Selections from 2018: Looking for the Shadow of Our Galaxy’s Black Hole

Editor’s note: In these last two weeks of 2018, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Detection of Intrinsic Source Structure at ~3 Schwarzschild Radii with Millimeter-VLBI Observations of SAGITTARIUS A*

Published May 2018

Main takeaway:

Radio observations of the supermassive black hole at the center of the Milky Way have reached their highest resolution yet, teasing out details of the structure of Sagittarius (Sgr) A* on a scale of just ~30 microarcseconds. This corresponds to a size of only ~3 times that of the black hole’s event horizon (the distance at which not even light can escape).

Why it’s interesting:

By observing our galaxy’s ~4-million-solar-mass black hole, Sgr A*, in unprecedented detail at radio wavelengths, astronomers hope to explore the structure of a supermassive black hole’s event horizon and learn about the physical processes that occur there. The observations presented in this study, led by Ru-Sen Lu (Max Planck Institute for Radio Astronomy, Germany; MIT Haystack Observatory), represent important advancement toward that goal, and they provide an tantalizing glimpse of Sgr A* that is thus far consistent with the picture of ring-like structure that is predicted for the black hole’s event horizon.

What we can expect ahead:

Event Horizon Telescope

The participating telescopes of the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA). A subset of these telescopes were used to make the observations in the Lu et al. 2018 study presented here. [ESO/O. Furtak]

The observations of Sgr A* were made by six stations for very long baseline interferometry (VLBI) located in Hawaii, California, Arizona, and Chile. These telescopes are linked together as part of the Event Horizon Telescope, a growing project that aims to assemble a VLBI network of millimeter wavelength dishes to image the event horizon of the Milky Way’s supermassive black hole. By combining observations from geographically distributed telescopes across the globe, the Event Horizon Telescope will behave like a telescope that is effectively the size of the Earth — which should provide an unprecedented view of Sgr A*’s shadow in the near future.


Ru-Sen Lu et al 2018 ApJ 859 60. doi:10.3847/1538-4357/aabe2e