We extend Holowinsky and Soundararajan's proof of quantum unique ergodicityfor holomorphic Hecke modular forms on SL(2,Z), by establishing it forautomorphic forms of cohomological type on GL_2 over an arbitrary number fieldwhich satisfy the Ramanujan bounds. In particular, we have uncondtionaltheorems over totally real and imaginary quadratic fields. In the totally realcase we show that our result implies the equidistribution of the zero divisorsof holomorphic Hecke modular forms, generalising a result of Rudnick over Q.
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