Falling outer rotation curves of star-forming galaxies at 0.6 < z < 2.6
  probed with KMOS^3D and SINS/ZC-SINF
  We exploit the deep resolved Halpha kinematic data from the KMOS^3D and
SINS/zC-SINF surveys to examine the largely unexplored outer disk kinematics of
star-forming galaxies (SFGs) out to the peak of cosmic star formation. Our
sample 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 stacking
approach we are able to constrain a representative rotation curve extending out
to ~4 effective radii. This average rotation curve exhibits a significant drop
in 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 and
R/R_turn. This result confirms that the fall-off seen previously in some
individual galaxies is a common feature of our sample of high-z disks. We show
that this outer fall-off strikingly deviates from the flat or mildly rising
rotation curves of local spiral galaxies of similar masses. We furthermore
compare our data with models including baryons and dark matter demonstrating
that the falling stacked rotation curve can be explained by a high mass
fraction of baryons relative to the total dark matter halo (m_d>~0.05) in
combination with a sizeable level of pressure support in the outer disk. These
findings are in agreement with recent studies demonstrating that star-forming
disks at high redshift are strongly baryon dominated within the disk scale, and
furthermore suggest that pressure gradients caused by large turbulent gas
motions are present even in their outer disks. We demonstrate that these
results are largely independent of our model assumptions such as the presence
of a central stellar bulge, the effect of adiabatic contraction at fixed m_d,
and variations in the concentration parameter.
