Ultraviolet images of the gravitationally lensed quadruple quasar Q2237+0305 with the HST WFPC2

We present and analyse observations of the quadruple lensed quasar Q2237+0305, obtained with the HST WFPC2 camera in the F336W and F300W filters. 25 exposures were performed within 15 h real time on 1995 November 3. On a time-scale of 3-4 h, we observe no variation in component A greater than 0.02 mag. The other components remain constant over a period of 10h to within about 0.05 mag. In the final 5h there is some evidence (not conclusive) for variation of component D by about 0.1 mag. The exposures indicate that component A is brighter than component B by about 0.3 mag. Components C and D are fainter than component A by about 1.3 and 1.4 mag respectively. Our results place an upper limit on any fifth (central) component of 6.5 mag fainter than component A. We determine the astrometric properties of the lens system, using only the exposures of the higher resolution Planetary Camera chip. We measure the relative distances of the four components with high accuracy. Our values are systematically larger than those of other investigators (by 0.1-2.0 per cent). We discuss the reasons why we believe our results are reliable. The F336W filter had been chosen for the observations because it corresponds to the redshifted Ly alpha line of the quasar. This filter might have allowed us to see extended Ly alpha emission from the broad-line region (BLR) of the quasar as Lya: arcs, and hence to determine the physical size of the BLR. However, the quasar components seen in this filter are consistent with a point source. We conclude that there cannot be any Lya: feature in the image plane brighter than about 23.5 mag in F336W and further from the quasar core than 100 mas. According to a lensing model by Rix, Schieder & Bahcall, this would preclude any such features in the source plane further than 20 mas (similar to 100 h(-1) pc, assuming q(0) = 0.5) from the quasar core and brighter than 25 mag before magnification.