Event Horizon of a Supermassive Black Hole
Observations of the supermassive black hole (SMBH) at the center of the supergiant elliptical galaxy M87 made using the Event Horizon Telescope and the Hubble Space Telescope indicates the presence of an event horizon and rules out the existence of a physical surface. Indeed, a black hole does not have a physical surface. Instead, the event horizon is a non-physical boundary around a black hole where the escape velocity becomes greater than the speed of light. Anything which crosses the event horizon, including light, is no longer visible to outside observers.
Accretion of material onto compact objects with physical surfaces such as white dwarfs and neutron stars lead to very different observational outcomes. As accreted material violently crashes down onto a physical surface, the remaining kinetic energy of the accreted mass is thermalized and radiated away as radiation, generating a considerable flux of infrared and optical emission.
The SMBH at the heart of M87 has an estimated ~6 billion times the Sun’s mass and it is accreting material at a prodigious rate. Measurements of the flux from the SMBH show it is at least an order of magnitude below what is expected if a physical surface were present. This indicates that the central compact object in M87 is a supermassive black hole with an event horizon and not some very massive exotic object with a physical surface. Once accreted material falls through the event horizon of a black hole, it is no longer visible to the outside universe.
Broderick et al. (2015), “The Event Horizon of M87”, arXiv:1503.03873 [astro-ph.HE]