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

Figure2showsthehistogramsofhalf-lightradii(rhl)ofsimulatedgalaxiesatz=3intheobservedg′-band.Ineachpanel,redsolidhistogramrepresentsgalaxieswithhighSFR(>10M⊙yr 1)thatweidentifywiththeobservedLBGs,whilegreendottedhistogramrepresentsthosewithlowSFR(<10M⊙yr 1).ThedistributionofthesimulatedLBGs(shadedinred)inpanel(c)ofFigure2di ersfromtheobservedone(bluehistogram).Inparticular,itappearsthatthesimulatedLBGsaresmallerthantheobservedLBGsbyafactorof～2.ThissuggeststhatthecurrentSFactivitiesaswellastheoverallstellarmassdistributionsinthesimulatedgalaxiesatz=3aretooconcentratednearthegalacticcenters.Thisfactcanbeshowninanalternativewayusingrotationcurves.Figure3showsrotationvelocitycurvesforthethreetopgalaxiesinFig.1.Thesecurvesseemtopeakattoohighavalue(300-1000kms 1)atsmallradii(～0.5kpc).

4.DISCUSSIONANDCONCLUSIONS

Thestandardcolddarkmattercosmologicalmodelisingoodagreementwitharichsetofobservationsonlargescales.Ourmainpurposeistosystematicallyexamineit,throughaseriesofpapers,inthecontextofgalaxyformationandevolution,aregimewhere,relativelyspeaking,ithasnotbeenseriouslycontested.Becauseastrophysicalprocessestendtoplayaprogressivelymoreimportantrolewithatsmallerscales,inparticular,ongalacticscalesandsmaller,itisvitaltoemployasfewadjustableastrophysicalparametersaspossible,tohaveatruetestofthecosmologicalmodel.

Inthispaper,the rstofaseries,wefocusonthesizesofgalaxies,primarilyLymanbreakgalaxies(LBGs;Steideletal.2003),atredshiftz=3,usingstate-of-the-artAMRcosmologicalhydrodynamicsimulationswithaspatialresolutionof～0.15h 1kpcinacosmologicalvolume.We ndthat,takingintoaccountbothdustextinctionandinstrumentalresolution,thecomputeddistributionofhalf-lightradiiofLBGsinthe