Galaxy Size Problem at z = 3: Simulated Galaxies are too Small

Abstract

Using state-of-the-art adaptive mesh refinement cosmological hydrodynamic simulations with a spatial resolution of proper 0.21h ^-1 ₇₃ kpc in refined subregions embedded within a comoving cosmological volume (27.4h ^-1 ₇₃ Mpc)³, we investigate the sizes of galaxies at z = 3 in the standard cold dark matter model. Our simulated galaxies are found to be significantly smaller than the observed ones: while more than one half of the galaxies observed by Hubble Space Telescope and Very Large Telescope ranging from rest-frame UV to optical bands with stellar masses larger than 2 × 10¹⁰ M _sun have half-light radii larger than ~2h ^-1 ₇₃ kpc, none of the simulated massive galaxies in the same mass range have half-light radii larger than ~2h ^-1 ₇₃ kpc, after taking into account dust extinction. Corroborative evidence is provided by the rotation curves of the simulated galaxies with total masses of 10¹¹-10¹² M _sun, which display values (300-1000 km s^-1) at small radii (~0.5h ^-1 ₇₃ kpc) due to high stellar concentration in the central regions that are larger than those of any well observed galaxies. Possible physical mechanisms to resolve this serious problem include: (1) an early reionization at z_ri Gt 6 to suppress gas condensation and hence star formation, (2) a strong, internal energetic feedback from stars or central black holes to reduce the overall star formation efficiency, or (3) a substantial small-scale cutoff in the matter power spectrum.