A Revised Model for the Formation of Disk Galaxies: Quiet History, Low Spin, and Dark-Halo Expansion
Abstract
(Abridged) A specific scenario of disk galaxy formation is singled out by the observed joint distribution of structural parameters: rotation velocity V, disk size R, and luminosity L. Within the LCDM cosmology, we employ a simple model involving four independent log-normal random variables: dark-halo concentration c, disk spin lam_gal, disk mass fraction m_gal, and stellar mass-to-light ratio M/L_I. While the slopes of the VL and RL relations crudely reflect the spherical virial relations Vvir \pr Rvir \pr Mvir^{1/3}, gas processes must compensate for the variations of c and M/L_I with mass. We require that the gas-to-stars ratio is decreasing with luminosity, as observed, and that m_gal is increasing with halo mass. A simultaneous match of the VL and RL zero points with adiabatic contraction requires low-c halos, but this model has V2.2~1.8 Vvir which will be unable to match the luminosity function (LF). Models in which disk formation induces an expansion rather than the commonly assumed contraction of the dark matter halos have V2.2~1.2 Vvir which allows a simultaneous fit of the LF. This may result from non-spherical, clumpy gas accretion, where dynamical friction transfers energy from the gas to the dark matter. This model requires low lam_gal and m_gal values, contrary to naive expectations. However, the low lam_gal is consistent with the notion that disk galaxies predominantly survive in halos with a quiet merger history, while a low m_gal is also indicated by galaxy-galaxy lensing. We also find that scatter in lambda_gal and in c need to be smaller than predicted for LCDM halos, again consistent with the idea that disk galaxies preferentially reside in halos with a quiet merger history.Keywords
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