Declining rotation curves at $z=2$ in $\Lambda$CDM galaxy formation
  simulations
  Selecting disk galaxies from the cosmological, hydrodynamical simulation
Magneticum Pathfinder we show that almost half of our poster child disk
galaxies at $z=2$ show significantly declining rotation curves and low dark
matter fractions, very similar to recently reported observations. These
galaxies do not show any anomalous behavior, reside in standard dark matter
halos and typically grow significantly in mass until $z = 0$, where they span
all morphological classes, including disk galaxies matching present day
rotation curves and observed dark matter fractions. Our findings demonstrate
that declining rotation curves and low dark matter fractions in rotation
dominated galaxies at $z=2$ appear naturally within the $\Lambda$CDM paradigm
and reflect the complex baryonic physics, which plays a role at the peak epoch
of star-formation. In addition, we find some dispersion dominated galaxies at
$z=2$ which host a significant gas disk and exhibit similar shaped rotation
curves as the disk galaxy population, rendering it difficult to differentiate
between these two populations with currently available observation techniques.
