  Outflows from quasars inevitably pollute the intergalactic medium (IGM) with
magnetic fields. The short-lived activity of a quasar leaves behind an
expanding magnetized bubble in the IGM. We model the expansion of the remnant
quasar bubbles and calculate their distribution as a function of size and
magnetic field strength at different redshifts. We generically find that by a
redshift z=3, about 5-20% of the IGM volume is filled by magnetic fields with
an energy density >10% of the mean thermal energy density of a photo-ionized
IGM (at T=10^4 K). As massive galaxies and X-ray clusters condense out of the
magnetized IGM, the adiabatic compression of the magnetic field could result in
the field strength observed in these systems without a need for further dynamo
amplification. The intergalactic magnetic field could also provide a nonthermal
contribution to the pressure of the photo-ionized gas that may account for the
claimed discrepancy between the simulated and observed Doppler width
distributions of the Ly-alpha forest.
