The detection of the lambda 2175 A feature, and further analysis of the BAL line profile structure in the gravitational lens candidate UM425

Abstract

We obtained MMT spectra of the gravitational lens candidate UM425 to compare the redshifts and line profile structures of lens components A and B, which are separated by ~6.5". The CIV lambda 1550 A emission in both A and B exhibits Broad Absorption Line (BAL) structure, consistent with the earlier detection of BAL structure in OVI lambda 1033 A and NV lambda 1240 A that was found with the International Ultraviolet Explorer (IUE) in component A. Cross-correlation of the spectra of A and B using emission lines of CIV lambda 1550 A, HeII lambda 1640 A, NIII lambda 1750 A, CIII] lambda 1909 A and MgII lambda 2800 A reveals a difference in the redshifts of A & B. However, the detailed BAL line profile structure found in spectra of A & B are strikingly similar to one another, which suggests the system is lensed. The spectra of A & B also indicate significant dust extinction, which we base on the presence of the lambda 2175 A absorption feature in the rest frame of the QSO (z_{QSO}=1.47). This feature is commonly seen in galactic sources, but is not generally observed in QSO spectra. Our spectra show the presence of the lambda 2175 A absorption feature in spectra of both images associated with the gravitational lens UM425. Based upon the strong similarity of BAL line profile structure exhibited by UM425A&B, particularly the presence of the lambda 2175 dust absorption feature in spectra of both images, we conclude that UM425 is a gravitational lens.