Cold gas disks in main-sequence galaxies at cosmic noon: Low turbulence, flat rotation curves, and disk-halo degeneracy
Federico Lelli, Zhi-Yu Zhang, Thomas G. Bisbas, Lingrui Lin, P. P. Papadopoulos, James M. Schombert, Enrico M. Di Teodoro, Antonino Marasco, Stacy McGaugh
Abstract
We study the dynamics of cold molecular gas in two main-sequence galaxies at cosmic noon (zC-488879 at z ≃ 1.47 and zC-400569 at z ≃ 2.24) using new high-resolution ALMA observations of multiple 12 CO transitions. For zC-400569 we also reanalyze high-quality H α 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 ∼8 kpc contrary to previous results pointing to outer declines in the rotation speed V rot ; (2) the intrinsic velocity dispersions are low ( σ CO ≲ 15 km s −1 for CO and σ Hα ≲ 37 km s −1 for H α ) and imply V rot / σ CO ≳ 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 ≃ 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 / σ V ratios, inner rotation curve shapes, bulge-to-total mass ratios) are remarkably similar to those of massive spirals at z ≃ 0, suggesting weak dynamical evolution over more than 10 Gyr of the Universe’s lifetime.