A spectacular poststarburst quasar

We report the discovery of a spectacular "poststarburst quasar" UN J1025-00 40 (B = 19; z = 0.634). The optical spectrum is a chimera, displaying the b road Mg II lambda 2800 emission line and strong blue continuum characterist ic of quasars, but is dominated in the red by a large Balmer jump and promi nent high-order Balmer absorption lines indicative of a substantial young s tellar population at similar redshift. Stellar synthesis population models show that the stellar component is consistent with a 400 Myr old instantane ous starburst with a mass of less than or similar to 10(11) M.. A deep, K-s -band image taken in similar to 0." 5 seeing shows a point source surrounde d by asymmetric extended fuzz, Approximately 70% of the light is unresolved , the majority of which is expected to be emitted by the starburst. While s tarbursts and galaxy interactions have been previously associated with quas ars, no quasar ever before has been seen with such an extremely luminous yo ung stellar population.