James Webb Space Telescope Feed Post

Galaxies Going Bananas: Inferring the 3D Geometry of High-Redshift Galaxies with JWST-CEERS

Example 3” × 3” false-color RGB (F356W+F200W+F115W) postage stamps of galaxies with a high (> 75%) probability of being prolate (top two rows), oblate (middle two rows) or spheroidal (bottom two rows). The inset text shows the photometric redshift, stellar mass, F200W b/a, Re and n, and the highest class probability. These galaxies fall in mass-redshift bins where our model was able to assign confident classification probabilities to different regions of the b/a vs. log a diagram. Abstract: The 3D geometry of high-redshift galaxies remains poorly understood. We build a differentiable Bayesian model and use Hamiltonian Monte Carlo to efficiently and robustly infer the 3D shapes of star-forming galaxies in JWST-CEERS observations with logM*/M?=9.0-10.5 at z=0.5-8.0. We reproduce previous results from HST-CANDELS in a fraction of the computing time and constrain the mean ellipticity, triaxiality, size and covariances with samples as small as ~50 galaxies. We find high 3D ellipticities for all mass-redshift bins suggesting oblate (disky) or prolate (elongated) geometries. We break that degeneracy by constraining the mean triaxiality to be ~1 for logM*/M?=9.0-9.5 dwarfs at z > 1 (favoring the prolate scenario), with significantly lower triaxialities for higher masses and lower redshifts indicating the emergence of disks. The prolate population traces out a ``banana' in the projected b/a-loga diagram with an excess of low b/a, large loga galaxies. The dwarf prolate fraction rises from ~25% at z=0.5-1.0 to ~50-80% at z=3-8. If these are disks, they cannot be axisymmetric but instead must be unusually oval (triaxial) unlike local circular disks. We simultaneously constrain the 3D size-mass relation and its dependence on 3D geometry. High-probability prolate and oblate candidates show remarkably similar Sérsic indices (n~1), non-parametric morphological properties and specific star formation rates. Both tend to be visually classified as disks or irregular but edge-on oblate candidates show more dust attenuation. We discuss selection effects, follow-up prospects and theoretical implications.