The double quasar Q2138-431: detection of a lensing galaxy
  This paper reviews the question of whether the wide separation double quasar
Q2138-431 is a gravitational lens. From early work, the two quasar images are
known to have almost identical spectra and redshifts, but no lensing galaxy has
so far been detected. In this paper we used recent deep surveys in infrared and
optical bands to search for the presence of a galaxy with the expected
properties of a gravitational lens. The search revealed a $5 \sigma$ detection
of a faint galaxy between the two quasar images on a deep $J$-band frame from
the VISTA Science Archive, with apparent magnitude $J = 20.68$. Non-detection
in the $I$-band implied a redshift $z > 0.6$, and mass modelling of the quasar
system 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 photographic
data from the UK 1.2m Schmidt telescope covering 25 years were used to
construct light curves for the two quasar images, which were then
cross-correlated to measure any time lag. This showed image B to lead image A
by around a year, consistent with 410 days from the mass model. Although the
similarity of the spectra and the detection of the lensing galaxy are the most
compelling arguments for the classification of Q2138-431 as a gravitational
lens, the time delay and mass-to-light ratio provide a consistent picture to
support this conclusion. The wide separation of the quasar images and the
simplicity of the mass model make Q2138-431 an excellent system for the
measurement of the Hubble constant.
