Invited Presentation

First M87 Event Horizon Telescope Results: The Shadow of the Supermassive Black Hole

Author(s): Geoff Bower (ASIAA)

Presenter: Geoffrey Bower (ASIAA)

When surrounded by a transparent emission region, black holes are expected
to reveal a dark shadow caused by gravitational light bending and photon
capture at the event horizon. To image and study this phenomenon, we have
assembled the Event Horizon Telescope (EHT), a global very long baseline
interferometry (VLBI) array observing at a wavelength of 1.3 mm. This allows
us to reconstruct event-horizon-scale images of the supermassive black hole
candidate in the center of the giant elliptical galaxy M87. We have resolved
the central compact radio source as an asymmetric bright emission ring with
a diameter of 42 +/- 3 microarcseconds, which is circular and
encompasses a central depression in brightness with a flux ratio >~ 10:1.
The emission ring is recovered using different calibration and imaging
schemes, with its diameter and width remaining stable over four different
observations carried out in different days. Overall, the observed image
is consistent with expectations for the shadow of a Kerr black hole as
predicted by general relativity. We compare
our images to an extensive library of ray-traced general-relativistic
magnetohydrodynamic simulations of black holes and derive a central mass
of M=(6.5 +/- 0.7) x 10^9 M_Sun. Our radio-wave observations thus provide
powerful evidence for the presence of supermassive black holes in centers
of galaxies and as the central engines of active galactic nuclei. They also
present a new tool to explore gravity in its most extreme limit and on
a mass scale that was so far not accessible. I will also discuss future prospects for
EHT imaging, including the Galactic Center black hole, Sagittarius A*.