The head of the Federal Reserve Bank of Boston yesterday countered growing criticism of the Fed’s purchase of $600 billion in long-term Treasury bonds, saying it’s needed to combat deflation and high unemployment. Republican congressional leaders and other critics are blasting the Fed’s move as nothing more than printing money to buy up bonds, effectively underwriting government spending and risking inflation. But Eric Rosengren, president of the Fed’s bank in Boston, insisted that the Federal Reserve has a dual role of both maintaining the stability of prices and keeping unemployment as low as possible.