Virialization of the Inner CGM and Implications for Disc Formation

Recent observations have highlighted the concept of 'disc settling', where the star-forming galaxy population becomes increasingly dominated by thin rotating discs with increasing mass and decreasing redshift, in contrast with disordered velocity fields and irregular morphologies at low masses and high redshifts. There is yet no established consensus on why rotation dominated-discs preferably form in massive SF galaxies and at low redshifts. Using a combination of analytic calculations, controlled numerical experiments, and the FIRE cosmological simulations, I will argue that disc settling is associated with a phase transition in the inner circumgalactic medium (CGM). I will show that in FIRE, the CGM of low mass irregular galaxies has sub-virial temperatures and large pressure fluctuations, while the CGM of more massive disc galaxies has virial temperatures and a smooth pressure distribution. Our results thus predict a tight connection between the thermal properties of the inner CGM and the kinematics of the central galaxy, a prediction which can be tested with upcoming surveys and facilities.