A new case of gravitational lensing

Abstract

Even before the discovery of the first gravitational lens system¹ in 1979 (see ref. 2 for a recent review of the other published candidates), it was recognized that a statistical evaluation of the occurrence of gravitational lensing within a well-defined sample of quasars is important to understand better the quasar luminosity function and possibly the QSO phenomenon itself³, to test cosmological models^4,5 and to probe the luminous and dark matter distribution on various scales in the Universe^4–6. A year ago, we began a systematic search from the European Southern Observatory (ESO) at La Silla, Chile, for gravitational lens systems in a selected sample of highly luminous quasars; M_v<–29.0. These objects are promising candidates for gravitationally lensed QSO images with arc-second and sub-arc-second separations (J.S. et al., manuscript in preparation and refs 7 and 8). Since December 1986, we have identified four possible cases. Here we give a brief description of our first identified gravitational lens system UM673 = Q0142–100 =PHL3703^9–11. It consists of two images, A (m_R = 16.9) and B (m_R = 19.1), separated by 2.2arcs at a redshift z_q = 2.719. The lensing galaxy (m_R≈ 19, Z_L≈0.49) has also been found. It lies very near the line connecting the two QSO images, ˜0.8 arc s from the fainter one. Application of gravitational optometry to this system leads to a value M₀≈2.4×10¹¹M⊙ for the mass of the lensing galaxy and to δt≈7 weeks for the most likely travel-time difference between the two light paths to the QSO (assuming H₀= 75 km s^–1 Mpc^–1,q₀ = 0).