Keywords: Gravitational Lensing - Dark Matter - Quasars
Identifying the real nature of dark matter in the Universe constitutes one of the most challenging goals in modern astrophysics (see Carr 1990 for a review). Gravitational lensing provides a very powerful tool to detect the various signatures of baryonic dark matter. Indeed, the distribution of dark matter associated with clusters of galaxies can be probed from the detailed analysis of giant luminous arcs and of the shear deformations of background sources (Fort & Mellier 1994); the effect of local compact dark matter, belonging to the disk or to the halo of our galaxy, can be detected in micro-lensing experiments such those being presently conducted by the DUO, EROS, MACHO and OGLE teams (see the recent review by Paczynski 1995). Finally, the cosmological density of isolated dark compact objects can be indirectly constrained by the (non) observation of `micro-lensing induced' flux variations in background QSOs or by the (non) detection of multiply imaged sources in a flux limited sample of QSOs. The first technique is specially sensitive to the effects of compact objects in the mass range -- (Canizares 1982, Schneider 1993).
The second technique consists in trying to resolve multiple macro-lensed QSO images using ground-based, radio telescopes and/or the HST with adequate high angular resolution imaging instruments. This class of techniques has a good sensitivity in the mass range --, depending on the instrument being used. The limits of this method are dictated by both the angular resolution and the dynamics of the instrument. Using the point-mass lens model, we show in this paper how statistical constraints can presently be derived in the above mass range; the contribution due to HST is emphasized. To be complete, let us mention here that by modelling the observed frequency of mutiply imaged Highly Luminous Quasars with galaxy lenses (some examples are well known), it has been possible to constrain the values of galactic parameters, the number counts of QSOs and the cosmological constant (Claeskens et al. 1995a).
In the next section, we present and discuss the expression of the probability for observing doubly imaged QSOs lensed by compact objects; we also emphasize the influence of the instrumental characteristics (angular resolution and dynamics). We present the data in section 3. The results and a discussion are given in section 4.
The generic expression for the probability of observing a lensing event along the line-of-sight to a QSO at redshift , due to a population of compact objects with mass , uniformly distributed in space, is:
where represents the cosmological density of compact objects with mass , in units of the present closure density of the Universe. Of course, the probability P increases with . In the above equation, is the so called magnification bias: because macro-lensing amplifies the flux of a background source, observing bright sources enhances the chance to discover a lens in a flux limited sample of quasars. Because of these two latter effects, the probability to discover a new lens is of course optimal when observing HLQs (typically ; see Turner 1984 & Surdej et al. 1988). Finally, in Eq. (1) represents the lensing cross-section for a compact object with mass at redshift z to produce double images from a QSO at redshift , and be detected by the given instrument.
At a given redshift, the general cross-section for multiple imaging is a ring whose thickness depends on three competing criteria:
The general expression for the lensing cross-section is the following:
where is the angular distance between the observer and the deflector. More formal details on this expression for the lensing probability can be found in Surdej et al. (1993a).
The present sample contains 1680 observations of 1207 different HLQs. It has been compiled by merging the HST snapshot survey (before refurbishment) and four selected ground-based imagery surveys of HLQs. Table 1 summarizes the properties of each subsample. In the case of multiple observations of a same QSO, the most efficient one has been kept. But this choice depends on the assumed lens mass. The two last columns of Table 1 show how many observations per sample have been kept, for two different lens masses. The percentages represent the internal efficiency of each subsample.
The sample presented in Table 1 only contains three lens candidates which satisfy the hypothesis of being possibly lensed by a dark compact object (i.e., two lensed images and no already detected galaxy-type lens) and which present an angular separation between their images smaller than . These are Q1208+1011 (Magain et al. 1992, Maoz et al. 1992), Q1009-025 (Surdej et al. 1993b) and J03.13 (Claeskens et al. 1995b).
Figure: Upper limit on versus (with a 99.7 % confidence level).
Comparing the observed and the predicted numbers of lenses in the sample, a firm upper limit on can be derived. With a 99.7% confidence level, and adopting an Einstein-de Sitter Universe (), we find that in the mass range --. The strongest constraint we have derived is for compact objects. The use of other cosmological models or the identification of a yet unseen galaxy-type lens among the three candidates would reinforce the present constraints. As shown in Fig. 2, our results are the most constraining ones for masses larger than , and a closure density of the Universe by any kind of compact objects in the mass range -- is ruled out.
The dotted line in Fig. 2 corresponds to a simulation of the constraints on if 500 additional observations of HLQs were obtained with the (presently refurbished) HST. This would drastically improve the constraint on in the mass range --. This would even surpass the best presently existing constraints derived from VLBI radio observations for compact objects more massive than (Kassiola et al. 1991). It is also important to note that the number counts of QSOs at radio wavelength is not known very accurately. So, the magnification bias for radio observations cannot be properly estimated and this makes optical observations presently more reliable in order to derive statistical values for the cosmological parameters. Therefore, HST can play a very important role in further constraining the cosmological density of compact dark matter. This goal could be achieved by carrying out a new snapshot survey.
Surdej, J., Remy, M., Smette, A. et al. 1993b, in the Proc. of the 31st Liège Int. Astroph. Coll. Gravitational Lenses in the Universe, eds. J. Surdej, D. Fraipont-Caro, E. Gosset, S. Refsdal, & M. Remy, pp 153--160
Surdej, J., Jaunsen, A., Claeskens, J.-F. et al. 1995, To appear in proc. IAU 173 symposium
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