Dynamical effects of satellite accretion on the Milky Way

Abstract

ΛCDM cosmology predicts that galaxies grow through the process of hierarchical structure formation. In the Milky Way (MW) galaxy, evidence of satellite accretion is present through observations of stellar streams in the outer halo, and chemo-kinematically distinct stellar populations in the inner regions. Recent surveys of the MW stars such as Gaia and the Sloan Digital Sky Survey (SDSS) provide an unprecedented view of the kinematic structure of stars in the MW halo and disc. In this thesis, I explore through simulations and data the small and large scale dynamical signatures arising in the kinematics of stars due to the infall of satellites.

In Chapter 2, I explore the dynamical effects of perturbations on the galactic disc. At relatively small galactic scales, perturbations to the galactic disc show distinct features in the vertical position-velocity structure. Data from the Gaia mission shows prominent phase-space spirals that are the signatures of disequilibrium in the MW disc. In my work, I explore a novel perspective of phase-space spiral in angular momentum (AM) space. Using Gaia DR3, I detected a prominent AM spiral in the solar neighbourhood. I demonstrate in this Chapter the relationship between the well-known z-vz spiral and the AM spiral. I show how z-vz maps to AM through simplifying assumptions. Further, by analytic modelling the orbits of stars in AM, I develop a generative model for the spiral where the disc is perturbed by a bulk tilt at an earlier time. My model successfully produces a winding spiral that varies with Lz and generally matches the salient features in the data across most Lz bins, though only a chi-by-eye fit. I find that modelling the phase spiral in AM is a promising method to constrain the timing and magnitude of the disc perturbation, and simultaneously fit MW disc potential parameters. The AM framework simplifies the interpretation of the phase spiral and offers a robust approach to modelling disequilibrium in the MW disc using all six dimensions of phase space.

Large scale perturbations, such as those arising from the infall of the Large Magellanic Cloud (LMC) into the MW, also leave distinct kinematic features in the outer halo of the MW. In Chapter 3, I measure the kinematic signature arising from the MW disc moving with respect to the outer stellar halo, which is observed as a dipole signal in the kinematics of stellar halo tracers and is called the reflex motion. I quantify how the reflex motion varies as a function of Galactocentric distance, finding that (i) the amplitude of the dipole signal increases as a function of radius, and (ii) the direction moves across the sky. I then compare the reflex motion signal against a compilation of published simulations that follow the MW–LMC interaction. These models show a similar trend of increasing amplitude of the reflex motion as a function of distance, but they do not reproduce the direction of the disc motion with respect to the stellar halo well. I also measure mean motions for the stellar halo as a function of distance, and discover radial compression in the outer halo and non-zero prograde rotation at all radii. By analysing a suite of n-body simulations in the literature, and measuring the reflex corrected bulk motions, I found that the compression signal is also present in MW–LMC models. On the other hand the rotation is not present, which suggests that it is not induced by the LMC. To validate the technique of measuring reflex, I constructed idealised tests and analysed the effects of sky coverage, binning and uncertainties in the data. I discuss prospects for directly constraining the mass and orbital history of the LMC through the impact on the motion of the MW stellar disc, and how the modelling of the reflex motion can be improved as more and better data become available.

Following the work in Chapter 3 that identified a wide range of reflex- and bulk- motions measure in literature simulations. In Chapter 4, I investigate the variation of the reflex and bulk motions in the MW halo as a function of the density profile in the outer MW, motivated to understand how the reflex motion responds to assumptions of the MW halo model. I model the MW halo as a broken NFW profile, where the outer halo is a power law beyond some characteristic break radius. I then used the analytic model to generate initial conditions and run a suite of simulations in basis function expansion code EXP . Each successive simulation had a MW halo with a steeper slope past the break radius. I then compute the reflex motion parameters for each simulation at the present day. I find that the amplitude of the reflex motion is not sensitive to a wide range of outer halo truncations with fixed LMC mass, while the MW–LMC mass ratios vary significantly due to the changing MW halo profile. To quantify the density perturbations, I analysed the basis function expansion coefficients, which encode the gravitational response. The result reveals systematic effects with stronger outer halo truncations. I find that a stronger outer halo truncation produces a smaller dipole distortion, while the quadrupole becomes stronger. I also find the signatures of a halo instability, arising from the truncation of the halo, whose oscillation frequency increases with increasing truncation slope.

In the final Chapter, I conclude the thesis and provide a future outlook on the research, building on the wealth of results and insights built throughout this thesis.