We present a measurement of the systemic proper motion of the Small Magellanic Cloud ( SMC ) made using the Advanced Camera for Surveys ( ACS ) on the Hubble Space Telescope ( HST ) .
We tracked the SMC ’ s motion relative to 4 background QSOs over a baseline of approximately 2 years .
The measured proper motion is : \mu _ { W } = -1.16 \pm 0.18 { { mas yr ^ { -1 } } } \ > , \mu _ { N } = -1.17 \pm 0.18 { { mas% yr ^ { -1 } } } \ > .
This is the best measurement yet of the SMC ’ s proper motion .
We combine this new result with our prior estimate of the proper motion of the Large Magellanic Cloud ( LMC ) from the same observing program to investigate the orbital evolution of both Clouds over the past 9 Gyr .
The current relative velocity between the Clouds is 105 \pm 42 { { km s ^ { -1 } } } \ > .
Our investigations of the past orbital motions of the Clouds in a simple model for the dark halo of the Milky Way imply that the Clouds could be unbound from each other .
However , our data are also consistent with orbits in which the Clouds have been bound to each other for approximately a Hubble time .
Smaller proper motion errors and better understanding of the LMC and SMC masses would be required to constrain their past orbital history and their bound vs. unbound nature unambiguously .
The new proper motion measurements should be sufficient to allow the construction of improved models for the origin and properties of the Magellanic Stream .
In turn , this will provide new constraints on the properties of the Milky Way dark halo .