Edward Forst, who co-heads Goldman Sachs' asset management unit, will retire, according to an internal memo sent Friday to employees.   He will be succeeded by Eric Lane, who will share responsibilities for running the division with Tim O’Neill, the other current co-head. Mr. Forst, 51, is the highest-ranking executive to leave the firm in recent months.  He's been a member of the firm’s influential management committee.  Mr. Forst has deep roots at Goldman, having joined the firm in 1994.  He's run a number of key divisions, including capital markets.  In 2008, he left Goldman for what turned out to be a year, to manage the Harvard University’s endowment.  By the following year, he surprised many by returning to Goldman as a senior strategy officer.  In 2010, Mr. Forst took over asset management, replacing Marc Spilker, who left the firm. As we reported last month, Goldman has seen an unusually large number of senior people announce their retirements.  In part, the departures are tied to the firm's cost cutting measures, though it's not known whether Mr. Forst’s departure was related to this effort. For further details, and to access the internal memos, go to:  [Dealbook 12/2/11].