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We exploit the deep resolved Halpha kinematic data from the KMOS^3D andSINS/zC-SINF surveys to examine the largely unexplored outer disk kinematics ofstar-forming galaxies (SFGs) out to the peak of cosmic star formation. Oursample contains 101 SFGs representative of the more massive (9.3 < log(M*/Msun)< 11.5) main sequence population at 0.6<z<2.6. Through a novel stackingapproach we are able to constrain a representative rotation curve extending outto ~4 effective radii. This average rotation curve exhibits a significant dropin rotation velocity beyond the turnover, with a slope of Delta(V)/Delta(R) =$-0.26^{+0.10}_{-0.09}$ in units of normalized coordinates V/V_max andR/R_turn. This result confirms that the fall-off seen previously in someindividual galaxies is a common feature of our sample of high-z disks. We showthat this outer fall-off strikingly deviates from the flat or mildly risingrotation curves of local spiral galaxies of similar masses. We furthermorecompare our data with models including baryons and dark matter demonstratingthat the falling stacked rotation curve can be explained by a high massfraction of baryons relative to the total dark matter halo (m_d>~0.05) incombination with a sizeable level of pressure support in the outer disk. Thesefindings are in agreement with recent studies demonstrating that star-formingdisks at high redshift are strongly baryon dominated within the disk scale, andfurthermore suggest that pressure gradients caused by large turbulent gasmotions are present even in their outer disks. We demonstrate that theseresults are largely independent of our model assumptions such as the presenceof a central stellar bulge, the effect of adiabatic contraction at fixed m_d,and variations in the concentration parameter.

Falling outer rotation curves of star-forming galaxies at 0.6 < z < 2.6 probed with KMOS^3D and SINS/ZC-SINF