An early dark matter-dominated phase in the assembly history of Milky Way-mass galaxies suggested by the TNG50 simulation and JWST observations

Understand

Whereas well-studied galaxies at cosmic noon are found to be baryon-dominated within the effective radius, recent JWST observations of $z\sim6-7$ galaxies with stellar masses of only $M_*\sim10^{8-9}\,{\rm M_\odot}$ surprisingly indicate that they are dark matter-dominated within $r_{\rm e}\approx 1\,$kpc.

• Here, we place these high-redshift measurements in the context of the TNG50 galaxy formation simulation, by measuring the central (within $1\,$kpc) stellar, gas, and dark matter masses of galaxies in the simulation.
• The central baryon fraction varies strongly with galaxy stellar mass in TNG50, and this $M_*$-dependence is remarkably constant across $0<z<6$: galaxies of low stellar mass ($M_*\sim10^{8-9}\,{\rm M_\odot}$) are dark matter-dominated, as $f_{\rm baryon}(<1\,{\rm kpc})\sim0.25$.
• At $z=6$, the baryonic mass in the centers of low-mass galaxies is largely comprised of gas, exceeding the stellar mass component by a factor $\sim4$.