THIRD-EPOCH MAGELLANIC CLOUD PROPER MOTIONS. I.HUBBLE SPACE TELESCOPE/WFC3 DATA AND ORBIT IMPLICATIONS

Abstract

We present proper motions for the Large & Small Magellanic Clouds (LMC & SMC) based on three epochs of Hubble Space Telescope data, spanning a ∼ 7 yr baseline, and centered on fields with background QSOs. The first two epochs, the subject of past analyses, were obtained with ACS/HRC, and have been reanalyzed here. The new third epoch with WFC3/UVIS increases the time baseline and provides better control of systematics. The three-epoch data yield proper motion random errors of only 1-2% per field. For the LMC this is sufficient to constrain the internal proper motion dynamics, as will be discussed in a separate paper. Here we focus on the implied center-ofmass proper motions: µ W,LMC = −1.910 ± 0.020 mas yr −1 , µ N,LMC = 0.229 ± 0.047 mas yr −1 , and µ W,SMC = −0.772 ± 0.063 mas yr −1 , µ N,SMC = −1.117 ± 0.061 mas yr −1 . We combine the results with a revised understanding of the solar motion in the Milky Way to derive Galactocentric velocities: v tot,LMC = 321 ± 24 km s −1 and v tot,SMC = 217 ± 26 km s −1 . Our proper motion uncertainties are now dominated by limitations in our understanding of the internal kinematics and geometry of the Clouds, and our velocity uncertainties are dominated by distance errors. Orbit calculations for the Clouds around the Milky Way allow a range of orbital periods, depending on the uncertain masses of the Milky Way and LMC. Periods 4 Gyr are ruled out, which poses a challenge for traditional Magellanic Stream models. First-infall orbits are preferred (as supported by other arguments as well) if one imposes the requirement that the LMC and SMC must have been a bound pair for at least several Gyr.

The VMC survey

“…Centre of mass proper motion measurements from the literature for both the SMC and 47 Tuc are indicated by empty squares whose sizes correspond to their statistical uncertainties. For the SMC the red rectangles (from smallest to largest) give the values from Kallivayalil et al (2013), Costa et al (2011), Vieira et al (2010, respectively, and for 47 Tuc the blue rectangle gives the value from Anderson & King (2003).…”

“…We note that the comparison always refers to proper motion measurements that are not corrected for solar reflex motion. The most accurate proper motion of the SMC to date has been measured using the HST (Kallivayalil et al 2013). The authors analysed five fields centred on one background quasar and aligned with the east-west axis of the galaxy at an angle of ∼45 • .…”

“…Sohn et al 2013;Massari et al 2013), including the Magellanic Clouds (e.g. Kallivayalil et al 2013); of the Sagittarius stream (Sohn et al 2015); and of globular clusters (e.g. Bellini et al 2014).…”

Local Group Proper Motion Dynamics

“…Given the individual LMC and SMC PM measurements, the first-infall models are the only ones in which the LMC and SMC form a bound pair (Kallivayalil et al 2013), consistent with their structures and star formation histories. The plausibility of a first-infall scenario is further supported by both cosmological simulations (Boylan-Kolchin et al 2011) and the gas-rich nature of the Clouds (van den Bergh 2006).…”

“…Kallivayalil et al (2006a,b) presented PMs based on two epochs of HST data with a 2-year time baseline. These measurements were recently refined by Kallivayalil et al (2013) using a third-epoch of HST data that extends the time baseline to 7 years (see Fig. 1).…”