James Webb Space Telescope Feed Post

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

Sample of six spatially-resolved high-redshift objects in JADES. Left panels show cutouts of the emission lines in the 2D rectified and combined spectra obtained with the high-resolution G395H grating. Negatives in the cutouts are the result of the background subtraction method used. Right panels show NIRCam image cutouts for each object (JADES, FRESCO), for the band that most closely resembles the emission line morphology (Section 2.2). The 3-shutter slits and 3-point nodding pattern used result in an effective area of 5 shutters: the shutter encompassing the source is shown in orange, and the shutters used for background subtraction are shown in purple. 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.