Lessons learned from studying H α galaxy kinematics with mock <i>JWST</i> /NIRSpec IFU observations at <i>z</i> &amp;gt; 6

ABSTRACT Galaxies with a disc morphology have been established at $z > 9$ with the James Webb Space Telescope (JWST). However, confirming their disc-like nature requires studying their gas kinematics, which can be challenging when relying solely on the warm gas observed by JWST. Unlike the cold gas traced by the Atacama Large Millimetre/Submillimetre Array (ALMA), warm gas is sensitive to outflows, complicating the interpretation of the disc dynamics. This elicits the question of how to compare information obtained from varied tracers, as well as how to physically interpret the low angular and spectral resolution observations generally available at high redshift. We address these challenges through comparative kinematic analysis of idealized and realistic JWST Near-Infrared Spectrograph integral field unit (NIRSpec/IFU) mock observations derived from two galaxies in the serra suite of cosmological zoom-in simulations. With these synthetic data, we determine the robustness of dynamical information recovered from typical IFU observations, and test widely used criteria for identifying discs and gaseous outflows at high redshift. We find that at the typical NIRSpec/IFU spectral and angular resolution ($\sim$0.05 arcsec pixel−1), non-circular motions due to inflows or outflows can mimic the smooth velocity gradient indicative of a disc, and bias measured velocity dispersion upwards by a factor of 2–3×. As a result, the level of rotational support may be underestimated in the NIRSpec/IFU observations. However, the recovered dynamical mass appears to be relatively robust despite biases in $v_\text{rot}$ and $\sigma$.