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

1.INTRODUCTION

Thestandardcosmologicalmodelhasbeenremarkablysuccessfulinaccountingforobservationsonscaleslargerthangalaxysizes(Krauss&Turner1995;Ostriker&Steinhardt1995;Bahcalletal.1999;Tegmarketal.2004;Spergeletal.2007).Weintendtotestthissamemodelwithregardtogalaxyformationandevolution,aregimewhereastrophysicalprocessesareimportantandhenceadetailedtestingofthecosmologicalmodelbecomesmoreintricate.Inthispaper,the rstofaseries,wefocusonthesizesofgalaxiesatredshiftz=3,primarilyLymanbreakgalaxies(LBGs;Steideletal.2003).Previousworksonthissubjectincludethosebasedonsemi-analyticmethods(e.g.,Mao&Mo1999).Here,wetakeabrute-forceapproachusinghigh-resolutionadaptivemeshre nement(AMR)cosmologicalsimulationstominimizethenumberofadjustableastrophysicalparametersandtherebymaximizethepredictabilityofthestandardmodel.

Aphysicalresolutionofoursimulationsof0.15h 1kpcatz=3permits,incosmologicalsimulationsofboxsize20h 1Mpccomoving,anaccuratecharacterizationofthesizesofgalaxiesatz=3.Weshowthattheintrinsichalf-lightradiiofsimulatedgalaxiesintheobservedg′-bandpeakat～0.3kpcatz=3.Takingintoaccountaself-consistenttreatmentofdustextinctionthatattenuatestheobserved uxbyafactorof～15,assuggestedbyobservations(Meureretal.1997;Pettinietal.1998;Sawicki&Yee1998),movesthepeakto～1.0kpc.Includingfurtherthepointspreadfunction(PSF)oftheobservationalinstrument,inparticular,FWHM≈0.05′′fortheAdvancedCameraforSurveys(ACS)onHST,slightlynarrowstheoveralldistributionbutleavesthepeaklocationlittlechangedat～1kpc,smallerthantheobservedsizes(Fergusonetal.2004)byafactorof～2.

Consistentwiththisintriguingresult,therotationcurvesofthesimulatedgalaxieswithtotalmassesof1011-1012M⊙haveunusuallyhighvalues(300-1000kms 1)atsmallradii(～0.5kpc).Suchhighrotationvelocitiesarisebecausestellarmassesarehighly