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This paper reviews the question of whether the wide separation double quasarQ2138-431 is a gravitational lens. From early work, the two quasar images areknown to have almost identical spectra and redshifts, but no lensing galaxy hasso far been detected. In this paper we used recent deep surveys in infrared andoptical bands to search for the presence of a galaxy with the expectedproperties of a gravitational lens. The search revealed a $5 \sigma$ detectionof a faint galaxy between the two quasar images on a deep $J$-band frame fromthe VISTA Science Archive, with apparent magnitude $J = 20.68$. Non-detectionin the $I$-band implied a redshift $z > 0.6$, and mass modelling of the quasarsystem gave a mass of $1.31 \times 10^{12} M_\odot$ for the lensing galaxy,with mass-to-light ratio $M_{\odot}/L_{\odot} = 9.0$. Archival photographicdata from the UK 1.2m Schmidt telescope covering 25 years were used toconstruct light curves for the two quasar images, which were thencross-correlated to measure any time lag. This showed image B to lead image Aby around a year, consistent with 410 days from the mass model. Although thesimilarity of the spectra and the detection of the lensing galaxy are the mostcompelling arguments for the classification of Q2138-431 as a gravitationallens, the time delay and mass-to-light ratio provide a consistent picture tosupport this conclusion. The wide separation of the quasar images and thesimplicity of the mass model make Q2138-431 an excellent system for themeasurement of the Hubble constant.

The double quasar Q2138-431: detection of a lensing galaxy