James Webb Space Telescope Feed Post

Ionised gas kinematics and dynamical masses of z >= 6 galaxies from JADES/NIRSpec high-resolution spectroscopy

Example of the fitting procedure for object JADES-NS-00016745 (Fig. 1). Although the final combination of all exposures (left) was used for our initial visual inspection and sample selection, the pixels in this spectrum are highly correlated. Instead of using this combined spectrum, we simultaneously fit to all individual exposures obtained. In the case of JADES-NS-00016745 two exposures were taken per nodding position, resulting in six independent measurements for one 3-point nodding pattern with NIRSpec. To combat the undersampled PSF of NIRSpec, we perform our modelling in the detector plane, propagating parametric models to the exact same location on the detector as the observed data. The likelihood is then computed from the combination of all residual images. Pixels flagged by the reduction pipeline as affected by cosmic rays are masked and shown in grey. Abstract: We explore the kinematic gas properties of six 5.5 < z < 7.4 galaxies in the JWST Advanced Deep Extragalactic Survey (JADES), using high-resolution JWST/NIRSpec multi-object spectroscopy of the rest-frame optical emission lines [Oiii] and Ha. The objects are small and of low stellar mass (~ 1 kpc; M* ~ 107-9 M?), less massive than any galaxy studied kinematically at z > 1 thus far. The cold gas masses implied by the observed star formation rates are ~ 10× larger than the stellar masses. We find that their ionised gas is spatially resolved by JWST, with evidence for broadened lines and spatial velocity gradients. Using a simple thin-disc model, we fit these data with a novel forward modelling software that accounts for the complex geometry, point spread function, and pixellation of the NIRSpec instrument. We find the sample to include both rotation- and dispersion-dominated structures, as we detect velocity gradients of v(re) ˜ 100 - 150 km s-1, and find velocity dispersions of s0 ˜ 30 - 70 km s-1 that are comparable to those at cosmic noon. The dynamical masses implied by these models (Mdyn ~ 109-10 M?) are larger than the stellar masses by up to a factor 40, and larger than the total baryonic mass (gas + stars) by a factor of ~ 3. Qualitatively, this result is robust even if the observed velocity gradients reflect ongoing mergers rather than rotating discs. Unless the observed emission line kinematics is dominated by outflows, this implies that the centres of these galaxies are dark-matter dominated or that star formation is 3× less efficient, leading to higher inferred gas masses.