Third-Epoch Magellanic Cloud Proper Motions I: HST/WFC3 data and Orbit Implications (1301.0832v1)

Abstract: We present proper motions for the Large & Small Magellanic Clouds (LMC & SMC) based on three epochs of \textit{Hubble Space Telescope} data, spanning a $\sim 7$ 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-of-mass proper motions: mu_W(LMC) = -1.910 +/- 0.020 mas/yr, mu_N(LMC) = 0.229 +/- 0.047 mas/yr, and mu_W(SMC) = -0.772 +/- 0.063 mas/yr, mu_N(SMC) = -1.117 +/- 0.061 mas/yr. 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 and v_{tot,SMC} = 217 +/- 26 km/s. 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 $\lesssim 4$ 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 paper refines center-of-mass proper motions for the LMC and SMC with 1-2% error using a 7-year HST/WFC3 dataset.
• Precise measurements reveal an LMC velocity of 321±24 km/s and an SMC velocity of 217±26 km/s, supporting a first-infall scenario.
• The results challenge traditional Magellanic Stream models, highlighting the role of binary interactions over repeated MW passages.

Analysis of Magellanic Cloud Proper Motions

The paper titled "Third-Epoch Magellanic Cloud Proper Motions I: HST/WFC3 Data and Orbit Implications" presents significant advancements in the measurement of proper motions (PMs) of the Large and Small Magellanic Clouds (LMC and SMC) by utilizing three epochs of Hubble Space Telescope (HST) data. This research focuses on analyzing HST/Wide Field Camera 3 (WFC3) data, enhancing constraints on the dynamics and orbital history of the Magellanic Clouds with respect to the Milky Way (MW).

The dataset spans approximately a 7-year baseline and integrates observations aimed at Quasi-Stellar Objects (QSOs) in the Clouds' fields. The improved precision and reduced systematic errors of this third epoch with WFC3 have led to errors of only 1-2% per field, allowing the authors to achieve highly refined center-of-mass (COM) motions for both the LMC and SMC.

Key Numerical Results and Their Implications

• LMC Proper Motion: The measured COM PMs for the LMC are $\mu_{W, LMC} = -1.910 \pm 0.020$ mas/yr and $\mu_{N, LMC} = 0.229 \pm 0.047$ mas/yr. This leads to a total Galactocentric velocity ($v_{\rm tot}$) of $321 \pm 24$ km/s.
• SMC Proper Motion: For the SMC, the COM PMs are $\mu_{W, SMC} = -0.772 \pm 0.063$ mas/yr and $\mu_{N, SMC} = -1.117 \pm 0.061$ mas/yr, corresponding to a $v_{\rm tot}$ of $217 \pm 26$ km/s.
• Relative Velocities: The relative velocity of the SMC to the LMC is found to be $128 \pm 32$ km/s, with a notably larger radial than tangential component.

These findings, coupled with revisions in our understanding of solar motion, align with a hypothesis that the Clouds are on their first infall into the MW. Notably, traditional models employing shorter orbital periods for the Magellanic Clouds fail to align with these updated velocities, posing challenges for older Magellanic Stream formation theories reliant on multiple pericentric passages.

Theoretical and Practical Implications

The results provide new constraints on the internal kinematics of the Magellanic Clouds and suggest that traditional models for the formation of the Magellanic Stream, which involve multiple MW-centric orbits, may need revision. The preferred interpretation involves first-infall scenarios, which align more closely with the observed orbital eccentricities and cosmological expectations of satellite dynamics in a $\Lambda$CDM universe.

These implications extend to understanding satellite interaction models, offering insights into the hierarchical assembly and dynamical friction within the MW halo. By reinforcing the importance of the LMC-SMC binary interaction over MW influences in forming the Stream, the paper proposes a paradigm shift in modeling this prominent Galactic feature.

Future Developments and Research Directions

The advent of numerous new QSO identifications behind the Clouds promises improved spatial coverage for PM studies. Further, the development of absolute PM measurement techniques via resolved background galaxies could refine these measurements, offering direct, absolute velocity perspectives.

In summary, this paper contributes critically refined measurements and interpretations to the kinematics of the Magellanic Clouds, challenging existing models and emphasizing the role of internal satellite dynamics and binary interactions. The transition towards innovative models of the Magellanic Stream underlines the evolving understanding of the profound cosmic ballet of galaxies in the vicinity of the MW.