The Dust-to-Gas Ratio in the Damped Lyα Clouds toward the Gravitationally Lensed QSO 0957+561

Abstract

We present HST/FOS1 spectra of the two bright images (A and B) of the gravitationally lensed QSO 0957+561 in the wavelength range 2200-3300 Å. We find that the absorption system (z_abs = 1.3911) near z_em is a weak, damped Lyα system with strong Lyα absorption lines seen in both images. However, the H I column densities are different, with the line of sight to image A intersecting a larger column density. The continuum shapes of the two spectra differ in the sense that the flux level of image A increases more slowly toward shorter wavelengths than that of image B. We explain this as the result of differential reddening by dust grains in the damped Lyα absorber. A direct outcome of this explanation is a determination of the dust-to-gas ratio, k, in the damped Lyα system. We derive k = 0.55 +/- 0.18 for a simple 1/λ extinction law and k = 0.31 +/- 0.10 for the Galactic extinction curve. For gravitationally lensed systems with damped Lyα absorbers, our method is a powerful tool for determining the values and dispersion of k, and the shapes of extinction curves, especially in the FUV and EUV regions. We compare our results with previous work.