Secular Public

Astrophysical Context

Most stars, perhaps all, are born in stellar discs. Major mergers destroyed some of these discs quite early in the history of the universe, but some discs have survived up to the present day, including the Milky Way. Understanding the secular dynamics of stellar discs therefore appears as an essential ingredient of cosmology, as the discs’ cosmological environments are now firmly established in the ΛCDM model.

Self-gravitating stellar discs are cold responsive dynamical systems in which rotation provides an important reservoir of free energy and where orbital resonances play a key role. The availability of free energy leads to some stimuli being strongly amplified, while resonances tend to localise their dissipation, with the net result that even a very small perturbation can lead to discs evolving to significantly distinct equilibria.

The problem of explaining the origin of thick discs in our Galaxy has been around for some time. The interest for this dynamical question has been revived recently in the light of Gaia data. Star formation within stellar discs typically occurs on the circular orbits of the gas, so that young stars should form a very thin disc (Wielen’77). However, chemo-kinematic observations of old stars within our Milky Way or in other galactic discs have all shown that thick components are very common. Yet the formation of thickened stellar discs remains a significant puzzle for galactic formation theory. Gaia data viewed in action-space is, as predicted, fairly structured in the solar neighbourhood. Some of these ridges are possibly transients, while others are of secular nature.

Upshot

The PhD candidate will follow up on Fouvry+’15 and evolve kinetic equations, accounting for mass and angular momentum inflow within the disc, and modeling self-consistently churning (drift in guiding center) and blurring (heating) over a Hubble time, while accounting for secular cold gas infall from the large-scale-structure environment (Pichon+’11).

The PhD candidate will quantify the expected cosmic evolution of the vertical structure of the disc and its population, relying on the multi-component generality of the kinetic formalism. This will involve building perturbatively thickened equilibria with a mapping in action-space from an integrable to a non-integrable model via fits of generating functions. The PhD candidate will then solve the exact field equations, construct an appropriate basis of potentials, and deal with the full response matrix in order to solve for the corresponding Balescu-Lenard and Fokker Planck equations. He/she will finally derive the corresponding global kinetic observables: age-vertical dispersion gradients and 3D velocity ellipsoid as a function of position and cosmic age and population, etc.

In connection with recent work on the inhomogeneous Balescu equation for the Hamiltonian mean field model (Benetti+’17), a toy model mimicking orbital resonances near a bar, the PhD candidate will study how collisional-driven separatrix crossing can explain dissolution and/or buckling of galactic bars.

Objectives

• Computation of the timescale for disc thickening, while accounting for self-gravity, as induced by, e.g. DM halo fluctuations, GMCs, passing-by satellites.
• Computation of the diffusion coefficients in orbital space for churning (diffusion in angular momentum) and blurring (diffusion in eccentricity), while accounting for self-gravity, as induced by different heating mechanisms, e.g. bars, spiral arms, GMCs, passing-by satellites, and DM halo fluctuations.
• Characterisation of diffusion in extended phase space, i.e. accounting for metallicity and age, and the respective signatures associated with the different heating sources.
• Global asymptotic and steady-state solution for exponential discs.
• Characterisation of the orbital diffusion signatures in the vicinity of the Sun, as traced by GAIA DR2 via marginals of DF(J,τ) and the corresponding kinetic estimators.
• Theory for bar dissolution and buckling.
• Self-consistent model for cosmic disc settling.

Requirement

Strong interest in theoretical astronomy, dynamics, analytical and numerical work.

Framework

The PhD will be co-supervised by Jean-Baptiste Fouvry and Christophe Pichon, as part of the SEGAL ANR (www.secular-evolution.org).