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

¹ INAF, Arcetri Astrophysical Observatory, Largo E. Fermi 5, 50125 Florence, Italy
e-mail: federico.lelli@inaf.it
² School of Astronomy and Space Science, Nanjing University, 163 Xianlin Avenue, Nanjing, Jiangsu 210023, PR China
³ Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Ministry of Education, Nanjing 210023, PR China
⁴ Research Center for Intelligent Computing Platforms, Zhejiang Laboratory, Hangzhou 311100, PR China
⁵ Universität zu Köln, I. Physikalisches Institut, Zülpicher Str. 77, 50937 Köln, Germany
⁶ Department of Physics, Section of Astrophysics, Astronomy and Mechanics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
⁷ School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
⁸ Department of Physics, University of Oregon, 461 Willamette Hall, Eugene, OR 97403, USA
⁹ Department of Physics & Astronomy, University of Florence, via G. Sansone 1, 50019 Sesto Fiorentino, Italy
¹⁰ INAF, Padova Astronomical Observatory, Vicolo Osservatorio 5, 35122 Padova, Italy
¹¹ Department of Astronomy, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA

Received: 30 September 2022
Accepted: 30 January 2023

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 ¹²CO transitions. For zC-400569 we also reanalyze 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 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 Hα) 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 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 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.