Young and turbulent: the early life of massive galaxy progenitors
  We present results from the "Ponos" simulation suite on the early evolution
of a massive, $M_{\rm vir}(z=0)=1.2\times 10^{13}$ M$_{\odot}$ galaxy. At
$z\gtrsim6$, before feedback from a central supermassive black hole becomes
dominant, the main galaxy has a stellar mass $\sim 2\times 10^{9}$ M$_{\odot}$
and a star formation rate $\sim 20$ M$_{\odot}$ yr$^{-1}$. The galaxy sits near
the expected main sequence of star-forming galaxies at those redshifts, and
resembles moderately star-forming systems observed at $z>5$. The high specific
star formation rate results in vigorous heating and stirring of the gas by
supernovae feedback, and the galaxy develops a thick and turbulent disc, with
gas velocity dispersion $\sim 40$ km s$^{-1}$, rotation to dispersion ratio
$\sim 2$, and with a significant amount of gas at $\sim 10^5$ K. The Toomre
parameter always exceeds the critical value for gravito-turbulence, $Q\sim
1.5-2$, mainly due to the contribution of warm/hot gas inside the disc. Without
feedback, a nearly gravito-turbulent regime establishes with similar gas
velocity dispersion and lower $Q$. We propose that the "hot and turbulent" disc
regime seen in our simulations, unlike the "cold and turbulent"
gravito-turbulent regime of massive clumpy disc galaxies at $z\sim 1-2$, is a
fundamental characterisation of main sequence galaxies at $z\gtrsim 6$, as they
can sustain star formation rates comparable to those of low-mass starbursts at
$z=0$. This results in no sustained coherent gas inflows through the disc, and
in fluctuating and anisotropic mass transport, possibly postponing the assembly
of the bulge and causing the initial feeding of the central black hole to be
highly intermittent.
