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 -
for the
external shear model and
for the two-plane model.
Generally, we found a nearly singular (with an upper limit on the core radius
of 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 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 of the SBb galaxy-lens is
estimated to be
, and the mass of the SBb galaxy inside
the mean ring of the images is
. 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 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 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

SIMBAD Objects

**©**

*ESO 2001*