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Issue A&A
Volume 370, Number 1, April IV 2001
Page(s) 43 - 52
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20010204



A&A 370, 43-52 (2001)
DOI: 10.1051/0004-6361:20010204

The external shear in the gravitationally lensed system Q 2237+0305: A two-plane lens modelling

B. M. Mihov

Institute of Astronomy, Bulgarian Academy of Sciences and Isaac Newton Institute of Chile, Bulgarian Branch 72 Tsarigradsko Chausse Blvd., 1784 Sofia, Bulgaria

(Received 13 July 1998 / Accepted 2 February 2001 )

Abstract
We present one-plane lens models (with and without an external shear added) and a two-plane lens model for the gravitationally lensed system Q 2237+0305. The first (the main) lens plane is at z=0.0394 and the second lens plane is at z=0.5664. We found the best lens solutions for this system up to now - $\chi^{2}_{\rm df}=0.35$ for the external shear model and $\chi^{2}_{\rm df}=0.44$ for the two-plane model. Generally, we found a nearly singular (with an upper limit on the core radius of $\approx$36 pc) pseudoisothermal mass distribution for the central part of the main lens that is consistent with the available observations of the Q 2237+0305 system (except the misalignment of about $13\degr$ between the mass and the light distributions). Furthermore, the main source of the external shear is possibly the object connected with the MgII absorption. In any case the external perturbations should be taken into account in the future models of the gravitationally lensed system Q 2237+0305. For the two-plane lens model the blue mass-to-light ratio for the central $1\arcsec$ of the SBb galaxy-lens is estimated to be $6.8^{ \scriptscriptstyle +0.3}_{ \scriptscriptstyle -0.1} {\cal M}_{\odot}/{\cal L}_{\odot,B}$, and the mass of the SBb galaxy inside the mean ring of the images is ${\cal M}(\lid0\farcs885)=1.471^{ \scriptscriptstyle +0.069}_{ \scriptscriptstyle -0.010} 10^{10} {\cal M}_{\odot}$. The introduction of the second lens plane leaves the core radius, the axis ratio and the position angle of the mass distribution in the main lens almost unchanged but decreases the mass inside the mean ring of the images with 1.2% and increases the total magnification of the images by a factor of $\approx$1.5. The parameters of the mass distribution in the second lens are not very well constrained. If the second lens is a single galaxy we set a lower limit on the radius of the MgII absorbing halo of $\approx$14 kpc. The probability for the two-plane lens model is estimated to be 8.0 10-9.


Key words: quasars: absorption lines -- quasars: individual: Q 2237+0305 -- gravitational lensing

Offprint request: B. M. Mihov, bmihov@astro.bas.bg

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© ESO 2001


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