Galaxies Ten Billion Years Ago Look A Lot Like Galaxies Today

The LambdaCDM model expects galaxies to take longer to appear when they do and to evolve significantly over time scales of billions of years. But, the latest observations of galaxies as much as 10 billion years old suggest that galaxies then had basically the same dynamics that they do now

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 12CO transitions. For zC-400569 we also re-analyze high-quality Hα 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 Vrot; (2) The intrinsic velocity dispersions are low (σCO≲15 km/s for CO and σHα≲37 km/s for Hα) and imply Vrot/σCO≳17−22 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 a0 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 Vrot/σV 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.

Federico Lelli, Zhi-Yu Zhang, Thomas G. Bisbas, Lingrui Lin, Padelis Papadopoulos, James M. Schombert, Enrico Di Teodoro, Antonino Marasco, Stacy S. McGaugh, "Cold gas disks in main-sequence galaxies at cosmic noon: Low turbulence, flat rotation curves, and disk-halo degeneracy" arXiv:2302.00030 (January 31, 2023) (Accepted for publication in Astronomy and Astrophysics).