Probing turbulence in the Inter-Galactic Medium
12 December 2013
The mean number density of baryons in intergalactic space is a low 2 x 10-7 cm-3 – roughly equivalent to an office cubicle that contains only 6 protons! Yet CAASTRO Associate Investigator Dr Jean-Pierre Macquart and his colleague Dr Kevin Koay at Curtin University have found that, even at such low densities, the cumulative effect of this matter over large intergalactic distances is enough to alter the properties of radio waves that propagate through it. If the Inter-Galactic Medium (IGM) is stirred up by the jets of active galaxies, winds from massive stars and supernovae, or shocks along void walls, the medium will be turbulent. Density fluctuations associated with this turbulence scatter the emission from an impulsive radio signals so that its radiation becomes smeared out in time.
The effect of temporal smearing appears to have already been seen in some radio transients at cosmological distances. If so, it may be providing the first glimpse of the turbulent structure of the IGM. In their current publication, the researchers generalise the theory of scattering to cosmological contexts and provide the theoretical framework for reverse-engineering the turbulent structure of the IGM using scattering measurements. They also estimated the approximate magnitude of scattering effects associated with the various components of the IGM, including the contribution from the diffuse IGM itself, intervening galaxies, and intra-cluster gas. They calculated that, in most cases, the amount of temporal smearing expected at 300MHz is typically ~1 ms – sufficiently small that the detectability of the recently-discovered Fast Radio Bursts (Lorimer et al. 2007; Thornton et al. 2013) would not be impaired at wavelengths comparable to a metre and are therefore potentially within the scope of low-frequency widefield arrays such as the MWA and LOFAR.
Macquart & Koay also examined the redshift dependence of the temporal smearing caused by Inter-Galactic turbulence and show that its properties are readily distinguishable from scattering in the host galaxy in which the transient event occurred. Thus, with the detection of more Fast Radio Bursts in the future, it ought to be possible to chart the evolution of turbulence in the IGM and clearly distinguish it from the effects of scattering in their host galaxies.
by J-P Macquart
The red and cyan lines on the top diagram represent alternate paths that the radiation may take after it is scattered back into the line of sight by the turbulent IGM. The delay associated with these paths gives rise to radiation that arrives late with respect to the unscattered pulse, and the arrival time of this radiation is colour-coded on the signal profile in the figure on the bottom left.