The emergence of Galactic Scaling Laws

Upshot

Galactic discs are currently observed everywhere by the James Web telescope. But why do such thin discs survive in the concordance model? This question has long been set aside as an obvious consequence of angular momentum conservation. The true answer is more subtle and enlightening for astronomy. It involves capturing gravity-driven baryonic processes operating on multiple scales, working to spontaneously set up a remarkably efficient level of self-regulation. This regulation is responsible for disc resilience & the tightness of observed scaling laws (Tully-Fisher, radial acceleration relations, etc.).

Astrophysical Context

An accurate modelling of galactic morphological diversity over cosmic time is critical to achieve high precision on cosmological parameter estimation with galactic surveys such as Euclid & LSST relying on morphology. A key missing piece of our understanding of the universe is the persistence of thin galactic discs and the role they plays in the emergence of tight scaling laws. The operating assumption for their long-term dynamics has been that the Universe reached a quiet period about 10 Gyrs ago. However, the standard cosmological model assumes a perturbed past environment, with traces of significant disturbances found by Gaia within the Milky Way. This PhD will test whether the tension between both assumptions can be resolved: how can galaxy formation conspire with cosmic flows to set up an efficient self-regulated machine to produce the thin discs that are observed with the James Web Telescope (JWST) at high and low redshifts? What are the implications of such self-regulation on the tightness of observed scaling relations? Why does it matter for morphological survey science?

Galactic discs are immersed in various sources of perturbations and inflow. The PhD will show that these processes, which in isolation would have a destructive impact on thin discs, in fact conspire to maintain their responsiveness. The emergence —broadly defined as the “arising of novel and coherent features through self-organisation in complex systems”— of an improbable ordered structure (a massive yet thin disc) is indeed paradoxically made possible by shocks and turbulence induced in the sub dominant gaseous component, which can radiate most of the entropy generated from the CGM, acting as an open reservoir of free rotational energy. The interplay between gravity and baryonic physics set up a self-regulating loop near marginal stability, whose efficiency increases with cosmic time: the thinner the disc, the more self-regulated; the tighter the internal coupling, the thinner the disc. This spontaneously emerging self-regulation in turn tightens most galactic scaling laws, as it glues baryonic properties of the galaxies (sSFR, metallicity, stellar surface density etc.) to their dynamical properties (halo mass, angular momentum distribution etc.). Such a framework therefore also naturally explains the induced emergence of tight scaling relations.

Methods

Thanks to earlier validation of kinetic theory applied to stellar systems, which captured the role of heating via orbital diffusion on discs’ secular evolution (Roule+’24), and very recent developments in large deviation theory (Feliachi+’24), we are now in a position to implement open dissipative quasi-linear models to also account for gas cooling, so as to reach a coherent understanding of homeostasis (a.k.a thermal regulation), achieved via gravitational-wake-accelerated feedback loops. The PhD student will capture the evolution of self-gravitating discs as emergent dissipative structures, while accounting for the regulating role of inflowing cold gas.

When completed, the student will have demonstrated in detail how gravity with baryons provides top-down causation, from the cosmic web, via the circumgalactic medium (CGM), down to wake-controlled turbulent star formation and feedback in the intra-galactic medium. The PhD student will explain the appearance, and most importantly the resilience over cosmic time of such fragile galactic structures. The student will co-jointly explain why most galactic scaling laws are so tight, thanks to this self-regulation. Eventually, the student will provide means to marginalize over the corresponding biases (e.g. intrinsic scatter, per type of environment) to improve cosmological parameter estimation. This will prove enlightening as an archetype of emergent tight scaling relations that can be analysed in detail, while also explaining galactic morphological diversity.

PhD Goals

Accounting for self-regulation will be the core science of this PhD. The student will make clear predictions on the disc settling epoch, and will also explain why and how galactic discs conspire to sustain this unlikely state, and what are the corresponding observational signatures.

The PhD’ scientific goals are:

  1. To demonstrate how gravity-driven baryonic processes operate on multiple scales to spontaneously set up a remarkably efficient level of self-regulation, tightening galactic scaling laws.
  2. To develop the theoretical models and the computational algorithms to follow the thinning of discs over cosmic (secular) times, using extended kinetic theories (open, dissipative, large deviation).
  3. To cast results in terms of observables (bar/bulge fraction, disc thickness, scaling laws and dispersions of metallicity-kinematic relation or RAR, TF, KS, etc) tailored to existing and upcoming facilities.

Requirement

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

Framework

The PhD will be co-supervised by Christophe Pichon , Corentin Cadiou (IAP, Paris) and Maxime Trebitsch (Obs, Paris) as part of the SEGAL ANR (https://www.secular-evolution.org).

References

  • Roule, M., Fouvry , J. B., Pichon, C, Chavanis, P .H. “Long-term relaxation of one-dimensional self-gravitating systems”, Physical Review E, Volume 106, Issue 4, (2022)
  • Park, M. ; Yi, S K. ; Peirani, S; Pichon, C ; Dubois, Y; Choi, H; Devriendt, J; Kaviraj, S; Kimm, T; Kraljic, K; Volonteri, M. “Exploring the Origin of Thick Discs Using the NewHorizon and Galactica Simulations” ApJS, 254, pp. May (2021) arXiv:2009.12373
  • Fouvry , J-B, Pichon, C, Chavanis, P .H., and Monk, L.: Resonant thickening of self-gravitating discs: imposed or self-induced orbital diffusion in the tightly wound limit, MNRAS,471,2642 (2017) 2017MNRAS.471.2642F
  • Fouvry , J.B; Prunet, S.: “Linear response theory and damped modes of stellar clusters”, (2021) 2021arXiv210501371F
  • Fouvry , J.B; Pichon, C.; Magorrian, J.; Chavanis P .H. “Secular diffusion in discrete self-gravitating tepid discs II: accounting for swing amplification via the matrix method” A&A (2015) 584 129
  • Hamilton,C.; Fouvry , J.B; Binney J. ; Pichon, C.; “Revisiting relaxation in globular clusters” MNRAS (2018).481.2041H
  • Fouvry , J.B; Pichon, C.; Chavanis P .H. “The secular evolution of discrete quasi-Keplerian systems. II. Application to a multi-mass axisymmetric disc around a supermassive black hole” A&A
  • Codis, S., Pichon, C., Devriendt, J., Slyz, A., Pogosyan, D., Dubois, Y ., Sousbie, T., 'Connecting the cosmic web to the spin of dark haloes: implications for galaxy formation' , 2012, MNRAS, 427, 3320
  • Pichon, C., Pogosyan, D., Kimm, T., Slyz, A., Devriendt, J., Dubois, Y ., 'Rigging dark haloes: why is hierarchical galaxy formation consistent with the inside-out build-up of thin discs?' , 2011, MNRAS 418, 2493
  • Pichon, C., Aubert, D., 'Dynamical flows through dark matter haloes: inner perturbative dynamics, secular evolution and applications', 2006, MNRAS, 368, 1657