Mid-Infrared Imaging of the Einstein Cross QSO

Observations of the Einstein Cross QSO reveal that the mid-infrared region is unaffected by microlensing. Thus, the infrared emission region must be extended on a scale >10¹⁷cm, ruling out syn-chrotron models, but consistent with dust emission models. Other constraints rule out starburst models. The flux ratios greatly constrain the lens model.