Weighing a galaxy bar in the lens Q2237 + 0305

In the gravitational lens system Q2237+0305 the cruciform quasar image geometry is twisted by 10° by the lens effect of a bar in the lensing galaxy. This effect can be used to measure the mass of the bar. We construct a new lensing model for this system with a power-law elliptical bulge and a Ferrers bar. The observed ellipticity of the optical isophotes of the galaxy leads to a nearly isothermal elliptical profile for the bulge, with a total quasar magnification of 16+5−4. We measure a bar mass of (7.5 ± 1.5) × 108h−175 M⊙ in the region inside the quasar images.

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