Constraining the Milky Way halo kinematics via its Linear Response to the Large Magellanic Cloud

Abstract

We model the response of spherical, non-rotating Milky Way (MW) dark matter and stellar halos to the Large Magellanic Cloud (LMC) using the matrix method of linear response theory. Our computations reproduce the main features of the dark halo response from simulations. We show that these features can be well separated by a harmonic decomposition: the large scale over/underdensity in the halo (associated with its reflex motion) corresponds to the = 1 terms, and the local overdensity to the ≥ 2 multipoles. Moreover, the dark halo response is largely dominated by the first order 'forcing' term, with little influence from self-gravity. This makes it difficult to constrain the underlying velocity distribution of the dark halo using the observed response of the stellar halo, but it allows us to investigate the response of stellar halo models with various velocity anisotropies: a tangential (respectively radial) halo produces a shallower (respectively stronger) response. We also show that only the local wake is responsible for these variations, the reflex motion being solely dependent on the MW potential. Therefore, we identify the structure (orientation and winding) of the in-plane quadrupolar (m = 2) response as a potentially good probe of the stellar halo anisotropy. Finally, our method allows us to tentatively relate the wake strength and shape to resonant effects: the strong radial response could be associated with the inner Lindblad resonance, and the weak tangential one with corotation.

On the Effect of the Large Magellanic Cloud on the Orbital Poles of Milky Way Satellite Galaxies

“…We note that this approach does only take the reflex motion and COM offset into account, but not any orbital pole enhancement due to the wake of the LMC or its direct torque effects on MW satellites galaxies. As recently shown analytically by Rozier et al (2022), the latter effect from the dark matter wake itself on the response of stars or satellites is a minor one. However, these additional effects will all be included in a self-consistent manner in the numerical simulations presented and analyzed from Section 3 onwards.…”

On the self-consistent time-dependent linearized response of stellar discs to external perturbations

“…In this paper we have presented a practical scheme for following the response of 2D stellar discs to internal and external perturbations by following Julian & Toomre (1966); Seguin & Dupraz (1994); Pichon & Aubert (2006) and Murali (1999) among others in recasting the linearised CBE as a Volterra integral equation of the first kind, which is easy to solve numerically. Rozier et al (2022) have recently presented a similar application to galactic halos.…”

“…Pichon & Cannon 1997;Evans & Read 1998;De Rijcke et al 2019), but for more general scenarios it is an unnecessary obfuscation and it makes more sense to solve for the time evolution directly (e.g. Julian & Toomre 1966;Seguin & Dupraz 1994;Murali 1999;Rozier et al 2022). We explain the difference between these two approaches in Section 2.…”