A Revised Model for the Formation of Disk Galaxies: Low Spin and Dark Halo Expansion

We use observed rotation velocity-luminosity (VL) and size-luminosity (RL)relations to single out a specific scenario for disk galaxy formation in theLCDM cosmology. Our model involves four independent log-normal randomvariables: dark-halo concentration c, disk spin lam_gal, disk mass fractionm_gal, and stellar mass-to-light ratio M/L_I. A simultaneous match of the VLand RL zero points with adiabatic contraction requires low-c halos, but thismodel has V_2.2~1.8 V_vir (where V_2.2 and V_vir are the circular velocity at2.2 disk scale lengths and the virial radius, respectively) which will beunable to match the luminosity function (LF). Similarly models withoutadiabatic contraction but standard c also predict high values of V_2.2/V_vir.Models in which disk formation induces an expansion rather than the commonlyassumed contraction of the dark-matter halos have V_2.2~1.2 V_vir which allowsa simultaneous fit of the LF. This may result from non-spherical, clumpy gasaccretion, where dynamical friction transfers energy from the gas to the darkmatter. This model requires low lam_gal and m_gal values, contrary to naiveexpectations. However, the low lam_gal is consistent with the notion that diskgalaxies predominantly survive in halos with a quiet merger history, while alow m_gal is also indicated by galaxy-galaxy lensing. The smaller than expectedscatter in the RL relation, and the lack of correlation between the residualsof the VL and RL relations, respectively, imply that the scatter in lam_gal andin c need to be smaller than predicted for LCDM halos, again consistent withthe idea that disk galaxies preferentially reside in halos with a quiet mergerhistory.