[ by Melanie Gretchen ] Goldman Sachs made official the hiring of its new PR chief, according to people familiar with the matter.  He accepted the job weeks ago, prior to the completion of background checks, according to the sources not authorized to speak on the record. Richard Siewert Jr., formerly counselor to Treasury Secretary Timothy Geithner, replace Lucas van Praag, long-time public relations chief who CEO Lloyd Blankfein recently announced is retiring.  While it is not known when he will start, Mr. Siewert will also be given the title of managing director, to report to John Rogers, Goldman chief of staff. For further details, go to [Dealbook, 3/13/12].