Continuum reddening due to dust in the spectrum of PHL 938.

Summary. The continuum spectrum of the radio-quiet QSO PHL 938 shows a clear change in gradient at 2175 A in the rest frame of emission, steepening at longer wavelengths. This can be explained by the influence of absorption due to dust over a broad band centred on 2175 A, as seen in local extinction curves. We obtain a best-fitting range of values for the colour excess E(B- V) lying between the limits 0.024 < E(B-V) < 0.067. An upper limit to the total hydrogen gas column density, NH, which may occupy the same region as the dust, is set by the absence of an absorption-line system at z Zem. The upper limit of the ratio NH lE (B-- V), a measure of the gas/dust ratio, is calculated for PHL938 and compared with its value in our Galaxy. It is seen that the abundance of dust in the region close to PHL 938 along the line of sight cannot be less than a tenth of the local value and is likely to be comparable if not anomalously high.