Galaxy Size Problem at z=3 Simulated Galaxies Are Too Small(11)

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

observedg′-bandpeaksat～1kpc,smallerthanthatobservedwithHSTACSthatpeaksat～2kpc.Intheabsenceofdustextinctionandwithin niteinstrumentalresolution,theintrinsichalf-lightradiiaswellasthehalf-stellar-massradiiactuallypeakat～0.3kpc.ConsistentwiththisapparentdiscrepancybetweensimulatedandobservedLBGs,therotationcurvesofthesimulatedgalaxiesoftotalmassesof5×1010-1012M⊙haveunusuallyhighvalues(300-1000kms 1)atsmallradii(～0.5kpc)forgalaxiesoftotalmassof1011-1012M⊙.Suchrotationcurvesarenotseeninlocalgalaxiesforwhichmeasurementsareavailable.

Theoriginofbothoftheseproblemsmaybethatstellarmassesaretoohighlyconcentratedinsmall(<1kpc)centralregionsofthesimulatedLBGs.Thisislikelycausedbyanover-abundanceofsmallergalaxiesthatformedathighredshifts(z>～6)andsubsequentlysanktothecentersoftheLBGsviadynamicalfriction.Thegalaxysizeproblematz=3foundinthepresentworkmayberelatedtoanapparentlargeexcessofpredictedbutunobserveddwarfhalos(Klypinetal.1999;Mooreetal.1999)andanover-concentrationofdarkmatterinsimulateddwarfgalaxiesonthescaleof～1kpc(Moore1994;Flores&Primack1994;Burkert1995;McGaugh&deBlok1998;Kravtsovetal.1998;Mooreetal.1999).

Likely,anypotentialviablesolutiontothisapparentproblemwouldhavetoreducetheamountofstarsthatformearlyandeventuallygetcollectedinthecentraldenseregionsofgalaxies.Possiblephysicalmechanismsinclude:(1)anearlyreionizationwithzri 6tosuppressgascondensationthatwillreduceearlierstarformation(e.g.,Bullock,Kravtsov,&Weinberg2000),(2)astrong,internalenergeticfeedbackfromstarsorcentralblackholestoreducetheoverallstarformatione ciency(e.g.,Sommer-Larsonetal.2003;Governatoetal.2007),or(3)asubstantialsmall-scalecuto inthematterpowerspectrum,forexample,ifthedarkmatterparticlesarewarmratherthancold(e.g.,Hogan&Dalcanton2000;