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Keywords:

  • galaxies: evolution;
  • intergalactic medium;
  • quasars: absorption lines;
  • cosmology: theory

ABSTRACT

We study supernova-driven galactic outflows as a mechanism for injecting turbulence in the intergalactic medium (IGM) far from galaxies. To this aim, we follow the evolution of a 1013-M galaxy along its merger tree, with carefully calibrated prescriptions for star formation and wind efficiencies. At z ≈ 3, the majority of the bubbles around galaxies are old (ages > 1 Gyr), that is, they contain metals expelled by their progenitors at earlier times; their filling factor increases with time, reaching about 10 per cent at z < 2. The energy deposited by these expanding shocks in the IGM is predominantly in kinetic form (mean energy density of 1 μeV cm−3, about two to three times the thermal one), which is rapidly converted in disordered motions by instabilities, finally resulting in a fully developed turbulent spectrum whose evolution is followed through a spectral transfer function approach. The derived mean IGM turbulent Doppler parameter, bt, peaks at z ≈ 1 at about 1.5 km s−1 with the maximum bt= 25 km s−1. The shape of the bt distribution does not significantly evolve with redshift but undergoes a continuous shift towards lower bt values with time, as a result of bubble ageing. We also find a clear trend of decreasing bt with inline image and a more complex dependence on Rs resulting from the age spread of the bubbles. We have attempted a preliminary comparison with the data, hampered by the scarcity of the latter and by the challenge provided by the subtraction of peculiar and thermal motions. Finally, we comment on the implications of turbulence for various cosmological studies.