Reflex motion in the Milky Way stellar halo resulting from the Large Magellanic Cloud infall

Abstract

We present the results of N-body models of the Milky Way and Large Magellanic Cloud system and study the kinematic reflex motion in the stellar halo owing to the barycentre displacement of the disc. In agreement with previous studies, we find that the Milky Way disc may be moving at 40 km s -1 relative to the barycentre prior to the Large Magellanic Cloud infall. The resulting reflex motion is visible in tangential velocities of the stellar halo as a simple dipole. The signal is strongest for stars with long dynamical times, identifiable in position-velocity data as stars with large apocentres, whose dynamical memory is still well-represented by the unperturbed Milky Way potential. The signal varies across the sky depending on the stellar tracer and may be up to the same magnitude as the velocity of the disc centre-of-mass, making reflex motion a source of bias for Milky Way potential determinations based on the modeling of stellar streams and/or smooth halo tracers such as blue horizontal branch or RR Lyrae stars.

Quantifying the Stellar Halo's Response to the LMC's Infall with Spherical Harmonics

“…show that the MW globular clusters and dwarf satellites also show net motion in v z (and no net motion in v x , v y ); however, they note the caveat that these tracers may not be phase mixed in the MW potential. We note that the velocity shifts in these simulations result from two sources: the DM overdensity in the north (which gets stronger with distance, because the LMC spends more time at larger radii), and the net acceleration of the MW disk toward the LMC (e.g., Petersen & Peñarrubia 2020). Disentangling the relative contributions to the overall velocity shift from these two sources is beyond the scope of this work.…”

“…Several studies of simulations of the LMC's infall demonstrate that a massive LMC invalidates the assumption of an inertial Galactocentric reference frame, as the center of mass (COM) can be substantially displaced (by as much as 30 kpc; Gómez et al 2015) from the center of the Galaxy, resulting in net motion of the halo with respect to the MW disk. This net COM motion is predicted to be ∼40 km s 1 (GC19, Erkal et al 2019;Petersen & Peñarrubia 2020); Gómez et al (2015) find that the net motion could be as high as 75…”

The mass of our Milky Way

“…In another study, Erkal et al in 2020 [121] pointed out that since the infall of the massive LMC can induce a substantial reflex motion in the MW [e.g. 120,155,302], this can make our MW, in particular the outer stellar halo, be out of equilibrium. Under this picture, [121] investigated how the non-equilibrium affects the performance of the Watkins et al mass estimator and how this reflex motion affects the mass estimated by using Leo I [55].…”

Detection of the Milky Way reflex motion due to the Large Magellanic Cloud infall

“…Just past its closest approach of about 50 kpc, and flying by the Milky Way at an astonishing speed of 327 km s −1 (ref. 6 ), the Large Magellanic Cloud can affect our Galaxy in a number of ways, including dislodging the Milky Way disc from the Galactic centre-of-mass [7][8][9] . Here, we report evidence that the Milky Way disc is moving with respect to stellar tracers in the outer halo (40 < r < 120 kpc) at v travel = 32 +4 −4 km s −1 , in the direction (ℓ, b) apex = (56 +9 −9 , −34 +10 −9 ) degrees, which points at an earlier location on the LMC trajectory.…”

“…Distant tracers have long dynamical times (see Supplementary Information) and are thus slow to react to the gravitational pull of the LMC 9 . As the MW disc moves in response to the LMC, but the outer halo does not, a reflex motion arises in which the distant halo appears to move relative to the MW disc, when in fact it is the MW disc moving relative to the distant halo stars, an effect akin to the apparent motion of rain drops seen from a car driving through a downpour.…”