PROPER MOTIONS OF THE LARGE MAGELLANIC CLOUD AND SMALL MAGELLANIC CLOUD: RE-ANALYSIS OFHUBBLE SPACE TELESCOPEDATA

Abstract

Kallivayalil et al. have used the Hubble Space Telescope to measure proper motions of the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) using images in 21 and five fields, respectively, all centered on known quasi-stellar objects (QSOs). These results are more precise than previous measurements, but have surprising and important physical implications: for example, the LMC and SMC may be approaching the Milky Way for the first time; they might not have been in a binary system; and the origin of the Magellanic Stream needs to be re-examined. Motivated by these implications, we have re-analyzed the original data in order to check the validity of these measurements. Our work has produced a proper motion for the LMC that is in excellent agreement with that of Kallivayalil et al., and for the SMC that is in acceptable agreement. We have detected a dependence between the brightness of stars and their mean measured motion in a majority of the fields in both our reduction and that of Kallivayalil et al. Correcting for this systematic error and for the errors caused by the decreasing charge transfer efficiency of the detector produces better agreement between the measurements from different fields. With our improved reduction, we do not need to exclude any fields from the final averages and, for the first time using proper motions, we are able to detect the rotation of the LMC. The best-fit amplitude of the rotation curve at a radius of 275 arcmin in the disk plane is 120 ± 15 km s−1. This value is larger than the 60–70 km s−1 derived from the radial velocities of H i and carbon stars, but in agreement with the value of 107 km s−1 derived from the radial velocities of red supergiants. Our measured proper motion for the center of mass of the LMC is (μα, μδ) = (195.6 ± 3.6, 43.5 ± 3.6) mas century−1; that for the SMC is (μα, μδ) = (75.4 ± 6.1, − 125.2 ± 5.8) mas century−1. The uncertainties for the latter proper motion are three times smaller than those of Kallivayalil et al.