The Shadow of a Black Hole

Slide 13 of 16 If we are so close to resolving the black hole already, what can we expect in the future? We still do not know whether we are really dealing with a black hole or something even more exotic. The defining feature of a black hole is its event horizon - the final barrier between our world and the hole. Even light which comes too close to the event horizon will be attracted by the black hole and disappear into the event horizon forever. This leads to an unusual optical appearance of any emission in the very vicinity around a black hole. Below we show some calculations obtained with a ray-tracing code for an optically thin emissioin region which exhibit an interesting feature: literally a "black hole" in the middle. This is what we call the shadow cast by the black hole - the photons (light) we do not see have vanished into the event horizon.
(Figure: Various models for the emission region around Sagittarius A*. Despite very different assumptions the shadow is very obvious and always of roughly the same size - from Falcke, Melia, & Agol 2000.)
Slide 13 of 16

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Contact: Heino Falcke

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