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 -173 kpc in refined subregions embedded within a comoving cosmological volume (27.4h -173 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 × 1010M have half-light radii larger than 2h -173 kpc, none of the simulated massive galaxies in the same mass range have half-light radii larger than 2h -173 kpc, after taking into account dust extinction. Corroborative evidence is provided by the rotation curves of the simulated galaxies with total masses of 1011-1012M, which display values (300-1000 km s-1) at small radii (0.5h -173 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 ≫ 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. © 2009. The American Astronomical Society. All rights reserved..