S. Refsdal

Discovery of a quadruply lensed quasar - The 'clover leaf' H1413 + 117

In November 1986 we began an optical search for examples of gravitational lensing in a sample of highly luminous quasars (HLQs, Mv < –29), with the aims of improving our knowledge of the quasar luminosity function, studying the dark matter content of the Universe, and redetermining some important cosmological parameters. This survey has found one new case of lensing1,2 and the general implications of the search have been summarized3. Here we report the discovery of a second gravitational lens system in the broad absorption line quasar H1413 + 117 (refs 4–6). Four images of comparable brightness are seen, separated by ˜1 arcsec. Spectra obtained of two of the images are identical apart from the presence of sharp absorption lines in one component, which are presumably due to gas clouds along the line of sight. The unique configuration of the images, together with the fairly rare occurrence of this type of quasar, makes it incontrovertible that this is a lensed system, not a cluster of quasars, and this second discovery made by imaging bright quasars establishes the power of the method for finding systems with small separations.

Detection of the lensing galaxy for the double QSO HE 1104-1805

Abstract High angular resolution ground-based direct imaging (V, R, Ic) of the double QSO HE 1104–1805 has been obtained with the NOT and NTT telescopes. Analysis of these data led to the first detection of the lensing galaxy. Direct imaging of HE 1104–1805 has subsequently been carried out with the Planetary Camera (WFPC2) through the F555W (nearly Johnson V) and F814W (nearly Kron-Cousins Ic) filters onboard HST. These images confirm the presence of the deflector between the two lensed quasar components (A & B). Direct imaging of this system in the near infrared (IR) at 2.2 μm (K′), with IRAC-2b at the Cassegrain focus of the ESO/MPI 2.2 m telescope, indicates that the additional component is indeed a very red extended object with magnitude K ≈ 16.5. We present deconvolutions of the HST and IR images using the 2-channel PLUCY method. From the HST observations, we also report very accurate relative positions and brightnesses of the QSO components. By comparison between the ground-based and the HST observations, from February–March 1994 to November 1995, we possibly detect the fading of component A by ∼0.3 magnitudes in the optical and a corresponding fading of B by about half this value. The observed monotonic decrease of the magnitude difference between B and A as a function of wavelength is consistent with a partial amplification of the A component by microlensing. Evolutionary models for galaxies show that, in order to produce the observed colour indices V − Ic > 2 and 4 11 h 50 M ⊙ L ⊙ ) for the galaxy is large but does not imply an excess of dark matter with respect to other well known lensing galaxies.

Foreground Galaxies Around Luminous Quasars

We compare galaxy counts in deep R-band exposures of the fields of 36 highly luminous, high redshift QSOs to those in control fields at a distance of 1 deg. We find indication for a weak overdensity of galaxies in the foreground of QSOs on scales of arcminutes on a low significance level. Counts inside rings around the quasars, stars in the quasar fields and stars in the control fields show evidence for an excess of galaxies on scales of several arcseconds around the quasars as well as for a stronger clustering of galaxies in the QSO fields than in the control fields. We interpret. this in terms of an amplification bias by gravitational lensing.

Foreground Galaxies and the Variability of Luminous Quasars

In order to look for an amplification bias (AB) by gravitational lensing caused by medium redshift (0.2 ≲ z ≲ 0.8) clusters or groups of galaxies, we compare galaxy counts in deep CCD images of highly luminous, high redshift QSOs with those in nearby control fields at a distance of 1 deg at the same galactic latitude. The total sample contains 37 objects up to now, from which one field had to be excluded because of a seeing difference between the QSO and control fields.

Gravitational Lensing as a Tool: Future Observational Prospects

Since the discovery in 1979 of the first gravitational lens system by Walsh, Carswell and Weymann, i.e. just a little more than one decade ago, more than 1000 scientific papers have been printed on this subject. In this review, we first recall the basic principles underlying the physics of gravitational lensing and provide the reader with a concise survey of the most important astrophysical and cosmological problems associated with gravitational lensing, particularly with a view to the observational capabilities of the present and next generations of space and ground based instrumentation. We refer the reader to the book of Schneider, Ehlers and Falco (1992) and to general reviews by (1992), (1992), (1993) and (1993) for a more complete and detailed presentation of various theoretical and observational aspects of gravitational lensing.