Secular Evolution of Galaxies – SEGAL

In the expanding universe, an over-dense perturbation passing a critical threshold will collapse onto itself. Through violent relaxation, it rapidly converges towards a stationary, phase-mixed and highly symmetric state. This perturbation will convert its cold gas into stars and, depending on how much entropy has been radiated away, produce either a spiral or an elliptical galaxy, with a partially frozen orbital structure. The galaxy is then locked in a quasi-stationary state imposed by its mean gravitational field. Yet such a self-gravitating relaxed system will continue to undergo a wide variety of dynamical processes, depending on its environment and on its internal structure and ‘temperature’– i.e. whether it is more pressure or centrifugally supported. Given time and stimuli, it has the opportunity to significantly reshuffle its orbital structure towards more likely configurations. SEGAL aims to understand this long-term reshuffling called gravity-driven secular evolution, described by so-called extended kinetic theories. They now allow us to gauge the respective roles of nature vs. nurture in establishing the observed dynamical properties of galaxies, relying on stochastic processes which capture all sources of fluctuations. This places the internal evolution of galaxies within the framework of cosmology.
The distinction between self- and externally-induced fluctuations allows us to disentangle their respective roles in sourcing secular evolution, as we quantify the diffusion signatures and the characteristic timescales associated with each source of fluctuations. The relative diffusive efficiency is sensitive to shot noise triggered by Giant Molecular Clouds, clumps within the halo, central bars, etc, and to external sources of fluctuations: flybys, etc. It is also sensitive to the system’s initial reservoir of free energy, which differs greatly between galactic components, and to the transformation of the underlying equilibria through infall. Once these mechanisms are statistically characterised and quantified, long-term implications can be compared to the novel features in phase space provided by GAIA and to the stellar orbits of GRAVITY.
In this context, the purpose of SEGAL is therefore to establish quasi-linear theory as an enlightening addition to N-body simulations to probe statistically the cosmic fate of galaxies on multiple scales over a good fraction of a Hubble time. Building up on practical advantage in capturing the long-term effect of self-gravity in an open multi-scale environment, we will proceed via the following computationally-demanding steps: i) we will quantify the environmental effects induced by the larger scale using ensemble average of simulations; compute the relevant power spectra driving externally induced fluctuations at each scale; ii) we will solve for the joint kinetic equations and quantifying each characteristic timescale, while accounting for the difference in temperature and degeneracies specific to each scale; identify the generic asymptotic solutions (resp. core halos, exponential discs, cusps...); iii) we will cast our results in terms of observables, and propose new observational diagnostics to test our theoretical predictions.
Eventually we will be able to gauge the roles of nature vs. nurture in establishing the observed properties of galaxy population on small and large scales, something currently out of reach of standard N-body techniques. More generally, gravity, with its rich phenomenology, is ideally suited to help us understand in detail the secular implications of collective modes, shot noise and resonances. It will continue to enlighten our understanding of the underlying mathematics, capturing generic processes such as entropy production, anisotropic resonant diffusion, secular phase transition etc. Hence gravitational kinetic theory will also carry on stimulating fascinating research in theoretical physics and beyond.