SIMBAD references

The presence and amount of dust in the halo matter producing narrow metal line absorption systems is still uncertain. We consider the parameter Σ_a=∑(1+z_a), summed over the detected aborption line system redshifts z_a, and ask whether radio-loud quasars at z<2 and with a large value of the parameter Σ_a are redder in the B-V colour than those with a smaller Σ_a. This is anticipated if the absorption lines originate in sites which contain ``normal'' dust causing selective extinction. The answer is positive, and a mean intrinsic reddening e₀ per halo is consistent with ≃0.05, or about 0.2 B-mag extinction, roughly as expected.