In this EAS Special Session, we aim to bring together observational and theoretical understanding of the multi-phase molecular ISM to build on the significant advances from JWST and ALMA made in the past few years.



The star-forming potential of a galaxy is fundamentally governed by molecular gas. Consequently, star formation in a galaxy can be regulated either by removing this gas from the host galaxy or by heating it to higher temperatures. Identifying which of these processes dominates in external galaxies, and whether this changes with redshift, remains an open question.

Molecular gas spans a wide temperature range but is typically divided into three phases: cold (T < 100 K), warm (100 < T < 1000 K), and hot (T > 1000 K). Cold molecular gas is primarily traced through sub-mm transitions such as CO and is considered the fuel for star formation, whereas the warm and hot phases are probed via rotational and ro-vibrational transitions in the mid- and near-infrared, respectively. Warm molecular gas can be excited by fluorescence, external UV radiation from star formation or AGN, or shocks driven by outflows and radio jets.

Over the past few decades, sub-mm spectroscopic facilities, particularly ALMA (also NOEMA, IRAM, SMA, VLA), have revolutionised our understanding of the cold molecular gas. These studies have revealed key insights into gas morphology, kinematics, and star formation efficiencies across normal star-forming galaxies, starbursts, quenched systems, and AGN hosts.

With the launch of JWST, a new window has opened on the warm and hot molecular gas in external galaxies. Early results already show warm gas occupying regions devoid of CO-traced cold molecular gas. Burgeoning spectroscopic datasets from NIRSpec and MIRI are now revealing the excitation mechanisms, morphology, and kinematics of warm molecular gas not only in the nearby universe but also at Cosmic Noon, capabilities that were beyond reach with Spitzer/IRS.

Meanwhile, high-resolution simulations such as STARFORGE can now resolve molecular gas directly, incorporating the effects of external radiation, shocks, and dust. Post-processing tools further allow extraction of molecular gas properties from simulations, providing a bridge to observations.

To date, communities focusing on cold versus warm/hot molecular gas in external galaxies have largely worked in parallel. With this EAS session on multi-phase molecular gas with JWST and ALMA, we will bring these groups together to discuss our current understanding of the molecular ISM, identify synergies, and explore how observations and simulations can be integrated to deliver comparable and complementary results.

Programme

• What have we learned about stellar and AGN feedback from tracing cold molecular gas across cosmic time? This includes gas content, morphology, kinematics, and the role of molecular outflows and coupling efficiencies.
• How is JWST infrared spectroscopy advancing our ability to trace warm and hot molecular gas, and in what ways do these phases differ from the cold gas?
• What predictions do simulations make for the content and composition of molecular gas, and how do they compare with current and future observations?

Invited speakers

• Richard Davies (Max Planck Institute for Extraterrestrial Physics, Germany)
• Pierre Guillard (Institut d'Astrophysique de Paris, France)
• Amelie Saintonge (Max Planck Institute for Radio Astronomy, Germany)

Scientific organisers

Darshan Kakkad (University of Hertfordshire, UK, co-chair), David Rosario (Newcastle University, UK, co-chair), Marina Bianchin (IAC, Spain), Vincenzo Mainieri (ESO, Germany), Timothy Davis (Cardiff University, UK)

Contact

Darshan Kakkad (d.m.kakkad @ herts.ac.uk), David Rosario (david.rosario @ ncl.ac.uk)