The physics of molecular hydrogen in space with JWST

IAP weekly specialised seminars / 14 June 2024
Pierre Guillard (Institut d'Astrophysique de Paris, Sorbonne Université & CNRS, France)
Our progress some fundamental questions in astrophysics, namely the formation of planets, stars and galaxies, depends critically on our ability to observe molecular hydrogen (H2). Initiator of complex chemistry in the interstellar medium, H2 is a tracer of molecular gas which complements carbon monoxide (CO) observations. It probes the low-metallicity molecular material in the outskirts of galaxies, and also unveils the energetic processes that shape the structure of the interstellar medium (ISM), from galactic scales to the scales of turbulent dissipation. H2 is an important coolant of shocked gas, including the virialized gas that cools to form early galaxies. The excitation conditions of H2, highly variable in the local Universe, encode information on the gas accretion modality, heating conditions, and AGN-driven feedback processes.
The James Webb Telescope (JWST) is opening a fully new perspective in our capability to observe H2 emission in the infrared, with a two order of magnitude leap forward in sensitivity, spatial and spectral resolution compared to the previous generation of telescopes. This talk will outline some of the recent observational results, with a focus on galaxy interactions and nearby active galaxies, in which the total H2 emission cannot be explained by the sole reprocessing of the available UV radiative energy. The modelling of molecular and atomic lines, which quantifies the dissipation rate and excitation of the irradiated and shocked gas, allows us to derive the radiative and mechanical energy budgets of galaxies, and determine in which phase of the gas the kinetic energy is dissipated. JWST data highlight in particular the link between AGN activity and turbulent dissipation, an exciting perspective to address the open question of the regulation of the gas content in galaxies.