| Abstract: | The similarity between the distributions of spins for galaxies (lambda_gal) and for dark matter haloes (lambda_halo), indicated both by simulations and observations, is commonly interpreted as a one-to-one correlation between the spins of the galaxy and its host halo. This is used to predict galaxy sizes in semi-analytic models via r_e ~ lambda_halo * r_vir, where r_e is the half-mass radius of the galaxy and r_vir is the halo radius. Utilizing two different suites of zoom-in cosmological simulations, we find that lambda_gal and lambda_halo are in fact barely correlated, especially at z > 1. A general smearing of correlation is expected based on the different spin histories, where the more recently accreted baryons through streams gain and then lose significant angular momentum compared to the gradually accumulated dark matter (Danovich et al. 2015). Expecting the spins of baryons and dark matter to be correlated at accretion into r_vir, the null correlation at the end reflects an anti-correlation between lambda_gal/lambda_halo and lambda_halo, which can partly arise from mergers and compaction events. On the other hand, the spin vectors tend to be aligned, with a median cos theta = 0.6-0.7 between galaxy and halo, consistent with instreaming along a preferred plane (Danovich et al. 2012). The galaxy spin is better correlated with the spin of the inner halo, but this is largely driven by the baryons themselves. Following the null spin correlation, the radius re is not correlated with lambda_halo. While our simulations reproduce a general relation of the sort r_e = A*r_vir, in agreement with observational estimates, the relation becomes tighter with A = 0.02(c/10)^(-0.7), where c is the halo concentration, associated with a redshift dependence. |