Towards sub-kpc scale kinematics of molecular and ionized gas of star-forming galaxies at $z\sim1$

TL;DR

This study compares the kinematics of molecular and ionized gas in two star-forming galaxies at z~1 using high-resolution observations, revealing differences in their rotation curves and disk structures, and highlighting the impact of disk thickness on observed kinematics.

Contribution

It provides the first sub-kiloparsec scale comparison of molecular and ionized gas kinematics at z~1, demonstrating the effects of disk thickness and distribution on rotation curves.

Findings

Molecular gas rotation curve is steeper than ionized gas in the Cosmic Snake.

Thicker and more extended gas disks smooth the observed rotation curves.

Ionized gas disks are thicker and more turbulent than molecular disks in these galaxies.

Abstract

We compare the molecular and ionized gas kinematics of two strongly lensed galaxies at $z\sim1$ based on observations from ALMA and MUSE. We derive the CO and [OII] 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 galaxies, the Cosmic Snake, for which we obtain a spatial resolution of few hundred parsecs 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 of few kpc 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 smooths the curve. We also find that the presence of a strongly inclined thick disk (>1 kpc) can smooth 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 and 19.5+-6 km/s 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 and 54+-11 km/s, 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 different spatial distribution of the molecular and ionized gas disks at $z\sim1$ and suggest the presence of thick ionized gas disks at this epoch.