Towards sub-kpc scale kinematics of molecular and ionized gas of star-forming galaxies at z ∼ 1^⋆

¹ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
e-mail: mgirard@swin.edu.au
² Observatoire de Paris, LERMA, Collège de France, CNRS, PSL Univ., Sorbonne University, UPMC, Paris, France
³ University of California – Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA
⁴ DARK, Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, 2100 Copenhagen, Denmark
⁵ Univ. Lyon, Univ. Lyon1, Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69230 Saint-Genis-Laval, France
⁶ CNRS, IRAP, 14 Avenue E. Belin, 31400 Toulouse, France

Received: 15 May 2019
Accepted: 11 September 2019

Abstract

We compare the molecular and ionized gas kinematics of two strongly lensed galaxies at z ∼ 1 that lie on the main sequence at this redshift. The observations were made with ALMA and MUSE, respectively. We derive the CO and [O II] rotation curves and dispersion profiles of these two galaxies. We find a difference between the observed molecular and ionized gas rotation curves for one of the two galaxies, the Cosmic Snake, for which we obtain a spatial resolution of a few hundred parsec along the major axis. The rotation curve of the molecular gas is steeper than the rotation curve of the ionized gas. In the second galaxy, A521, the molecular and ionized gas rotation curves are consistent, but the spatial resolution is only a few kiloparsec on the major axis. Using simulations, we investigate the effect of the thickness of the gas disk and effective radius on the observed rotation curves and find that a more extended and thicker disk smoothens the curve. We also find that the presence of a strongly inclined (> 70°) thick disk (> 1 kpc) can smoothen the rotation curve because it degrades the spatial resolution along the line of sight. By building a model using a stellar disk and two gas disks, we reproduce the rotation curves of the Cosmic Snake with a molecular gas disk that is more massive and more radially and vertically concentrated than the ionized gas disk. Finally, we also obtain an intrinsic velocity dispersion in the Cosmic Snake of 18.5 ± 7 km s⁻¹ and 19.5 ± 6 km s⁻¹ for the molecular and ionized gas, respectively, which is consistent with a molecular disk with a smaller and thinner disk. For A521, the intrinsic velocity dispersion values are 11 ± 8 km s⁻¹ and 54 ± 11 km s⁻¹, with a higher value for the ionized gas. This could indicate that the ionized gas disk is thicker and more turbulent in this galaxy. These results highlight the diversity of the kinematics of galaxies at z ∼ 1 and the different spatial distribution of the molecular and ionized gas disks. It suggests the presence of thick ionized gas disks at this epoch and that the formation of the molecular gas is limited to the midplane and center of the galaxy in some objects.