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A recent article on high-resolution 86 GHz observations with the GlobalMillimeter VLBI Array, the phased Atacama Large Millimeter/submillimeter Array,and the Greenland Telescope describes the detection of a limb-brightenedcylindrical jet, $25 \mu\rm{as}< z< 100 \mu\rm{as}$, where $z$ is the axialdisplacement from the supermassive black hole in the sky plane. It was shown tobe much wider and much more collimated than 2D simulations of electromagnetic(Blandford-Znajek) jets from the event horizon predicted. This was anunanticipated discovery. The claimed detection of a jet connected to theaccretion flow provides a direct observational constraint on the geometry andphysics of the jet launching region for the first time in any black hole jettedsystem. This landmark detection warrants further analysis. This Letter focuseson the most rudimentary properties, the shape and size of the source of thedetected jet emission, the determination of which is not trivial due toline-of-sight effects. Simple thick-walled cylindrical shell models for thesource were analyzed to constrain the thickness of the jet wall. The analysisindicates a tubular jet source with a radius $R\approx 144 \mu\rm{as}\approx38M$ and that the tubular jet walls have a width $W \approx 36\mu\rm{as}\approx 9.5 M$, where $M$ is the geometrized mass of the black hole (a volumecomparable to that of the interior cavity). The observed cylindrical jetconnects continuously to the highly limb-brightened jet (previously describedas a thick-walled tubular jet) that extends to $z> 0.65$ mas, and the two arelikely in fact the same outflow (i.e., from the same central engine).

The cylindrical jet base of M87 within 100$\mu\rm{as}$ of the central engine