Substructure at High Speed. I. Inferring the Escape Velocity in the Presence of Kinematic Substructure

Lina Necib,Tongyan Lin

doi:10.3847/1538-4357/ac4243

Abstract

Abstract The local escape velocity provides valuable inputs to the mass profile of the galaxy, and requires understanding the tail of the stellar speed distribution. Following Leonard & Tremaine, various works have since modeled the tail of the stellar speed distribution as ∝ ( v esc − v ) k , where v esc is the escape velocity, and k is the slope of the distribution. In such studies, however, these two parameters were found to be largely degenerate and often a narrow prior is imposed on k in order to constrain v esc. Furthermore, the validity of the power-law form can breakdown in the presence of multiple kinematic substructures or other mis-modeled features in the data. In this paper, we introduce a strategy that for the first time takes into account the presence of kinematic substructure. We model the tail of the velocity distribution as a sum of multiple power laws as a way of introducing a more flexible fitting framework. Using mock data and data from FIRE simulations of Milky Way-like galaxies, we show the robustness of this method in the presence of kinematic structure that is similar to the recently discovered Gaia Sausage. In a companion paper, we present the new measurement of the escape velocity and subsequently the mass of the Milky Way using Gaia eDR3 data.

Highlights

Evidence of the theory of hierarchical galaxy formation (White & Rees 1978) has been abundant in recent years
Following Leonard & Tremaine (1990), various works have since modeled the tail of the stellar speed distribution as ∝k, where vesc is the escape velocity, and k is the slope of the distribution
We argue for an approach that can more robustly determine where the “tail” of the stellar speed distribution is, and that takes into account the presence of kinematic substructure

Summary

Introduction

Evidence of the theory of hierarchical galaxy formation (White & Rees 1978) has been abundant in recent years. The Gaia mission (Lindegren et al 2016; Gaia Collaboration et al 2018) has shed light on some of these substructures, and in particular led to the identification of a large debris flow called the Gaia Sausage (Belokurov et al 2018), or Gaia Enceladus (Helmi et al 2018). Such a structure extends to ∼ 30 kpc, including stars on highly eccentric orbits. It is most likely the product of a merging satellite of a stellar mass 108−9M that was disrupted at about redshift z ∼ 1 − 3 (Myeong et al 2018b; Deason et al 2018; Lancaster et al 2018)

Objectives

Results

Conclusion