Sagittarius A* (Sgr A*) is the radio source associated with the dynamical center of the Galaxy. It is quite bright in absolute terms, but relative to the Eddington luminosity, roughly the maximum luminosity possible from an accretion powered object, it is many orders of magnitude underluminous. That is, our supermassive black hole is something of an underachiever! It is also a light weight, weighing in at around 4 million solar masses (compared to more typical supermassive black hole masses of a billion solar masses). Nevertheless, it provides the first opportunity we have to image the black hole with a resolution sufficient to resolve its event horizon.
This would be done with a global network of sub-millimeter radio telescopes, phased together into a large array to form a telescope whose effective size is the entire earth. This is similar to the existing Very Long Baseline Array which operates at wavelengths above 3 mm. Significant progress has already been made on this, with a recent observation by Doeleman and collaborators actually resolving Sgr A* on a Hawaii-Arizona baseline.
Here you can find some figures and movies of theoretical images of the supermassive black hole in the center of the Milky Way, Sgr A*. That is, you can see some pictures of what we might expect to find as this new observational capability is developed further!
The observed luminosity of Sgr A* is presumably produce by the liberation of gravitational potential energy as gas is captured and falls into the black hole. Here we show a few images of the Radiatively Inefficient Accretion Flow (RIAF), presumed to exist around Sgr A*.
|
| An image of a RIAF around a non-spinning (Schwarzschild) black hole viewed from an inclination of 10° . To show the extreme lensing, the coordinate grid in the equatorial plane are shown by the green grid (radius and azimuthal angle). Note how the strong lensing around the black hole bends the back of the disk up and into view. (Click image for hi-res version.) |
|
| An image of a RIAF around a moderately spinning black hole viewed from an inclination of 10° . Note how the spin of the black hole twists the coordinates around it. (Click image for hi-res version.) |
|
| An image of a RIAF around a moderately spinning black hole viewed from an inclination of 5° . (Click image for hi-res version.) |
|
| An image of a RIAF around a rapidly spinning black hole viewed from an inclination of 10° . (Click image for hi-res version.) |
|
| An image of a RIAF around a maximally spinning black hole viewed from an inclination of 10° . (Click image for hi-res version.) |
Sgr A* is strongly variable, showing enhancements in its infrared luminosity by orders of magnitude on timescales comparable to the light-crossing time of the horizon. These are inferred to be the result of stochastic dissipative events within the accretion flow, such as strong shocks or magnetic reconnection events (like those that produce solar flares on the Sun). Such events will also necessarily remain strongly localized, producing hot spots in the accretion flow. These hot spots can be used to learn about General Relativity in much the same way the orbits of the planets shed light upon gravity in the Newtonian regime. Ultimately, we should be able to verify if General Relativity properly describes gravity around black holes!
| Hot spot orbiting a non-rotating black hole. This shows the hot spot from a variety of inclinations, first zooming in and then panning from along the orbital axis to edge on in the orbital plane. Note the presence of a secondary, and sometimes tertiary, image. These are produced by light-rays that spend more time near the horizon, and thus encoded in them is information about strong gravity. (Right-click image to download high-res version (17M!), which can be viewed in QuickTime.) |
| Hot spot orbiting a rapidly-rotating black hole. This shows the hot spot from a variety of inclinations, first zooming in and then panning from along the orbital axis to edge on in the orbital plane. In this case the secondary image persists throughout the entire orbit. This is a consequence of two things: the twisting up of space around the black hole due to its rotation and the much smaller inner-most stable circular orbit. (Right-click image to download high-res version (15M!), which can be viewed in QuickTime.) |
| Hot spot orbiting a non-rotating black hole. This shows the hot spot from an inclination of 45° with polarization tickmarks. Note that the polarization tickmarks in the higher-order images are generally not aligned with the primary, resulting in a net depolarization of the source. (Right-click image to download high-res version (2.7M!), which can be viewed in QuickTime.) |
| Hot spot orbiting a rapidly-rotating black hole. This time the hot-spot orbit is the at the same coordinate distance as for the non-rotating black hole. (Right-click image to download high-res version (2.2M!), which can be viewed in QuickTime.) |
| Hot spot orbiting a non-rotating black hole in an accretion flow at 7mm. The opacity of the accretion flow hides small spots at long wavelengths, highlighting the importance of moving to the sub-millimeter. However, for hot spots on large orbits these can be seen, in principle, at long wavelengths as well. (Right-click image to download high-res version (4.9M!), which can be viewed in QuickTime.) |
| Hot spot orbiting a non-rotating black hole in an accretion flow at 7mm. In this case the hot spot is bigger, corresponding to roughly the hot-spot luminosities observed at 7 mm. (Right-click image to download high-res version (5.3M!), which can be viewed in QuickTime.) |
Recent radio & infrared observations have now made it all but impossible for Sgr A* to not have an event horizon. This represents the first conclusive test of General Relativity's most shocking result, the existence of compact horizons, regions of spacetime that pinch off from the exterior universe.