Most operating systems use a memory dump (os x does. Should be around the same size as the physical memory on the installed computer.) This is a method for preserving a data hierarchy (cpu registers > l2 cache > RAM > hdd [this is not a complete hierarchy of memory, but you get the idea.]) I noticed that memory dump the other day too, hadn't seen it previously, but it is a pretty common thing when dealing with operating systems. It shouldnt exist anymore when the VM is shut down (perhaps maybe if the system has been suspended, this file would probably contain the contents of the VM RAM at that time.)
