The angular momentum of baryons and dark matter halos revisited

Recent theoretical studies have shown that galaxies at high redshift are fed by cold, dense gas filaments, suggesting angular momentum transport by gas differs from that by dark matter. Revisiting this issue using high-resolution cosmolog ical hydrodynamics simulations with adaptive mesh refinement (AMR), we find that at the time of accr etion, gas and dark matter do carry a similar amount of specific angular momentum, but that it is systematically higher than that of the dark matter halo as a whole. At high redshift, fres hly accreted gas rapidly streams into the central region of the halo, directly depositing thi s large amount of angular momentum within a sphere of radius r = 0.1Rvir. In contrast, dark matter particles pass through the central region unscathed, and a fraction of them ends up populating the outer regions of the halo ( r/Rvir > 0.1), redistributing angular momentum in the process. As a result, large-scale motions of the cosmic web have to be considered as the origin of gas angular momentum rather than its virialised dark matter halo host. This generic resu lt holds for halos of all masses at all redshifts, as radiative cooling ensures that a significant f raction of baryons remain trapped at the centre of the halos. Despite this injection of angular mo mentum enriched gas, we predict an amount for stellar discs which is in fair agreement with observations at z=0. This arises because the total specific angular momentum of the baryons (g as and stars) remains close to that of dark matter halos. Indeed, our simulations indicate that any differential loss of angular momentum amplitude between the two components is minor even though dark matter halos continuously lose between half and two-thirds of their specific angular momentum modulus as they evolve. In light of our results, a substantial revisi on of the standard theory of disc formation seems to be required. We propose a new scenario where gas efficiently carries the angular momentum generated by large-scale structure motions deep inside dark matter halos, redistributing it only in the vicinity of the disc.