Supermassive black hole
Outside the Milky Way

Unambiguous dynamical evidence for supermassive black holes exists only for a handful of galaxies; these include the Milky Way, the Local Group galaxies M31 and M32, and a few galaxies beyond the Local Group, such as NGC 4395. In these galaxies, the root mean square (or rms) velocities of the stars or gas rises proportionally to 1/ r near the center, indicating a central point mass. In all other galaxies observed to date, the rms velocities are flat, or even falling, toward the center, making it impossible to state with certainty that a supermassive black hole is present.

Nevertheless, it is commonly accepted that the center of nearly every galaxy contains a supermassive black hole. The reason for this assumption is the M–sigma relation, a tight (low scatter) relation between the mass of the hole in the 10 or so galaxies with secure detections, and the velocity dispersion of the stars in the bulges of those galaxies. This correlation, although based on just a handful of galaxies, suggests to many astronomers a strong connection between the formation of the black hole and the galaxy itself.

On March 28, 2011, a supermassive black hole was seen tearing a mid-size star apart. That is the only likely explanation of the observations that day of sudden X-ray radiation and the follow-up broad-band observations. The source was previously an inactive galactic nucleus, and from study of the outburst the galactic nucleus is estimated to be a SMBH with mass of the order of a million M. This rare event is assumed to be a relativistic outflow (material being emitted in a jet at a significant fraction of the speed of light) from a star tidally disrupted by the SMBH. A significant fraction of a solar mass of material is expected to have accreted onto the SMBH. Subsequent long-term observation will allow this assumption to be confirmed if the emission from the jet decays at the expected rate for mass accretion onto a SMBH.
oulipo:avoid_letter[t], language:response_language[Spanish], combination:two_responses