Gravitational lensing in quasar samples

The first cosmic mirage was discovered approximately 20 years ago as the double optical counterpart of a radio source. This phenomenon had been predicted some 70 years earlier as a consequence of General Relativity. We present here a summary of what we have learnt since. The applications are so numerous that we had to concentrate on a few selected aspects of this new field of research. This review is focused on strong gravitational lensing, i.e. the formation of multiple images, in QSO samples. It is intended to give the reader an up-to-date status of the observations and to present an overview of its most interesting potential applications in cosmology and astrophysics, as well as numerous important results achieved so far. The first section follows an intuitive approach to the basics of gravitational lensing and is developed in view of our interest in multiply imaged quasars. The astrophysical and cosmological applications of gravitational lensing are outlined in Sect. 2 and the most important results are presented in Sect. 5. Sections 3 and 4 are devoted to the observations. Finally, conclusions are summarized in the last section. We have tried to avoid duplication with existing (and excellent) introductions to the field of oravitational lensing. For this reason, we did not concentrate on the individual properties of specific lens models, as these are already well presented in Narayan and Bartelmann (1996) and on a more intuitive ground in Refsdal and Surdej (1994). Wambsganss (1998) proposes a broad view on gravitational lensing, in astronomy; the reviews by Fort and Mellier (1994) and Hattori et al. (1999) deal with lensing by galaxy clusters, microlensing in the Galaxy and the local group is reviewed by Paczynski (1996) and a general panorama on weak lensing is given by Bartelmann and Schneider (1999) and Mellier (1999). The monograph on the theory of gravitational lensing by Schneider, Ehlers and Falco (1992) also remains a reference in the field

Close projected QSO-Galaxy associations: are they real?

peer reviewedThe present status of close projected associations between high redshift quasars and foreground galaxies is reviewed in the framework of gravitational lensing, adopting both an observational and a theoretical approach. The Non Singular Isothermal Spherical lens model (Hinshaw & Krauss 1987) is used. We confirm that a stronger overdensity q of bright galaxies is expected at small projected angular separations (<= 3arcsec ) from high redshift and bright (thus highly luminous) quasars (HLQs). The effects of a non singular galactic core radius and/or microlensing only slightly enhance this expected overdensity. The predictions reproduce pretty well the scarce observations which are presently available, with the exception of two rather high overdensities previously claimed in the literature (e.g. q=2.9 for theta <= 6arcsec , Webster & Hewett 1990). Uncontrolled morphological selections may affect these controversial results so that additional unbiased observations are badly needed before rejecting gravitational lensing as the mechanism to produce the observed close angular QSO-galaxy associations. We describe three new selected and bias-free samples consisting of a total of 219 different HLQs. Although an apparent overdensity is detected in two of them, these galaxy excesses are found to be statistically not significant. Firm confirmation of the reality of close projected QSO-Galaxy associations is hampered by small number statistics: the expected and observed absolute numbers of such QSO-galaxy associations are still presently found to be very small. Therefore, the corresponding error bars are quite large. We conclude that the best observational strategy to significantly detect (at 3sigma ) an overdensity of galaxies near HLQs is to observe about 1500 HLQs (M_V ~ -29) down to a limiting magnitude Rlim ~ 23. Future automated surveys may provide us with such a large and unbiased database. In conclusions, we find that the lensing-induced correlation between galaxies and HLQs at small angular separations consists of a very interesting but weak effect, much less sensitive to cosmological or astrophysical parameters than the number of multiply imaged sources expected within a sample of HLQs. Based on data collected at the European Southern Observatory, La Silla, Chil

Gravitational lensing studies with the 4-m International Liquid Mirror Telescope (ILMT)

A 4-m International Liquid Mirror Telescope (ILMT) is being built in the north of Chile by an international consortium and will become operational in two years from now. We present here a short description of the telescope as well as estimates of the microlensing, macrolensing and weak lensing effects expected from a deep, multicolor imaging survey made with such a telescope

HST observations of gravitationally lensed QSOs

Thanks to its sharp view, HST has significantly improved our knowledge of tens of gravitationally lensed quasars in four different respects: (1) confirming their lensed nature; (2) detecting the lensing galaxy responsible for the image splitting; (3) improving the astrometric accuracy on the positions of the unresolved QSO images and of the lens; (4) resolving extended lensed structures from the QSO hosts into faint NIR or optical rings or arcs. These observations have helped to break some degeneracies on the lens potential, to probe the galaxy evolution and to reconstruct the true shape of the QSO host with an increased angular resolution

Weighing nearby stars with GAIA ?

Microlensing consists in two major effects: (1) variation in the apparent position of the background sources (astrometric component) and (2) flux variations of the background sources (photometric component). While the latter has been extensively used in the search for dark objects in the Galactic disk and halo (projects like MACHO (Alcock et al, 1997), EROS (Derue et al, 1999), OGLE (Paczynski et al, 1994)), the first effect has not yet been part of a systematic observational program, simply because the observations of very slight displacements in the positions of background sources requires an astrometric accuracy which current telecopes do not yet provide. We investigate here whether the astrometric accuracy of GAIA could enable such measurements and, as a consequence, enable new, direct and original measurements of the mass of nearby stars

Surveys with a 4 m Liquid Mirror Telescope

We describe an international project of construction and operation of a 4 m Liquid Mirror Telescope (LMT) led by E. Borra. A LMT, whose main advantage is its very low cost, is particularly well suited for the search and study of gravitational lenses, type Ia supernovae, faint nearby red, brown and white dwarfs, halo stars with high proper motions and, more generally, all variable phenomena like quasars, variable stars, micro-lensing effects, etc

Quasars with Gaia: Identification and Astrophysical Parameters

Gaia will provide astrometric and photometric observations for about 500 000 quasars distributed over the whole sky. The latter would constitute an isotropic grid of fixed sources perfectly suited to determine the referential frame. However, they must first be properly identified among stars, whose population is about 2000 times larger. Using broad and medium band photometry, we first compare the efficiency of two analysis methods (Ï 2 fitting and Artificial Neural Networks, ANNs) to produce the QSO catalogue with the lowest amount of contamination by stars. We then investigate whether the Gaia photometry could also provide precise values of the QSO astrophysical parameters (APs, like the redshift, the continuum slope, the emission line strength and possibly the extinction). To reach that purpose, we also compare the performances of the Spectral Principal Component Analysis (SPCA) with those of the Ï 2 fitting and ANN analysis

HST observations of the gravitational lens systems HE 1104-1805 and J03.13

High angular resolution Principle Component Image (PC1) images of the gravitational lens systems HE 1104-1805 and J03.13 are presented. Using a method described in Remy et al. (1997a), optimal TinyTim Point Spread Function (PSFs) are constructed to fit at best the lensed point-like components. The derived photometry of the GL components and the detection of the lens galaxy for HE 1104-1805 are discussed. Textbook case Faint Object Spectrograph (FOS) spectra of J03.13 A and B clearly show that this double QSO is a cosmic mirage

Microlensing probes the AGN structure of the lensed quasar J1131-1231

We present the analysis of single epoch long slit spectra of the three brightest images of the gravitationally lensed system J1131-1231. These spectra provide one of the clearest observational evidence for differential micro-lensing of broad emission lines (BELs) in a gravitationally lensed quasar. The micro-lensing effect enables us: (1) to confirm that the width of the emission lines is anti-correlated to the size of the emitting region; (2) to show that the bulk of Fe II is emitted in the outer parts of the Broad Line Region (BLR) while another fraction of Fe II is produced in a compact region; (3) to derive interesting informations on the origin of the narrow intrinsic Mg II absorption doublet observed in that system