Turbulent gas-rich discs at high redshift: the origin of early massive stellar bars

Recent observations combining the power of ALMA and JWST have revealed large ($3-7$ kpc), massive ($3-10\times {10}^{10}{\mathrm{M}}_{\odot }$) stellar bars at $z=4-5$ when the Universe was only 1.2-1.6 Gyr old. At this early epoch, the host galaxy was baryon-dominated (typically 75% gas, 25% stars) within the observed extent of the disc ($8-15$ kpc). Using NEXUS $N$-body/hydrodynamic simulations, we show that such bars can form promptly (400$-$800 Myr), provided the disc mass fraction is high ($f_{\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{c}}\gtrsim 70\mathrm{\%}$) and the bar is gas-dominated at the time of its formation, consistent with the observations. In this limit, gas-free bars are unstable to vertical bending modes, but a dominant gas component suppresses this instability. Unlike massive bars in the local Universe, these early bars were sites of vigorous star formation, as we show. Remarkably, for gas-rich models with $f_{\mathrm{g}\mathrm{a}\mathrm{s}}\lesssim 60\mathrm{\%}$, the bars develop X-shaped boxy bulges; at higher gas fractions ($f_{\mathrm{g}\mathrm{a}\mathrm{s}}>60\mathrm{\%}$), diffusion suppresses resonant orbit trapping and the emerging bar collapses within 1 Gyr to form a classical bulge. The bar formation time, length, mass, and $m=2$ Fourier amplitude are all inversely related to $f_{\mathrm{g}\mathrm{a}\mathrm{s}}$. We present a simple analytic model for how stochastic forcing shifts the bar onset time, defined as the time at which the growing bar amplitude reaches a specified threshold.