Stellar bulges and regularly rotating disks in high-z galaxies

I present a novel strategy to dynamically detect central mass concentrations ("bulges") in rotating galaxies at high z, using high-resolution ALMA observations of cold gas (high-J CO lines and/or the [CII]-158 um line). The ALMA data allow us to trace rotation curves at kpc or even sub-kpc spatial resolution, and to build mass models taking into account the gravitational contributions of the gas disk, the stellar disk, and eventually a stellar bulge. The method has been firstly applied to a star-forming galaxy at z=4.75 with [CII] data at 700-pc resolution (Lelli et al. 2021, Science, 371, 713). The galaxy rotation curve rises steeply in the inner parts, similarly to Sa-S0 galaxies in the nearby Universe, requiring the presence of a stellar bulge in addition to the star-forming disk. Mass models give a bulge mass of 2x10^10 Msun and a bulge-to-total mass ratio of about 0.4. This indicates that massive bulges and rotating disks can form very rapidly in the early Universe, just 1.2 Gyr after the Big Bang, challenging galaxy formation models. The same strategy is applied to two main-sequence galaxies at z=1.5 and z=2.2 using CO observations. Preliminary results suggest that these two galaxies have dynamically cold disks (Vrot/sigma>10) and compact, low-mass bulges (bulge-to-total mass ratios of 0.05-0.1) which were not clearly identified in available HST images