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Astron. Astrophys. 334, L33-L36 (1998)

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3. Optical and infrared follow-up; lensing galaxies

All the systems listed in Table 1 have been observed with the HST, either with WFPC2 (in V and I-bands) or with NICMOS (in H-band). Full details of the HST observations will be presented elsewhere. Fig. 2 shows an example of one of the lensing galaxies detected, that of B1608+656. In all but one case the lensing galaxy has been detected in a position consistent with the gravitational lens hypothesis, the exception being B2114+022 where two galaxies are detected (Augusto et al. 1998). The resulting magnitudes are listed in Table 1. We give the magnitude within the Einstein radius except for B0218+357 and B1422+231 where values for total luminosity are taken from the literature. Multiple optical counterparts corresponding to two or more of the radio images are detected in all systems except B2114+024.

[FIGURE] Fig. 2. The MERLIN 5-GHz image (contours) of B1608+656 (made from a 12-hour track on 1996 January 29; noise level of 130µJy/beam; contours are at 300µJy/beam [FORMULA] [-1,1,2,4,8,16,32]) superimposed on the HST/WFPC2/814-nm image (greyscale) of B1608+656. The lensing galaxy is clearly visible in the optical image, as are lensed arcs due to the extension of the lensed image, which is a post-starburst galaxy (Fassnacht et al. 1996).

In most cases the redshifts of the lensing galaxy and the lensed object are determined and this enables us to estimate a lens mass. In the cases where the redshift information is incomplete we assume a lens redshift of 0.5 and a source redshift of 1.5. In each case we have adopted a singular isothermal sphere model for the mass distribution of the lens. The mass-to-light ratios so calculated 1 are given in Table 1, and are all consistent with the values expected if the lenses are normal luminous elliptical galaxies ([FORMULA] 20, e.g. Oegerle & Hoessel 1991). Both mass and light are estimated within the Einstein radius: the (non-SIS) correct mass distribution to adopt outside this radius is beyond the scope of this paper.

No K-corrections or evolutionary corrections have been applied to give the values presented in Table 1. However, these corrections are relatively unimportant in the present context as for an E galaxy at redshift 0.6 the combined value of these corrections give correction factors of [FORMULA] 0.35, [FORMULA] 1.15 and [FORMULA] 1.85 in V, I, and H, respectively, for the mass-to-light ratios (Poggianti 1997; the correction factors for spirals are not very different). It can be seen, therefore, that the corrected mass-to-light ratios for our lensing galaxies range from [FORMULA] 1 to around 20. Of the two systems with the largest [FORMULA], B1600+434 is an edge-on spiral (Jaunsen & Hjorth 1997; Koopmans, de Bruyn & Jackson 1998) and some internal reddening is therefore expected, which will boost the inferred mass-to-light ratio. The other high- [FORMULA] object is B2045+265, which has a large separation and contains a relatively faint (but not dark) lensing galaxy.

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© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998