Rutgers University Astrophysics Seminar

I will present surprising observational results on the 3D geometry of high-redshift star-forming galaxies based on Early Release Science data from the James Webb Space Telescope. Using a differentiable Bayesian model with Hamiltonian Monte Carlo, I will show that there are many more flat, elongated dwarf galaxies than there are round, circular dwarf galaxies seen in projection at high redshift. This puzzle can be explained if ~50-80% of dwarfs at z~2-8 with stellar masses ~10^9-10^10 Msun (including Milky Way progenitors) are not axisymmetric (oblate) disks or spheroids. Instead, they may be significantly flattened along two axes either as prolate ellipsoids (cigars) or as triaxial ellipsoids (oval, surfboard-shaped disks). Both prolate and triaxial ellipsoids trace out a "banana" on the projected b/a-log(a) diagram with an excess of low b/a (edge-on) and deficit of high b/a (round) objects, as observed. Empirical simulations show that the deficit of round dwarfs at high-redshift is real, i.e., we are complete to face-on disks over a reasonable range of sizes and magnitudes. I will argue that imaging alone may be insufficient for distinguishing prolate, oblate and triaxial systems. Instead, deep high-resolution spectroscopy will be needed to constrain the orbits of the stars which are expected to be highly non-circular in prolate/triaxial systems. This is suggested by high-resolution cosmological simulations in which the formation of these elongated galaxies is tied to mergers along cosmic web filaments. I will discuss prospects for both observational and theoretical follow-up, possible connections to our own Milky Way, and implications for galaxy formation more generally.