A microlensing measurement of the size of the broad emission-line region in the lensed quasar QSO 2237+0305

We present spatially resolved spectroscopic images of the gravitationally lensed quasar QSO 2237+ 0305 taken with the GMOS integral field unit (IFU) on the Gemini North Telescope. These observations have the best spatial resolution of any IFU observations of this object to date, and include the redshifted C III] and MgII QSO broad lines. Unlike Mediavilla et al. some years ago, we find no evidence for an arc of resolved broad-line emission in either the C III] or MgII lines. We calculate the image flux ratios of both the integrated emission lines and the surrounding continua. The flux ratios of the C III] and MgII emission lines are consistent with each other but differ substantially from their corresponding continuum flux ratios and the radio/mid-infrared flux ratios previously published. We argue that the broad emission-line region must be microlensed and the C III] and MgII emission regions must be approximately the same size and co-located along the line of sight. Assuming a simple model for the broad emission-line region and the continuum region, we show that the size of the C III]/Mg II broad-line region is similar to 0.06 h(70)(1/2) pc and that of the continuum region is <= 0.02 h(70)(1/2) pc.