Using state-of-the-art adaptive mesh refinement cosmological hydrodynamic simulations with a spatial resolution of 0.15 h-1kpc on a cosmological volume (20 h-1Mpc), we investigate the sizes of galaxies at z=3 in the standard cold dark matter model where re

ABSTRACT

Usingstate-of-the-artadaptivemeshre nementcosmologicalhydrodynamic

simulationswithaspatialresolutionof～0.15h 1kpconacosmologicalvolume(20h 1Mpc)3,weinvestigatethesizesofgalaxiesatz=3inthestandardcolddarkmattermodelwherereionizationisassumedtocompleteatzri～6.OursimulatedLymanbreakgalaxies(LBGs)aresmallerthantheobservedonesbyafactorof2.Speci cally,takingintoaccountbothdustextinctionandinstrumen-talresolution,we ndthatthedistributionofhalf-lightradiiofthesimulatedLBGsintheobservedg′-bandpeaksat～1kpc,whereasthedistributionoftheLBGsobservedwiththeAdvancedCameraforSurveysonHSTpeaksat～2kpc.Withoutdustextinctionandwithin niteinstrumentalresolution,boththein-trinsichalf-lightandhalf-stellar-massradiipeakat～0.3kpc.Corroborativeevidenceisprovidedbytherotationcurvesofthesimulatedgalaxieswithtotalmassesof1011-1012M⊙,whichdisplayvalues(300-1000kms 1)atsmallradii(～0.5kpc),largerthanthoseofanyobservedgalaxies.Thesehighrotationve-locitiesarisebecausestellarmassesareconcentratedinsmall(<1kpc)centralregionsofthesimulatedLBGs.Anypotentialsolutiontothisseriousproblemwouldhavetoreducetheamountofstarsthatformearlyandeventuallygetcollectedinthecentraldenseregionsofgalaxiesviadynamicalfriction.Possiblephysicalmechanismsinclude:(1)anearlyreionizationatzri 6tosuppressgascondensationhencestarformation,(2)astrong,internalenergeticfeedbackfromstarsorcentralblackholestoreducetheoverallstarformatione ciency,or(3)asubstantialsmall-scalecuto inthematterpowerspectrum.

Subjectheadings:hydrodynamics—galaxies:formation—galaxies:kinematicsand

dynamics—cosmology:theory—methods:numerical—ultraviolet:galaxies