## 4. The reconstruction of the cluster mass distribution## 4.1. Inversion relations for a single source redshiftThe reconstruction of the surface mass density of a cluster from
lensed images of background sources has been described in several
papers (see introduction) for sources at the same redshift Hence, the surface mass density can be obtained in terms of the shear by integrating this first-order differential equation. However, the shear is not an observable in general (Kochanek 1990; Paper I), but the quantity , if we confine our considerations to non-critical clusters. (For critical clusters only is observable.) Inserting into (4.1) yields (Kaiser 1995) where Hence, it is possible to derive the gradient of the quantity
was explicitly constructed to account for this `rotational noise component' and has been demonstrated in Seitz & Schneider (1996) to work better than the other proposed methods. Here, is a vector field explicitly constructed in Seitz & Schneider (1996), and is the average of over the data field , i.e., the region where image ellipticities have been measured. Of course, is an undetermined constant, so that the surface mass density is determined up to the transformation ## 4.2. General inversion methodIn the general case of a redshift distribution of the sources, we again make use of (4.1). The formal integration of (4.1) proceeds in the same way as that of (4.2), i.e., where is the average of over the data field . The vector field is defined in (4.1) and given by first partial derivatives of the shear. The shear in turn is related to the mean image ellipticity via (3.5), Note that Eq. (4.7) is a local relation, valid at every point . The complicated dependence of on and suggests an iterative approach for the solution of the inversion problem: let be an `measured' estimate for and let and be an estimate for the shear field and the surface mass density. From that, an updated estimate for the shear field can be obtained, using (4.7): Then, by differentiation, the vector field can be calculated from (4.1), by using the shear field . And finally, an updated estimate for the surface mass density field is obtained from (4.6), This iteration process is started by chosing , . It is clear that the integration constant is still a free variable, i.e., with the method described here there remains a global invariance transformation of the resulting surface mass density field; in contrast to the case considered in Sect. 4.1, this transformation cannot be explicitly determined, due to the highly nonlinear relations occurring here. For critical clusters we need at most 10 steps to achieve a convergence of the iteration algorithm. For less massive clusters about 5 iteration steps are sufficient. We find that the iteration algorithm is more stable and converges faster than in the case of a single source redshift (see Paper II), mainly because there are no well-defined critical curves as function of , since their location depends on the source redshift. ## 4.3. The case for non-critical clustersIn the case of weak lensing (,
), the mass reconstruction depends only on the
mean value of If the cluster is non-critical for all redshifts of the sources,
the inversion problem can be simplified because
for all Then, (3.5) simplifies to As a result, the iteration procedure described in the last subsection can be applied in a somewhat simpler way, by using (4.11) instead of (4.7); in addition, one can use the approximation (A2.4) derived in the Appendix 2 for which yields This approximation is found to be sufficiently accurate to describe for (generic) non-critical clusters (see Fig. 8 in Appendix 2). Combining (4.11) for , (4.12), and replacing the expectation value with the observed local average , we obtain with the definition Inserting this expression for into (4.1), one obtains after some manipulations with the matrix Thus, with the inverse If we now define (4.16) can be written as Note the similarity between (4.19) and (4.2). Thus, the vector
field can be constructed directly from the
observable , as in the case of all sources
being at the same redshift, and the same inversion equation (4.4)
should be used, but with the current definitions of in other words, can be determined only up to a multiplicative constant. © European Southern Observatory (ESO) 1997 Online publication: July 3, 1998 |