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Astron. Astrophys. 350, 349-367 (1999)

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6. Gravitational lensing

In a recent paper Marri & Ferrara (1998) study the effects of gravitational lensing (GL) on high-z objects, in particular Type II SNe, for a hierarchical model of galaxy formation. For the three flat cosmologies they study (SCDM with [FORMULA] = 1, LCDM with [FORMULA] = 0.4, [FORMULA] = 0.6 and CHDM with [FORMULA] = 0.7, [FORMULA] = 0.3), they find that there is at least a 10% chance that objects with [FORMULA] 4 are magnified by a factor [FORMULA] 3. To estimate the effects of GL on our results we have therefore used their magnification probabilities on our model (i.e. their Figs. 4 and 5).

For the SCDM model, which yields the highest magnification, we find that the increase in the total number of core collapse SNe is only a few percent when using the NGST limits ([FORMULA] = 30.2 and [FORMULA] = 31.4). The effects are even smaller for the shorter wavelength bands. The reason for the small effect is that with the faint limits of the NGST almost all SNe up to [FORMULA] 4 are detected even without magnification, and that the number of SNe with even higher z, where the magnification has largest effect, is relatively small.

More interesting, for SNe with [FORMULA]4 we find, when using the NGST M´ limit, an increase by [FORMULA] 20% of the predicted counts. For SNe with [FORMULA] 9 the increase is [FORMULA] 40%. It should, however, be noted that estimates presented by Porciani & Madau (1998) give a much lower probability for substantial magnification compared to the Marri & Ferrara results used in the estimates above. The reason for this seems to be that Marri & Ferrara assume point-like lenses, whereas Porciani & Madau use a more realistic mass distribution characterized by singular isothermal spheres. Therefore, the effects of gravitational lensing presented here could be overestimated.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999
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