2024ApJ...960L..10L

Although a thick disk is a structure prevalent in local disk galaxies and also present in our home Galaxy, its formation and evolution are still unclear. Whether the thick disk is born thick and/or gradually heated to be thick after formation is under debate. To disentangle these two scenarios, one effective approach is to inspect the thickness of young disk galaxies in the high-redshift Universe. In this work we study the vertical structure of 191 edge-on galaxies spanning redshift from 0.2 to 5 using JWST NIRCAM imaging observations. For each galaxy, we retrieve the vertical surface brightness profile at 1 R_e and fit a sech² function that has been convolved with the line spread function. The obtained scale height of galaxies at z > 1.5 shows no clear dependence on redshift, with a median value in remarkable agreement with that of the Milky Way's thick disk. This suggests that local thick disks are already thick when they were formed in early times and secular heating is unlikely to be the main driver of thick disk formation. For galaxies at z < 1.5, however, the disk scale height decreases systematically toward lower redshift, with low-redshift galaxies having comparable scale height with that of the Milky Way's thin disk. This cosmic evolution of disk thickness favors an upside-down formation scenario of galaxy disks.