In the past decade advances in observing techniques and in galaxy formation simulations drew the attention to the role of the circumgalactic medium (CGM) as a key ingredient to understand how material is exchanged between galaxies and their Cosmological environment.
To trace processes in the CGM it is critical to test its state in Galaxy Clusters (CGs). In GCs the CGM is expected to be different from the Interstellar Medium (ISM) inside galaxies because of the presence of bright cluster galaxies (BCGs).
Recent observations have indeed shown that the CGM molecular gas state is much more extreme with cases of extreme thermal decoupling of gas and dust reservoirs, where large amounts of lower-density, galaxy-expelled, molecular gas and dust are detected.
I present observations towards two clusters at low (z=0.35) and high (z=2.2) redshift targeting the cold gas through its different tracers in the sub-millimetre regime: Carbon Monoxide and atomic Carbon. These data combined with upcoming JWST MIRI observations (approved in Cycle 2) including the direct detections of the H2 lines tracing the warmer gas, will be used to examine possible extreme gas-dust thermal decoupling over large H2 and dust mass reservoirs.
Beyond cluster studies these observations are important because the traditional way to estimate the amount of molecular gas, M(H2), and the initial conditions of star formation might be different impacting the type of the resulting stellar Initial Mass Function (IMF).