Angular momentum evolution of disk galaxies at high redshifts

Taku Okamura

The stellar disk size of a galaxy depends on the stellar disk to dark halo mass ratio, m* equiv M*/Mdh, and the fraction of the dark-halo angular momentum transfered to the disk, j* equiv J*/Jdh.Since j* is also determined by various star-formation related baryon processes (but in different ways from m*), measuring j* and m* at various redshifts is essential to obtain a consistent understanding of the formationof disk galaxies. However, such studies at high redshifts are still lacking.We use the 3D-HST GOODS-S and COSMOS imaging data and stellar-mass catalog to examine j* and m* for star-forming galaxies at z ~ 2,3, and 4, when disks are actively forming.For each redshift, after carefully removing merging galaxies, we divide the catalog into three M* bins and infer Mdh from clustering analysis, thus obtaining m* for each bin.We also measure, for each bin, effective radii at rest 5000AAwith Galfit and fit a log-normal distribution to them to derive an average radius which, combined with Mo et al.'s (1998) analytic disk formation model and Mdh and m* values, gives j*. We find that the j*/m* ratio is ~0.7 (typical errors ~30%) for all three redshifts over the entire mass range examined, 8 * 10^10 < Mdh/h^-1 Mstar < 2 * 10^12,with a possible (<30%) decrease with mass.This high ratio is close to those of local disk galaxies, descendants of our galaxies in Mdh growth, implying a nearly constant j*/m* over the past 12 Gyr.These results are remarkable, because mechanisms controlling the angular momentum of disks such as inflows and feedbacksdepend on both redshift and halo mass and reproducing these results appears non-trivial for galaxy formation simulations.We also discuss disk instabilities using j* and m*.