GALAXY SIZE PROBLEM AT z=3: SIMULATED GALAXIES ARE TOO SMALL

Using state-of-the-art adaptive mesh refinement cosmological hydrodynamic simulations with a spatial resolution of proper 0.21h(73)(-1) kpc in refined subregions embedded within a comoving cosmological volume (27.4h(73)(-1) 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 x 10(10) M(circle dot) have half-light radii larger than similar to 2h(73)(-1) kpc, none of the simulated massive galaxies in the same mass range have half-light radii larger than similar to 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 10(11)-10(12) M(circle dot), which display values (300-1000 km s(-1)) at small radii (similar to 0.5h(73)(-1) 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.