Joseph Cartagena, the rapper known as Fat Joe, has turned himself in at the Federal Detention Center in Miami to begin his four-month sentence for tax evasion. TMZ confirmed the news with prison officials Monday. Cartagena pleaded guilty in December to neglecting to pay more than $1 million in taxes for the years 2007 and 2008. In addition to the prison sentence, he was also ordered to pay a $15,000 fine and had to agree to be supervised for a year, TMZ said.