Testing the inference of kinematics from mock JWST NIRSpec/MSA observations of TNG50 galaxies at $z\sim2-6$

Abstract

We use the TNG50 galaxy formation simulation to generate mock JWST NIRCam and NIRSpec microshutter array (MSA) observations of H$\alpha$-emitting gas in $M_*=10^8-10^{11.5}\,M_\odot$ star-forming galaxies at $z=2-6$. We measure morphological properties from the mock imaging through Sersic profile fitting, and gas rotational velocities ($v$) and velocity dispersions ($\sigma$) by fitting the mock spectra as thin, rotating discs. To test the efficacy of such simple parametric models in describing complex ionised gas kinematics, we compare the best-fit quantities to intrinsic simulation measurements. At $z=3$, we find that $v$ and $\sigma$ for aligned and resolved sources generally agree well with intrinsic measurements, within a factor of $\sim$2 and $\sim$1.5, respectively. The recovery of kinematics is robust for smooth, disc-like systems, but $v$ and $\sigma$ can be over- or underestimated by more than a factor of 2, respectively, for intrinsically elongated systems. The scatter in the recovery accuracy is larger at higher redshift, as TNG50 galaxies at $z>3$ deviate more strongly from the thin rotating disc assumption. Despite uncertain measurements for individual galaxies, we find that key population trends, such as the weak redshift evolution of $\sigma$ and $v/\sigma$ as well as the dependence of $\sigma$ on the global star formation rate, are broadly recovered by our kinematic modelling. Our work provides the end-to-end framework needed to compare NIRSpec MSA observations to cosmological simulations and to quantify observational biases in measuring ionised gas kinematics, highlighting the need for the development of dedicated models for high-redshift galaxies.