This study uses LAMOST spectroscopic data to identify and quantify substructure in the Milky Way halo, revealing that at least 10% of halo stars are part of stellar debris structures indicative of past accretion events.

Contribution

It introduces a statistical method to detect stellar debris structures (SHARDS) in the Galactic halo using synthetic models and observational data, advancing understanding of the Milky Way's merger history.

Findings

At least 10% of halo stars are part of SHARDS.

Excess substructure observed across all radii < 40 kpc.

Substructure levels match models from accreted satellites.

Abstract

We derive the fraction of substructure in the Galactic halo using a sample of over 10,000 spectroscopically-confirmed halo giant stars from the LAMOST spectroscopic survey. By observing 100 synthetic models along each line of sight with the LAMOST selection function in that sky area, we statistically characterize the expected halo populations. We define as SHARDS (Stellar Halo Accretion Related Debris Structures) any stars in >3-sigma excesses above the model predictions. We find that at least 10% of the Milky Way halo stars from LAMOST are part of SHARDS. By running our algorithm on smooth halos observed with the LAMOST selection function, we show that the LAMOST data contain excess substructure over all Galactocentric radii R_GC < 40 kpc, beyond what is expected due to statistical fluctuations and incomplete sampling of a smooth halo. The level of substructure is consistent with the fraction of stars in SHARDS in model halos created entirely from accreted satellites. This work illustrates the potential of vast spectroscopic surveys with high filling factors over large sky areas to recreate the merging history of the Milky Way.