NEW YORK (Reuters) – Bernard Madoff, a long-time fixture and powerful adviser on Wall Street, was arrested and charged on Thursday with allegedly running a $50 billion Ponzi scheme, U.S. authorities said. The former chairman of the Nasdaq Stock Market who remains a member of Nasdaq OMX Group Inc's nominating committee, is best known as the founder of Bernard L. Madoff Investment Securities LLC, the closely-held market-making firm he founded in 1960. But the alleged fraud involved a hedge fund he ran from a separate floor of the building where his brokerage is based. Madoff told senior employees of his...