The matter content of the jet in M87: evidence for an electron-positron jet

Recentobservationshave allowed the geom etry and kinem atics ofthe M 87 jet to be tightly constrained.W e com bine these constraintswith historicalVery Long Baseline Interferom etry (VLBI)resultsand thetheory ofsynchrotron self-absorbed radio cores in orderto investigate the physicalpropertiesofthe jet.O urresultsstrongly suggest thejetto bedom inated by an electron-positron (pair)plasm a.Although ourconserva-tiveconstraintscannotconclusively dism issan electron-proton plasm a,theviability of thissolution isextrem ely vulnerable to furthertightening ofVLBIsurface brightness lim its.The argum entspresented,coupled with future high-resolution m ulti-frequency VLBIstudiesofthe jetcore,willbe able to (cid:12)rm ly distinguish these two possibilities.