New Analysis of the Proper Motions of the Magellanic Clouds using HST/WFPC2

TL;DR

This study measures the proper motions of the Magellanic Clouds using HST data, revealing higher velocities than expected and suggesting they may be on their first orbit around the Milky Way, with implications for galaxy interaction models.

Contribution

It provides the first high-precision proper motion measurements of the Magellanic Clouds using multiple HST epochs, challenging previous assumptions about their orbital history.

Findings

Proper motions are larger than previously thought.

The Clouds may be on their first passage around the Milky Way.

The LMC and SMC may not be gravitationally bound.

Abstract

In HST Cycles 11 and 13 we obtained two epochs of ACS/HRC data for fields in the Magellanic Clouds centered on background quasars. We used these data to determine the proper motions of the LMC and SMC to better than 5% and 15% respectively. The results had a number of unexpected implications for the Milky Way-LMC-SMC system. The implied three-dimensional velocities were larger than previously believed and close to the escape velocity in a standard 10^12 solar mass Milky Way dark halo, implying that the Clouds may be on their first passage. Also, the relative velocity between the LMC and SMC was larger than expected, leaving open the possibility that the Clouds may not be bound to each other. To further verify and refine our results we requested an additional epoch of data in Cycle 16 which is being executed with WFPC2/PC due to the failure of ACS. We present the results of an ongoing analysis of these WFPC2 data which indicate good consistency with the two-epoch results.