UM 425 - A new gravitational lens candidate

G. Meylan, S. Djorgovski
1989 Astrophysical Journal  
We report the discovery of a probable new gravitational lens, associated with the quasar UM 425 = QSO 1120 + 019. This object was specifically selected in an optical imaging survey for gravitational lenses because of its relatively high redshift and apparently high luminosity. Multicolor (BVR) images were obtained with a charge-coupled device (CCD) at the Cerro Tololo Interamerican Observatory (CTIO) 60 inch (1.5 m) telescope in 1987 March, showing three close companions around the bright image
more » ... of the quasar. The follow-up spectroscopy in marginal weather conditions, with the Mount Palomar 200 inch (5 m) and Las Campanas 100 inch (2.5 m) telescopes (in 1988 March and April), suggested that the brightest companion has the same emission lines as the quasar. Additional spectra and additional imaging were obtained with the European Southern Observatory (ESO) 3.6 m telescope at La Silla, in 1988 May. UM 425 and its brightest companion (which we denote as components A and B) have similar BVR colors. Spectra of both objects, in the range 3600-8000 Â, with resolution of about 7 Â pixel" 1 , show the same emission lines, i.e., C iv 1549, C m] 1909, and Mg n 2799, with the same redshift z = 1.465 ± 0.005. The spectra are also similar in shape and in some details, including, possibly some associated absorption in the Mg n 2799 and C iv 1549 lines. The velocity difference between the two brightest components from the cross-correlation technique is AJ^_ B = 200 ± 100 km s" 1 and is consistent with zero. Subtracting a scaled spectrum of the brighter component (A) from that of the brightest companion (B), leaves a residual which may be interpreted as the spectrum of a lensing galaxy at z ^ 0.6. Whereas we cannot exclude the possibility that we are seeing a pair of physically distinct AGNs, the overall data are in favor of the gravitational lens hypothesis.
doi:10.1086/185386 fatcat:jhf77kwnxnhzdoysctiea2i5xe