Dark matter merging induced turbulence as an efficient engine for gas cooling

Abstract

We have performed a cosmological numerical simulation of primordial baryonic\ngas collapsing onto a $3\\times10^7$M$_{\\odot}$ dark matter (DM) halo. We show\nthat the large scale baryonic accretion process and the merger of few\n$\\sim10^6$ M$_{\\odot}$ DM halos, triggered by the gravitational potential of\nthe biggest halo, is enough to create super sonic ($\\mathcal{M}>10$) shocks and\ndevelop a turbulent environment. In this scenario the post shocked regions are\nable to produced both H$_2$ and HD molecules very efficiently reaching maximum\nabundances of $n_\\mathrm{H_2}\\sim10^{-2}n_\\mathrm{H}$ and $n_\\mathrm{HD}\\sim\n\\mathrm{few}\\times10^{-6}n_\\mathrm{H}$, enough to cool the gas below 100K in\nsome regions. The kinetic energy spectrum of the turbulent primordial gas is\nclose to a Burgers spectrum, $\\hat{E}_k\\propto k^{-2}$, which could favor the\nformation of low mass primordial stars. The solenoidal to total kinetic energy\nratio is $0.65\\la R_k\\la0.7$ for a wide range of wave numbers; this value is\nclose to $R_k\\approx 2/3$ natural equipartition energy value of a random\nturbulent flow. In this way turbulence and molecular cooling seem to work\ntogether in order to produce potential star formation regions of cold and dense\ngas in primordial environments. We conclude that both the mergers and the\ncollapse process onto the main DM halo provide enough energy to develop super\nsonic turbulence which favor the molecular coolants formation: this mechanism,\nwhich could be universal and the main route toward formation of the first\ngalaxies, is able to create potential star forming regions at high redshift.\n