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Astron. Astrophys. 339, L13-L16 (1998)

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1. Introduction

Gravitational lensing of distant extragalactic sources (e.g., QSOs) by intervening large masses at low to moderate redshift (e.g., galaxies) has become a powerful tool of modern cosmology. The multiple images of lensed QSOs are expected to be separated by a few arc seconds in the sky, implying transverse line-of-sight (LOS) separations of up to few tens of kpc. The detection and study of the lensing agent provide an independent way to (i) probe the dark mass involved in the lensing potential (e.g., Natarajan et al. 1998); (ii) put constraints on the the Hubble parameter [FORMULA] in case of QSO variability (Refsdal 1964). However, the position of the lensing agent, normally much fainter than the lensed source, is fairly difficult to determine. To date, about 20 lens redshifts have been determined out of twice as many known gravitational lens events.

In this letter we present new BVR images of the gravitationally lensed QSO pair HE 2149-2745 A,B aimed at detecting the galaxy responsible for the double image. The QSO redshift is [FORMULA] and the angular separation between the QSO images is [FORMULA]. As the QSO pair was discovered, the spectroscopic and R-band observations by Wisotzki et al. (1996) already revealed its lensing nature, but the detection of the lensing galaxy remained uncertain. Our new images confirm the presence of the lensing galaxy. The sampling is well below the small separation between the two QSO images, so an estimate of galaxy parameters is possible. They are discussed in the framework of possible lens scenarios.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998