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Astron. Astrophys. 339, L73-L76 (1998)

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6. Implications for the lensing galaxy

If the redshift of the lens [FORMULA] were known, we could compute the Hubble parameter from the time delay using the fundamental equation connecting the time delay [FORMULA] and the parameter T

[EQUATION]

As [FORMULA] is still unknown, we cannot use this formula to get [FORMULA]. However, simply adopting a canonical value for [FORMULA] allows us to predict the redshift of the lens, or better, to constrain the range of possible values for [FORMULA]. Fig. 4 shows the product of time delay and Hubble parameter as a function of [FORMULA]. For [FORMULA], [FORMULA], [FORMULA], and [FORMULA], the SIST model predicts [FORMULA], and the velocity dispersion of the galaxy for this model is [FORMULA]. This corresponds to a mass of [FORMULA] inside of one Einstein radius, well within the range expected for a reasonably massive galaxy. With an [FORMULA] band magnitude of 20.9 according to R98, the mass-to-light ratio would then be of the order of 10 solar units, again quite consistent with the expectations for such a galaxy (cf. Keeton et al. 1998).

[FIGURE] Fig. 4. Time delay scaled by [FORMULA] for the SIST (thick) and SIEMD (thin) model. Standard Einstein-de Sitter cosmology is shown as solid lines, a low-density universe ([FORMULA]) as dashed lines. Flat low-density world models are located between these two curves.

Recently, values for the time delay have been predicted based on the assumption that one of the two strong metal absorption line systems at [FORMULA] or [FORMULA] can be identified with the deflector. R98 give [FORMULA] yrs, Courbin et al. (1998) even [FORMULA] yrs. Since we can reliably exclude [FORMULA] yr, our results are not compatible with [FORMULA] significantly larger than 1; in particular, the absorbers at 1.32 and 1.66 can be ruled out.

We have searched our higher resolution NTT spectra of HE 1104-1805 (cf. Lopez et al. 1998) for absorption lines within the redshift range permitted by Fig. 4. An additional demand is that the lines should be stronger in A, as this component is located closer to the deflector. Two Mg II absorption systems, at [FORMULA] and 0.73, meet the criteria. Of these, [FORMULA] is acceptable only for a time delay as short as [FORMULA] yrs, and is furthermore not compatible with the [FORMULA] colour estimate of R98. This leaves the system at [FORMULA] as candidate; however, the lens could also be an elliptical galaxy for which Mg II absorption would be a poor indicator. The very red colours measured by R98 support such a notion.

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© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998
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