Halo gas thermodynamics from the cosmic microwave background: implications for large-scale structure and galaxy formation

Understanding galaxy formation is an outstanding problem in Astrophysics. The feedback processes that drive it, exploding stars and accretion onto supermassive black holes, are poorly understood. This results in an order unity uncertainty in the distribution of the gas inside halos, the ``missing baryon problem''. Because baryons are 15% of the total mass in the universe, this baryonic uncertainty is the largest theoretical systematics for percent precision weak lensing surveys like DES, HSC, Rubin Observatory, Roman Observatory and Euclid.
By measuring the kinematic and thermal Sunyaev-Zel'dovich effects (kSZ and tSZ), high resolution and high sensitivity CMB experiments can solve these issues by measuring the gas thermodynamics in galaxy groups and clusters, at high redshift and out to the outskirts of the halo. I will present joint tSZ, kSZ and dust measurement of BOSS (CMASS) galaxy groups, for which clustering and lensing data is also available. Using data from the Atacama Cosmology Telescope (ACT), we produced the highest significance kSZ measurement to date. This measurement shows with high statistical confidence that the gas is more spread out than the dark matter. It informs the modeling of the CMASS galaxy-galaxy lensing data, and shows that the small-scale ``lensing is low'' tension is not entirely caused by baryonic effects. Finally, comparing the observed kSZ and tSZ to hydrodynamical simulations reveals insight about the modalities of feedback.