The Event Horizon of M 87

The 6× 10M⊙ supermassive black hole at the center of the giant elliptica l galaxy M87 powers a relativistic jet. Observations at millimeter wavelengths with the Event Horizon Telescope have localized the emission from the base of this jet to angular scales comparable to the putat ive black hole horizon. The jet might be powered directly by an accretion disk or by electromagnetic extract ion of the rotational energy of the black hole. However, even the latter mechanism requires a confining thick accreti on disk to maintain the required magnetic flux near the black hole. Therefore, regardless of the jet mechanism, the obs rved jet power in M87 implies a certain minimum mass accretion rate. If the central compact object in M87 wer e not a black hole but had a surface, this accretion would result in considerable thermal near-infrared and opt ical emission from the surface. Current flux limits on the nucleus of M87 strongly constrain any such surface emiss ion. This rules out the presence of a surface and thereby provides indirect evidence for an event horizon. Subject headings: black hole physics – galaxies: individual (M87) – gravitati on – radio continuum: galaxies – infrared: galaxies – ultraviolet: galaxies