EHT 2014

Black holes are the engines that power quasars and active galactic nuclei throughout cosmic time. The masses of black holes in nearby galaxies define clear correlations with the properties of their host galaxies. These results suggest that black holes, while a thousand times lighter than the galaxy, grow alongside their hosts during its cosmic evolution. I will discuss the growth of black holes, and the establishment of the connection between galaxies and black holes.

We discuss how polarized emission can place constraints on the properties of accretion flows and jets in the strong gravity regime for systems like SgrA* and M87 being observed by the Event Horizon Telescope.

Over the last 20 years, advances in high angular resolution imaging technology has enabled the motions of individual stars to be tracked at the Galactic Center. This has provided the best evidence to date not only for the presence of a supermassive black hole at the center of our Galaxy, but for the existence of black holes in general. These high resolution measurements have also revealed an environment surrounding the black hole that is quite unexpected in a number of ways, challenging our understanding of the physical processes between black holes and their surround stars and gas.

Sgr A* is one of the few black holes whose capture radius is just resolvable with the <0.5” spatial resolution of the Chandra X-ray Observatory. Motivated by this potential we proposed for the deepest ever view of our Galactic center in 2012, via a Chandra-HETGS “X-ray Visionary Project”. Involving over 60 members of the Galactic center community, we obtained ~35 days of observations within a single year, for the first time also using the spectral gratings.

Long wavelength measurements provide sensitive probes of the intrinsic structure of Sgr A* and of the scattering properties of the line-of-sight interstellar medium. At this wavelength, scattering dominates the apparent size of the source but careful closure amplitude techniques can provide highly accurate structural information. We present new results from the VLBA at 7mm wavelength that for the first time reveal two-dimensional intrinsic structure while also demonstrating the stability of the intrinsic size during periods of significant activity at NIR and X-ray wavelengths.

I summarize several physical processes that can produce efficient particle acceleration in radiatively inefficient accretion flows. I then describe the implications for non-thermal emission and EHT observations of Sgr A*.