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

Abstract: We study the dynamics of cold molecular gas in two main-sequence galaxies at cosmic noon (zC-488879 at $z\simeq1.47$ and zC-400569 at $z\simeq2.24$) using new high-resolution ALMA observations of multiple $^{12}$CO transitions. For zC-400569 we also re-analyze 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 $\sim$8 kpc contrary to previous results pointing to outer declines in the rotation speed $V_{\rm rot}$; (2) The intrinsic velocity dispersions are low ($\sigma_{\rm CO}\lesssim15$ km/s for CO and $\sigma_{\rm H\alpha}\lesssim37$ km/s for H$\alpha$) and imply $V_{\rm rot}/\sigma_{\rm CO}\gtrsim17-22$ yielding no significant pressure support; (3) Mass models using HST images display a severe disk-halo degeneracy: 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; (4) Milgromian dynamics (MOND) can successfully fit the rotation curves with the same acceleration scale $a_0$ measured at $z\simeq0$. 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 $V_{\rm rot}/\sigma_{\rm V}$ ratios, inner rotation curve shapes, bulge-to-total mass ratios) are remarkably similar to those of massive spirals at $z\simeq0$, suggesting weak dynamical evolution over more than 10 Gyr of the Universe's lifetime.