Secular Bars Public

Upshot

Understanding the diversity of galaxy shapes across cosmic history is essential for making the most of major sky surveys such as Euclid and LSST. These surveys rely heavily on galaxy morphology (their shapes and structures) to extract precise information about the universe. Yet one of the main unsolved puzzles is why thin galactic discs, delicate structures of stars and gas, persist over billions of years, and why they so consistently obey tight scaling laws (the regular patterns that link galaxy properties together).

Astrophysical Context

In this context, the candidate will address a set of fundamental questions: how do galaxy interact with the large-scale cosmic flows to build an efficient, self-regulating system that produces these ubiquitous thin discs? How does this gravity-driven regulation compare with the widely used picture of galaxies shaped mainly by stellar or black hole feedback? And why does the persistence of these fragile discs matter for the precision of galaxy surveys?

They will challenge the mainstream understanding of galactic disc formation and stability. They will posit that thin galactic discs are not sustained by fine-tuned feedback mechanisms, but rather represent emergent systems driven by gravity and baryonic processes to a state of homeostasis near marginal gravitational stability. The thesis will provide quantitative analytical insight into "why" galaxies behave as they do, complementing the "what" shown by simulations, and ultimately improve cosmological parameter estimation by reducing biases related to galaxy morphology.

With this PhD, the candidate will model galactic discs as self-organising structures that are constantly fed and regulated by streams of cold gas flowing in from the cosmic web. The outcome will be a detailed explanation of how gravity, acting together with normal matter, imposes (twice) a top-down order on galaxies via shocks: from the largest scales of the cosmic web, through the circumgalactic medium surrounding galaxies, down to the shock-driven processes of star formation inside discs. This framework will explain not only the emergence of thin discs, but also their remarkable resilience over cosmic time – and why galactic scaling laws are so tight. Beyond galaxy formation itself, this investigation will also shed light on how complex, self-regulating structures emerge in nature under the simple yet universal influence of gravity.

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) as part of the GALBAR ANR (https://www.secular-bars.org).

References

• This 2025 ERC application Δ and this 2025 ANR application Δ,
• and a corresponding presentation at the Cargese turbulent workshop.
• The late 2024 presentation Δ and the corresponding 60 minutes video (in english).
• This IPI 2024 funding application Δ
• This other ERC funding application Δ
• The late 2022 presentation Δ and the corresponding 15 minutes video (in english).
• An (early) 2022 presentation Δ and the corresponding 30 minutes video (in english) which gives a bit more details about the mathematics
• This general audience video (in french)
  
• PhDs of former students available here.

Attach:NHround.jpg Δ

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