Gary L. Gottlieb, MD, the president of Brigham and Women's Hospital in Boston, will become chief executive of Partners HealthCare System Inc. at the end of the year. Partners' board of directors unanimously selected Gottlieb, ending five months of speculation. In September, Partners said James J. Mongan, MD, would retire at the end of 2009 and named Gottlieb and three other system executives as finalists for his job. A search committee also evaluated outside candidates.