We present results from the Ponos simulation suite on the early evolution of a massive, $M_{rm vir}(z=0)=1.2times 10^{13}$ M$_{odot}$ galaxy. At $zgtrsim6$, before feedback from a central supermassive black hole becomes dominant, the main galaxy has a stellar mass $sim 2times 10^{9}$ 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, $Qsim 1.5-2$, 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 $zsim 1-2$, is a fundamental characterisation of main sequence galaxies at $zgtrsim 6$, 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.
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