Turbulence in the Intergalactic Medium

TL;DR

This study models supernova-driven galactic outflows as a source of turbulence in the intergalactic medium, analyzing its evolution, properties, and potential observational signatures through simulations and spectral analysis.

Contribution

It introduces a detailed simulation of turbulence generation in the IGM from galactic outflows and characterizes its evolution and observational implications.

Findings

Turbulent energy density peaks at z~1 with b_t ~1.5 km/s.

Turbulence causes a shift towards lower Doppler parameters over time.

The shape of the turbulence distribution remains relatively stable across redshifts.

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 10^13 Msun 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 >1Gyr), i.e. they contain metals expelled by their progenitors at earlier times; their filling factor increases with time reaching about 10% at z<2. The energy deposited by these expanding shocks in the IGM is predominantly in kinetic form (mean energy density of 1 \mu eV cm^-3, about 2-3 x 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, b_t, peaks at z~1 at about 1.5 km/s with maximum b_t = 25 km/s. The shape of the b_t distribution does not significantly evolve with redshift but undergoes a continuous shift towards lower b_t values with time, as a result of bubble aging. We find also a clear trend of decreasing b_t with N_HI and a more complex dependence on R_s 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.