Abstract

We have embarked on a search for temporal velocity shifts, on timescales of a year or so, in QSO broad lines. Eighteen quasars in a total sample of thirty have been analysed so far, and no evidence for velocity shifts has been found. If no shifts are found in the remaining twelve QSOs, then our results will call into question the identification of certain QSO pairs as lensed systems and the limits on the sizes and spatial distribution of Lyman α clouds derived from such systems.

Introduction

Steidel and Sargent (1991) recorded high signal–to–noise ratio spectra of the QSO pairs Q1634+267A,B and Q2345+007A,B in order to determine whether the systems were gravitationally lensed. They concluded that the systems were both lensed on the strength of detailed comparisons of the line profile and continuum shapes. However, they also discovered that the Ly α, N V λ1240, C IV λ1549, and Si IV λ1400 emission lines of the two images of Q1634+267A,B exhibit a relative velocity shift of as much as ∼ 1000 km s−1. The authors favored the explanation that the redshifts of individual lines in QSO spectra vary with time and that, due to the roughly one year time delay between the light paths of the two images, one is seeing the individual QSO at two different times. Here, we describe our efforts to verify this prediction. We have reobserved 30 objects from the Mg II λ2800 survey of Steidel and Sargent (1992), being careful to use an identical instrument configuration and to obtain similar quality spectra.