Unique probe of dark matter in the core of M87 with the Event Horizon
  Telescope
  We demonstrate the unprecedented capabilities of the Event Horizon Telescope
(EHT) to image the innermost dark matter profile in the vicinity of the
supermassive black hole at the center of the M87 radio galaxy. We present the
first model of the synchrotron emission induced by dark matter annihilations
from a spiky profile in the close vicinity of a supermassive black hole,
accounting for strong gravitational lensing effects. Our results show that the
EHT should readily resolve dark matter spikes if present. Moreover, the photon
ring surrounding the silhouette of the black hole is clearly visible in the
spike emission, which introduces observable small-scale structure into the
signal. We find that the dark matter-induced emission provides an adequate fit
to the existing EHT data, implying that in addition to the jet, a dark matter
spike may account for a sizable portion of the millimeter emission from the
innermost (subparsec) region of M87. Regardless, our results show that the EHT
can probe very weakly annihilating dark matter. Current EHT observations
already constrain very small cross sections, typically down to a few 10^{-31}
cm^{3} s^{-1} for a 10 GeV candidate, close to characteristic values for
p-wave-suppressed annihilation. Future EHT observations will further improve
constraints on the DM scenario.
