# The total mass of the Large Magellanic Cloud from its perturbation on the Orphan stream

## TL;DR

This paper uses the Orphan stream's perturbation to measure the Large Magellanic Cloud's mass, revealing its significant gravitational influence on Milky Way structures and challenging static galaxy models.

## Contribution

It provides the first measurement of the LMC's mass based on stream perturbations, demonstrating its substantial impact on the Milky Way's halo dynamics.

## Key findings

- LMC mass estimated at approximately 1.38 x 10^{11} solar masses
- Milky Way mass within 50 kpc is about 3.80 x 10^{11} solar masses
- Predicted bulk upward motion of the Milky Way's stellar halo outskirts

## Abstract

In a companion paper by Koposov et al., RR Lyrae from \textit{Gaia} Data Release 2 are used to demonstrate that stars in the Orphan stream have velocity vectors significantly misaligned with the stream track, suggesting that it has received a large gravitational perturbation from a satellite of the Milky Way. We argue that such a mismatch cannot arise due to any realistic static Milky Way potential and then explore the perturbative effects of the Large Magellanic Cloud (LMC). We find that the LMC can produce precisely the observed motion-track mismatch and we therefore use the Orphan stream to measure the mass of the Cloud. We simultaneously fit the Milky Way and LMC potentials and infer that a total LMC mass of $1.38^{+0.27}_{-0.24} \times10^{11}\,\rm{M_\odot}$ is required to bend the Orphan Stream, showing for the first time that the LMC has a large and measurable effect on structures orbiting the Milky Way. This has far-reaching consequences for any technique which assumes that tracers are orbiting a static Milky Way. Furthermore, we measure the Milky Way mass within 50 kpc to be $3.80^{+0.14}_{-0.11}\times10^{11} M_\odot$. Finally, we use these results to predict that, due to the reflex motion of the Milky Way in response to the LMC, the outskirts of the Milky Way's stellar halo should exhibit a bulk, upwards motion.