<P> Some of the best evidence for the presence of black holes is provided by the Doppler effect whereby light from nearby orbiting matter is red - shifted when receding and blue - shifted when advancing . For matter very close to a black hole the orbital speed must be comparable with the speed of light, so receding matter will appear very faint compared with advancing matter, which means that systems with intrinsically symmetric discs and rings will acquire a highly asymmetric visual appearance . This effect has been allowed for in modern computer generated images such as the example presented here, based on a plausible model for the supermassive black hole in Sgr A * at the centre of our own galaxy . However the resolution provided by presently available telescope technology is still insufficient to confirm such predictions directly . </P> <P> What already has been observed directly in many systems are the lower non-relativistic velocities of matter orbiting further out from what are presumed to be black holes . Direct Doppler measures of water masers surrounding the nuclei of nearby galaxies have revealed a very fast Keplerian motion, only possible with a high concentration of matter in the center . Currently, the only known objects that can pack enough matter in such a small space are black holes, or things that will evolve into black holes within astrophysically short timescales . For active galaxies farther away, the width of broad spectral lines can be used to probe the gas orbiting near the event horizon . The technique of reverberation mapping uses variability of these lines to measure the mass and perhaps the spin of the black hole that powers active galaxies . </P> <P> Gravitation from supermassive black holes in the center of many galaxies is thought to power active objects such as Seyfert galaxies and quasars . </P> <P> An empirical correlation between the size of supermassive black holes and the stellar velocity dispersion σ (\ displaystyle \ sigma) of a galaxy bulge is called the M - sigma relation . </P>

Supermassive black holes at the center of galaxies