COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses

Free access article

Issue		A&A Volume 451, Number 3, June I 2006


Page(s)		759 - 766
Section		Cosmology (including clusters of galaxies)
DOI		http://dx.doi.org/10.1051/0004-6361:20054454

A&A 451, 759-766 (2006)
DOI: 10.1051/0004-6361:20054454

III. Redshift of the lensing galaxy in eight gravitationally lensed quasars

A. Eigenbrod¹, F. Courbin¹, G. Meylan¹, C. Vuissoz¹ and P. Magain²

¹ Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire, 1290 Sauverny, Switzerland
² Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 août 17, Sart-Tilman, Bat. B5C, 4000 Liège, Belgium

(Received 1 November 2005 / Accepted 21 December 2005)

Abstract
Aims.We measure the redshift of the lensing galaxy in eight gravitationally lensed quasars in view of determining the Hubble parameter H₀ from the time delay method.
Methods.Deep VLT/FORS1 spectra of lensed quasars are spatially deconvolved in order to separate the spectrum of the lensing galaxies from the glare of the much brighter quasar images. A new observing strategy is devised. It involves observations in Multi-Object-Spectroscopy (MOS) which allows the simultaneous observation of the target and of several PSF and flux calibration stars. The advantage of this method over traditional long-slit observations is a much more reliable extraction and flux calibration of the spectra.
Results.For the first time we measure the redshift of the lensing galaxy in three multiply-imaged quasars: SDSS J1138+0314 ( $z_{\rm lens}$ = 0.445), SDSS J1226-0006 ( $z_{\rm lens}$ = 0.517), SDSS J1335+0118 ( $z_{\rm lens}$ = 0.440), and we give a tentative estimate of the redshift of the lensing galaxy in Q 1355-2257 ( $z_{\rm lens}$ = 0.701). We confirm four previously measured redshifts: HE 0047-1756 ( $z_{\rm lens}$ = 0.407), HE 0230-2130 ( $z_{\rm lens}$ = 0.523), HE 0435-1223 ( $z_{\rm lens}$ = 0.454) and WFI J2033-4723 ( $z_{\rm lens}$ = 0.661). In addition, we determine the redshift of the second lensing galaxy in HE 0230-2130 ( $z_{\rm lens}$ = 0.526). The spectra of all lens galaxies are typical for early-type galaxies, except for the second lensing galaxy in HE 0230-2130 which displays prominent [OII] emission.

Key words: gravitational lensing -- cosmological parameters -- quasars: general

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