Semi-analytic models of galaxy formation postulate the existence of virialized gaseous halos around galaxies at high redshifts .
A small fraction of the light emitted by any high-redshift quasar is therefore expected to scatter off the free electrons in the halo of its host galaxy .
The broadening of the scattered emission lines of the quasar can be used to measure the temperature of these electrons .
For gas in virial equilibrium , the velocity width of the scattered line profile is larger by a factor of \sim ( m _ { p } / m _ { e } ) ^ { 1 / 2 } = 43 than the velocity dispersion of the host galaxy and reaches \ga 10 , 000 ~ { } { km~ { } s ^ { -1 } } for the massive galaxies and groups in which bright quasars reside .
In these systems , the scattered width exceeds the intrinsic width of the quasar lines and hence can be used to measure the virial temperature of the quasar host .
The high degree of polarization of the scattered radiation can help filter out the extended scattered light from the central emission by the quasar and its host galaxy .
The signal-to-noise ratio of the spectral broadening can be enhanced by matching the full spectrum of the scattered radiation to a template of the unscattered quasar spectrum .
Although the central fuzz around low-redshift quasars is dominated by starlight , the fuzz around high-redshift quasars might be dominated by scattering before galaxies have converted most of their gas reservoirs into stars .