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Issue A&A
Volume 386, Number 1, April IV 2002
Page(s) 12 - 30
Section Cosmology
DOI http://dx.doi.org/10.1051/0004-6361:20020226



A&A 386, 12-30 (2002)
DOI: 10.1051/0004-6361:20020226

Lens magnification by CL0024+1654 in the $\vec{U}$ and $\vec{R}$ band

S. Dye1, A. N. Taylor2, T. R. Greve3, 4, Ö. E. Rögnvaldsson5, E. van Kampen2, P. Jakobsson6, V. S. Sigmundsson6, E. H. Gudmundsson6 and J. Hjorth3

1  Astrophysics Group, Blackett Laboratory, Imperial College, Prince Consort Road, London SW7 2BW, UK
2  Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
3  Astronomical Observatory, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
4  Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
5  NORDITA, Blegdamsvej 17, 2100 Copenhagen Ø, Denmark
6  Science Institute, Dunhaga 3, 107 Reykjavik, Iceland

(Received 28 August 2001 / Accepted 8 February 2002)

Abstract
We estimate the total mass distribution of the galaxy cluster CL0024+1654 from the measured source depletion due to lens magnification in the R band. Within a radius of $0.54\,h^{-1}\,{\rm Mpc}$, a total projected mass of $(8.1\,\pm\, 3.2)\times 10^{14}~h^{-1}\,{M}_{\odot}$ (EdS) is measured. The $1\sigma$ error here includes shot noise, source clustering, uncertainty in background count normalisation and contamination from cluster and foreground galaxies. This corresponds to a mass-to-light ratio of $M/L_B=470\pm180$. We compute the luminosity function of CL0024+1654 in order to estimate contamination of the background source counts from cluster galaxies. Three different magnification-based reconstruction methods are employed: 1) an estimator method using a local calculation of lens shear; 2) a non-local, self-consistent method applicable to axi-symmetric mass distributions; 3) a non-local, self-consistent method for derivation of 2D mass maps. We have modified the standard single power-law slope number count theory to incorporate a break and applied this to our observations. Fitting analytical magnification profiles of different cluster models to the observed number counts, we find that CL0024+1654 is best described either by a NFW model with scale radius $r_{\rm s}=334\pm191\,h^{-1}\,{\rm kpc}$ and normalisation $\kappa_{\rm s}=0.23\pm0.08$ or a power-law profile with slope $\xi=0.61\pm0.11$, central surface mass density $\kappa_0=1.52\pm0.20$ and assuming a core radius of $r_{\rm core}=35\,h^{-1}\,{\rm kpc}$. The NFW model predicts that the cumulative projected mass contained within a radius  R scales as ${M}({<}R)=2.9\times 10^{14}(R/1')^{1.3-0.5\lg (R/1')}~h^{-1}\,{M}_{\odot}$ . Finally, we have exploited the fact that flux magnification effectively enables us to probe deeper than the physical limiting magnitude of our observations in searching for a change of slope in the U band number counts. We rule out both a total flattening of the counts with a break up to $U_{\rm AB}\leq 26.6$ and a change of slope, reported by some studies, from ${\rm d}\log N / {\rm d}m=0.4 \rightarrow 0.15$ up to $U_{\rm AB}\leq26.4$ with 95% confidence.


Key words: gravitational lensing -- galaxies: clusters: individual: CL0024+1654 -- cosmology: dark matter

Offprint request: S. Dye, sdye01@ic.ac.uk

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