TOKYO, May 15 (Reuters) - Japan's Nikkei share index broke above 15,000 and hit fresh 5-1/2 year highs on Wednesday, bolstered by a strong performance from Wall Street and further weakness in the yen. Sony Corp was untraded with a glut of buy orders after a proposal from an activist fund to list its entertainment unit sent its U.S.-listed shares soaring 10 percent. Its shares were indicated at 1,957 yen, up 4.3 percent from the previous day's closing price. The Nikkei rose 1.8 percent to 15,028.36 after hitting as high as 15,030.26 soon after the opening bell, the highest since January 2008. The broader Topix gained 1.6 percent to 1,250.60.