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ABSTRACT We present a model for the formation of the Magellanic Stream (MS) due to ram pressure stripping. We model the history of the Small and Large Magellanic Clouds in the recent cosmological past in a static Milky Way (MW) potential with diffuse halo gas, using observationally motivated orbits for the Magellanic Clouds derived from HST proper motions within the potential of the MW. This model is able to reproduce the trailing arm but does not reproduce the leading arm feature, which is common for models of the stream formation that include ram pressure stripping effects. While our model does not outperform other models in terms of matching the observable quantities in the MS, it is close enough for our ultimate goal – using the MS to estimate the MW mass. By analysing our grid of models, we find that there is a direct correlation between the observed stream length in our simulations and the mass of the MW. For the observed MS length, the inferred MW mass is 1.5 ± 0.32 × 1012$$\, \mathrm{M}_\odot$$, which agrees closely with other independent measures of the MW mass. We also discuss the MS in the context of H i streams in galaxy clusters, and find that the MS lies on the low-mass end of a continuum from Hickson groups to the Virgo cluster. As a tracer of the dynamical mass in the outer halo, the MS is a particularly valuable probe of the MW’s potential. 

Abstract We show that a small but measurable shift in the eclipse midpoint time of eclipsing binary (EBs) stars of ∼0.1 s over a decade baseline can be used to directly measure the Galactic acceleration of stars in the Milky Way at ∼kiloparsec distances from the Sun. We consider contributions to the period drift rate from dynamical mechanisms other than the Galaxy’s gravitational field and show that the Galactic acceleration can be reliably measured using a sample of Kepler EBs with orbital and stellar parameters from the literature. The contribution from tidal decay we estimate here is an upper limit assuming the stars are not tidally synchronized. We find there are about 200 detached EBs that have estimated timing precision better than 0.5 s, and for which other dynamical effects are subdominant to the Galactic signal. We illustrate the method with a prototypical, precisely timed EB using an archival Kepler light curve and a modern synthetic HST light curve (which provides a decade baseline). This novel method establishes a realistic possibility to constrain dark matter substructure and the Galactic potential using eclipse timing to measure Galactic accelerations, along with other emerging new methods, including pulsar timing and extreme-precision radial velocity observations. This acceleration signal grows quadratically with time. Therefore, given baselines established in the near future for distant EBs, we can expect to measure the period drift in the future with space missions like JWST and the Roman Space Telescope. 

ABSTRACT We show that smoothed particle hydrodynamics (SPH) simulations of dwarf galaxies interacting with a Milky Way-like disc produce moving groups in the simulated stellar disc. We analyse three different simulations: one that includes dwarf galaxies that mimic the Large Magellanic Cloud, Small Magellanic Cloud, and the Sagittarius dwarf spheroidal; another with a dwarf galaxy that orbits nearly in the plane of the Milky Way disc; and a null case that does not include a dwarf galaxy interaction. We present a new algorithm to find large moving groups in the VR, Vϕ plane in an automated fashion that allows us to compare velocity substructure in different simulations, at different locations, and at different times. We find that there are significantly more moving groups formed in the interacting simulations than in the isolated simulation. A number of dwarf galaxies are known to orbit the Milky Way, with at least one known to have had a close pericentre approach. Our analysis of simulations here indicates that dwarf galaxies like those orbiting our Galaxy produce large moving groups in the disc. Our analysis also suggests that some of the moving groups in the Milky Way may have formed due to dynamical interactions with perturbing dwarf satellites. The groups identified in the simulations by our algorithm have similar properties to those found in the Milky Way, including similar fractions of the total stellar population included in the groups, as well as similar average velocities and velocity dispersions.