Dual-disk galaxies and thermal states of circumgalactic medium

Disk galaxies often have dual-disk structures composed of thin disks and thick disks. Recent observations reveal that such duality is ubiquitous across broad galaxy mass and cosmic time. It is suggested that more massive galaxies have smaller mass fractions of thick disks and undergo a transition from thick (or single) disk states to dual-disk states at earlier times. Previous studies suggest that the circumgalactic medium (CGM) in halos above the critical mass is heated by the development of stable shocks and feedback from active galactic nuclei. It has been argued that this thermal change of the CGM causes the transition of the accretion of CGM from the fast cold mode to the slow hot mode driven by the radiative cooling of heated CGM. We consider here the hypothesis that the cold and hot modes promote the formation of thick and thin disks, respectively. Previous theoretical studies showed that this hypothesis explains the age and chemical bimodality of the Milky Way disk as well as the mass dependence of disk duality observed for local galaxies. Using simple galaxy evolution models, we show that this hypothesis also reproduces the thick-to-thin disk mass ratios observed for high-redshift galaxies up to z = 2 . Earlier transition to dual disks in more massive galaxies is explained as the outcome of earlier crossing of the critical halo mass by more massive halos. The caveat here is that this interpretation is not applicable to bulge-dominated galaxies at the high-mass end because present models do not include galaxy mergers, which likely act as the primary route to bulge formation. It should also be noted that the link between the disk duality and gas accretion modes may be indirect and multiple processes may transform accreted material into different types of disks in real galaxies. Past theoretical studies suggest that the mode change in CGM accretion underlies earlier and stronger quenching of star formation in more massive galaxies and the increasing bend with time of the star-forming galaxy main sequence at the high-mass end. We infer that the change in the CGM thermal state is inscribed in diverse characteristics of galaxies.