M 87 is one of the nearest radio galaxies with a prominent jet extending from sub-pc to kpc-scales .
Because of its proximity and large mass of the central black hole , it is one of the best radio sources to study jet formation .
We aim at studying the physical conditions near the jet base at projected separations from the BH of \sim 7 - 100 Schwarzschild radii ( R _ { sch } ) .
Global mm-VLBI Array ( GMVA ) observations at 86 GHz ( \lambda = 3.5 mm ) provide an angular resolution of \sim 50 \mu as , which corresponds to a spatial resolution of only 7 ~ { } R _ { sch } and reach the small spatial scale .
We use five GMVA data sets of M 87 obtained during 2004–2015 and present new high angular resolution VLBI maps at 86 GHz .
In particular , we focus on the analysis of the brightness temperature , the jet ridge lines , and the jet to counter-jet ratio .
The imaging reveals a parabolically expanding limb-brightened jet which emanates from a resolved VLBI core of \sim ( 8 - 13 ) R _ { sch } size .
The observed brightness temperature of the core at any epoch is \sim ( 1 - 3 ) \times 10 ^ { 10 } K , which is below the equipartition brightness temperature and suggests magnetic energy dominance at the jet base .
We estimate the diameter of the jet at its base to be \sim 5 R _ { sch } assuming a self-similar jet structure .
This suggests that the sheath of the jet may be anchored in the very inner portion of the accretion disk .
The image stacking reveals faint emission at the center of the edge-brightened jet on sub-pc scales .
We discuss its physical implication within the context of the spine-sheath structure of the jet .