Galaxies can only form and evolve because of the larger environment that nurtures them. The circumgalactic medium (CGM) is an oasis that comprises less than 1% of the volume of the universe, but it is essential as the sole source of baryonic material, sustaining the formation of new stars in an otherwise barren landscape.

Yet, despite its central importance, the CGM is difficult to observe directly, and such observations are even more difficult to interpret. Gas is often detected with a mix of species that cannot be explained by a single temperature, with implied densities that are too low to maintain hydrodynamic pressure, and with prominent absorption from ionization states that are expected to be short-lived. At the same time, theoretical models vary widely in their assumptions about the composition, microphysics, and astrophysical processes that shape the CGM.

This meeting will bring together experts in theory and observation to help resolve these questions and develop a deeper understanding of the structure of the CGM, addressing the following key questions:

1. What is the multiphase structure of the CGM, and how can we best constrain it observationally?   

2. How turbulent is the CGM, how is this connected to the kinematics of CGM cold clouds, and how does this depend on galaxy mass and environment?        

3. How does the CGM deviate from equilibrium assumptions- hydrostatic (for hot gas), ionization (for UV species), thermal (for cool gas), pressure (hot/cool)? 

4. How are gas and angular momentum accreted from the CGM onto galaxies? How important is rotational support in the CGM relative to turbulent support? 

5. How do metals cycle in and out of galaxies, and how are they distributed in the CGM? How well mixed are metals in the CGM?   

6. What effect do B-fields / cosmic rays / thermal conduction have on the CGM, and when must they be taken into account when interpreting observations?

7. What can we learn from the CGM of the Milky Way and nearby galaxies, and how do these differ from populations at higher redshifts?    

8. How do we leverage new observational capabilities to better probe the CGM, and as a community, what should be our next steps (observations/theory/instrumentation) to take our understanding of the CGM to the next level?

Sanchayeeta Borthakur (Arizona State University, Chair)

Evan Scannapieco (Arizona State University, Co-chair)

Hsiao-Wen Chen (University of Chicago)

Nicolas Lehner (Univeristy of Notre Dame)

Benjamin Oppenheimer (University of Colorado)

Brian O’Shea (Michigan State University)

Joop Schaye (Leiden University)

Chuck Steidel (California Institute of Technology)

Jessica Werk (University of Washington)

Kimberly Baptista

Tyler McCabe

Mansi Padave

Sanchayeeta Borthakur

Evan Scannapieco