Low angular momentum in clumpy, turbulent disk galaxies

We measure the stellar specific angular momentum $j_s=J_s/M_s$ in four nearby (z ≈ 0.1) disk galaxies that have stellar masses $M_s$ near the break $M_s^{\ast }$ of the galaxy mass function but look like typical star-forming disks at z ≈ 2 in terms of their low stability (Q ≈ 1), clumpiness, high ionized gas dispersion (40−50 $km{s}^{-1}$), high molecular gas fraction (20%–30%), and rapid star formation ($\sim 20M_{\odot }{yr}^{-1}$). Combining high-resolution (Keck-OSIRIS) and large-radius (Gemini-GMOS) spectroscopic maps, only available at low z, we discover that these targets have $\sim 3$ times less stellar angular momentum than typical local spiral galaxies of equal stellar mass and bulge fraction. Theoretical considerations show that this deficiency in angular momentum is the main cause of their low stability, while the high gas fraction plays a complementary role. Interestingly, the low $j_s$ values of our targets are similar to those expected in the $M_s^{\ast }$ population at higher z from the approximate theoretical scaling $j_s\propto {(1+z)}^{-1/2}$ at fixed $M_s.$ This suggests that a change in angular momentum, driven by cosmic expansion, is the main cause for the remarkable difference between clumpy $M_s^{\ast }$ disks at high z (which likely evolve into early-type galaxies) and mass-matched local spirals.