Cold gas disks in main-sequence galaxies at cosmic noon: Low turbulence, flat rotation curves, and disk-halo degeneracy

We study the dynamics of cold molecular gas in two main-sequence galaxies at cosmic noon (zC-488879 at z similar or equal to 1.47 and zC-400569 at z similar or equal to 2.24) using new high-resolution ALMA observations of multiple (CO)-C-12 transitions. For zC-400569 we also reanalyze high-quality H alpha data from the SINS/zC-SINF survey. We find that (1) both galaxies have regularly rotating CO disks and their rotation curves are flat out to similar to 8 kpc contrary to previous results pointing to outer declines in the rotation speed V-rot; (2) the intrinsic velocity dispersions are low (sigma(CO) less than or similar to 15 km s(-1) for CO and sigma(H alpha) less than or similar to 37 km s(-1) for H alpha) and imply V-rot/sigma(CO) greater than or similar to 17 - 22 yielding no significant pressure support; (3) mass models using HST images display a severe disk-halo degeneracy, that is models with inner baryon dominance and models with "cuspy" dark matter halos can fit the rotation curves equally well due to the uncertainties on stellar and gas masses; and (4) Milgromian dynamics (MOND) can successfully fit the rotation curves with the same acceleration scale a(0) measured at z similar or equal to x2004;0. The question of the amount and distribution of dark matter in high-z galaxies remains unsettled due to the limited spatial extent of the available kinematic data; we discuss the suitability of various emission lines to trace extended rotation curves at high z. Nevertheless, the properties of these two high-z galaxies (high V-rot/sigma(V) ratios, inner rotation curve shapes, bulge-to-total mass ratios) are remarkably similar to those of massive spirals at z similar or equal to 0, suggesting weak dynamical evolution over more than 10 Gyr of the Universe's lifetime.