The ultraviolet spectrum of the gravitational lens candidate UM 425 = QSO 1120+019: Evidence for broad absorption line (BAL) structure

Abstract

The UV line profile structure of high-ionization resonance lines found with the International Ultraviolet Explorer (IUE) in the brightest of four multiply imaged sources (image-A) in the `candidate gravitational lens UM 425 = QSO 1120+019 indicates broad absorption line (BAL) structure. The deep-broad trough associated with the O VI line extends to velocities ~-12,000 km s^-1^, and contains discrete features that suggest multicomponent velocity structure. This structure may include contributions from C IV absorption from the early-type galaxy that is believed to lens UM 425. A strong absorption feature in the blue wing of the Lyman-α λ1216 emission line may be a Lyman-α absorption system at a z_Lya_ = 1.437 +/- 0.003, or it may be formed by the superposition of the broad N V λλ1238, 1242 absorption trough on the extended blue emission wing of the QSO Lyman-α line. We obtained a redshift of z_QSO_ = 1.471 +/- 0.003 from Lyman-α λ1215, consistent with the redshift found by Meylan and Djorgovski in the optical. The Lyman-α line appears unusually weak due to the presence of N V λ1240 BAL absorption. A Lyman-limit absorption system at λ912 was not observed in the QSO rest frame. The detection of BAL structure in the other weaker ground-state resonance lines of N II(1) and S IV (1) was not found, suggesting these lines are formed in a region that is distinct from the BAL component. Detection of BAL structure in the other fainter images in this system with HST instrumentation, similar to structure observed here in image A, could provide evidence that UM 425 is a gravitational lens.