Lack of dust in quasar absorption line systems

Abstract

THE origin of the absorption line systems in quasars is still uncertain¹. Most such systems apparently have column densities of atomic hydrogen of the order of 10¹⁹cm⁻², but, at least two quasars, 1331 + 170 (ref. 2) and PHL957 (ref. 3), have such strong Lyman a absorption lines that atomic hydrogen column densities of the order of 10²¹ cm⁻² are indicated. It should be possible to observe the dust produced 2,200 Å extinction bump as it is redshifted into the visible⁴, and to determine whether the absorption line systems are produced in spiral galaxies where the dust content is similar to that in the interstellar medium. McKee and Petrosian⁴ argue that the emission line regions of quasars generally lack dust, and that toward PHL957 the 2,200 Å feature is absent. From recent published data, I argue that dust similar to that found in the interstellar medium is not found toward the quasars 1331 + 170 and PHL 957. This could explain why H₂ is not found toward PHL 957, and it also indicates that the absorption line systems in quasars are not produced in spiral galaxies similar to ours.