MAGNIF: A Tentative Lensed Rotating Disk at z = 8.34 detected by JWST NIRCam WFSS with Dynamical Forward Modeling

We report galaxy MACS0416-Y3 behind the lensing cluster MACSJ0416.1-2403 as a tentative rotating disk at $z = 8.34$ detected through its $\mathrm{[OIII]}\lambda5007$ emission in JWST NIRCam wide-field slitless spectroscopic observations. The discovery is based on our new grism dynamical modeling methodology for JWST NIRCam slitless spectroscopy, using the data from “Median-band Astrophysics with the Grism of NIRCam in Frontier Fields” (MAGNIF), a JWST Cycle-$2$ program. The $\mathrm{[OIII]}\lambda5007$ emission line morphology in grism data shows velocity offsets compared to the F480M direct imaging, suggestive of rotation. Assuming a geometrically thin disk model, we constrain the rotation velocity of $v_{\mathrm{rot}} = 58_{-35}^{+53}\ \mathrm{km/s}$ via forward modeling of the two-dimensional (2D) spectrum. We obtain the kinematic ratio of $v_{\mathrm{rot}}/\sigma_{v} = 1.6_{-0.9}^{+1.9}$ , where $\sigma_{v}$ is the velocity dispersion, in line with a quasi-stable thin disk. The resulting dynamical mass is estimated to be $\mathrm{log}_{10}(M_{\mathrm{dyn}}/M_{\odot}) = 8.4_{-0.7}^{+0.5}$ . If the rotation confirmed, our discovery suggests that rotating gaseous disks may have already existed within $600$ million years after Big Bang.

eprint arXiv:2310.09327