Rotational Support of Giant Clumps in High-z Disc Galaxies
  We address the internal support against total free-fall collapse of the giant
clumps that form by violent gravitational instability in high-z disc galaxies.
Guidance is provided by an analytic model, where the proto-clumps are cut from
a rotating disc and collapse to equilibrium while preserving angular momentum.
This model predicts prograde clump rotation. This is confirmed in hydro-AMR
zoom-in simulations of galaxies in a cosmological context. In most high-z
clumps, the centrifugal force dominates the support, R=Vrot^2/Vcirc^2 > 0.5,
where Vrot is the rotation velocity and Vcirc is the circular velocity. The
clump spin indeed tends to be in the sense of the global disc angular momentum,
but substantial tilts are frequent. Most clumps are in Jeans equilibrium, with
the rest of the support provided by turbulence. Simulations of isolated
gas-rich discs that resolve the clump substructure reveal that the cosmological
simulations may overestimate R by ~30%, but the dominance of rotational support
at high-z is not a resolution artifact. In turn, isolated gas-poor disc
simulations produce at z=0 smaller gaseous non-rotating transient clouds,
indicating that the difference in rotational support is associated with the
fraction of cold baryons in the disc. In our current cosmological simulations,
the clump rotation velocity is typically Vrot~100 km/s, but when beam smearing
of \geq 0.1 arcsec is imposed, the rotation signal is reduced to a small
gradient of \leq 30 km/s/kpc across the clump. The velocity dispersion in the
simulated clumps is comparable to the disc dispersion so it is expected to
leave only a marginal signal. Retrograde minor-merging galaxies could lead to
massive clumps that do not show rotation.Testable predictions of the scenario
as simulated are that the mean stellar age of the clumps, and the stellar
fraction, are declining linearly with distance from the disc center.
