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The cosmic accretion of both dark matter and baryons into halos typically ismeasured using some evolving virial relation, but recent work suggests thatmost halo growth at late cosmic time (z < 2) is not physical but is rather theby-product of an evolving virial radius ("pseudo-evolution"). Using Omega25, asuite of cosmological simulations that incorporate both dark matter and gasdynamics with differing treatments of gas cooling, star formation, and thermalfeedback, we systematically explore the physics that governs cosmic accretioninto halos and their galaxies. Physically meaningful cosmic accretion of bothdark matter and baryons occurs at z > 1 across our halo mass range: M_200m =10^{11-14} M_sun. However, dark matter, because it is dissipationless, isdeposited (in a time-average sense) at > R_200m(z) in a shell-like manner, suchthat dark-matter mass and density experience little-to-no physical growth atany radius within a halo at z < 1. In contrast, gas, because it is able to coolradiatively, experiences significant accretion at all radii, at a rate thatroughly tracks the accretion rate at R_200m, at all redshifts. Infalling gasstarts to decouple from dark matter at ~2 R_200m and continues to accrete tosmaller radii until the onset of strong angular-momentum support at ~ 0.1R_200m. Thus, while the growth of dark matter is subject to pseudo-evolution,the growth of baryons is not. The fact that the accretion rate of gas ongalactic scales tracks the accretion rate near R_200m provides insight into thetight relations between the masses/sizes of galaxies and those of their hosthalos across cosmic time.

The physical nature of the cosmic accretion of baryons and dark matter into halos and their galaxies