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Astron. Astrophys. 330, 57-62 (1998)

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1. Introduction

It is well established that gravitationally lensed quasars are unique natural rulers for measuring the Universe and for deriving the cosmological parameters (Refsdal, 1964a,b). Measuring the time delay from the images of a lensed QSO can provide an estimate of the Hubble parameter [FORMULA], independent of any other classical method. However, a good knowledge of the geometry of the lensed system is mandatory for the method to be effective (e.g. Schechter et al, 1997; Keeton & Kochanek, 1996; Courbin et al, 1997). In spite of this crucial requirement and although the number of known gravitationally lensed quasars does not stop increasing (see for a review Keeton & Kochanek, 1996), the precise geometry of most lensed QSOs remains poorly known. In most cases, even the matter responsible for the lensing, whether it be in the form of a single galaxy or several galaxies, is not detected. The high redshifts of these galaxies (hence their faint apparent magnitudes) and the strong blending with the nearby much brighter QSO images are the main reasons for their non-detection.

Imaging in the near IR (1 to 2.5 microns) has the advantage that the relative brightness between the lensed QSO and any lensing galaxy decreases, making the galaxy easier to detect. The disadvantage is that the IR sky is considerably brighter. This forces one to take many images to avoid detector saturation; however, this turns out to be an advantage (see Section 3).

This paper presents IR observations of the quasar HE 1104-1805. The strong similarity between the optical spectra obtained for its two components (Wisotzki et al, 1993) makes HE 1104-1805 a good gravitational lens candidate. The high redshift of the object ([FORMULA], Smette et al, 1995) and the relatively wide angular separation between the lensed images (3.2 [FORMULA]) indicate that a large mass is involved in the lensing potential. If the deflector is a high redshift galaxy or a galaxy cluster, deep IR observations should reveal it.

We used a recently developed image deconvolution algorithm (Magain, Courbin & Sohy, 1997; hereafter MCS) to optimally combine the numerous IR frames and obtain deep, sharp images of HE 1104-1805. The present paper describes how this powerful technique allows us to study the immediate environment of HE 1104-1805 and detect the lensing galaxy.

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© European Southern Observatory (ESO) 1998

Online publication: January 8, 1998