A companion galaxy to the poststarburst quasar UN J1025-0040

UN J1025-0040 is a quasar at z = 0.6344 that shows an extremely bright post starburst population of age similar to 400 Myr (Brotherton et al.). Images of UN J1025-0040 show a nearly stellar object 4 ".2 south-southwest of the quasar. We present imaging and spectroscopy that confirm that this object is a companion galaxy at redshift z = 0.6341. We estimate an age of similar to 800 Myr for the dominant stellar population in the companion. The compa nion appears to be interacting with the quasar host galaxy, and this intera ction may have triggered both the starburst and the quasar activity in UN J 1025-0040.