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-vir(z = 0) = 1.2 x 10(13) M-circle dot galaxy. At z greater than or similar to 6, before feedback from a central supermassive black hole becomes dominant, the main galaxy has a stellar mass similar to 2 x 10(9) M-circle dot and a star formation rate similar to 20 M-circle dot 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 similar to 40 km s(-1), rotation to dispersion ratio similar to 2, and with a significant amount of gas at similar to 10(5) K. The Toomre parameter always exceeds the critical value for gravito-turbulence, Q similar to 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 similar to 1-2, is a fundamental characterization of the main-sequence galaxies at z greater than or similar to 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