The Federal Reserve Bank of New York said Thursday it has created an arm to oversee the parts of its balance sheet acquired in efforts to bail out failing Wall Street firms. In a press release, the bank said this new group would be called the Special Investment Management Group, and it promoted Sarah Dahlgren to executive vice president to lead the group. Dahlgren, a long-time staffer, was previously in charge of the New York Fed’s relationship with American International Group Inc. (AIG), the troubled insurer that was…