Massive Dark Objects Detected in Galaxies

**Massive Dark Objects Detected in Galaxies**
Galaxy	Const	Type	Distance	Luminosity	Mass	Comment
Milky Way	[Sgr]	Sbc	0.028	1.9	2	Sgr A, Galactic Center
M31	And	Sb	2.9	5.2	30	Andromeda Galaxy
M32	And	E2	2.9	0.25	3	Satellite of M31
NGC 3115	Sex	SO	27	14.2	2
M 106	CVn	Sbc	24	1.3	40	Maser Detection
NGC 4261	Vir	E2	90	33	400
M87	Vir	E0	60	56	3000
M104	Vir	Sa	50	47	1000	Sombrero Galaxy
NGC 3377	Leo	E5	38	5.2	100
M105	Leo	E1	38	13	50
NGC 4486B	Vir	E0	60	0.82	500	Satellite of M87
M77	Cet	Sb	60			Caltech IR detection
M84	Vir	S0	60	56	300	HST STIS (May 1997)

Key:

Galaxy: Identification/Catalog number of galaxy
Const: Constellation
Type: Hubble type
Distance: Distance in million light years (of massive dark object)
Luminosity: Galaxy luminosity in billion solar luminosities; bulge only for spirals
Mass: Dark Object Mass in million solar masses
Comments: Common name of galaxy or other comment

This table summarizes data for some of the well-established massive dark objects in the nuclei of galaxies. The masses of these objects are usually estimated from the orbital velocity of stars or gas orbiting the objects at distances of some tens or hundreds of light years, which shows that enormous masses are concentrated in small volumes. Many of these central objects exhibit other signs of violent gravitational action: Accretion disks, emission of strong radio and/or high-energy radiation, or jets of highly accelerated material.

What may be the nature of these mysterious objects ? Many astronomers take it for granted that these objects are supermassive black holes, but none of them is really resolved. To get truely compelling evidence, or better, to reveal the true nature of the massive dark objects, a resolution would be required of the order of the Schwarzschild radius (the "horizon" radius of a black hole acording to Einstein's General Relativity theory). The Schwarzschild radius is simply correlated to the mass of an object, as it is defined as that radius where the escape velocity from the mass equals light velocity; this means that it is propoertional to the mass. The formal relation can be given as

R_S = 2.94 km * M [solar masses] Even for the most massive central object (in M87), this yields a value of R_S = 9 billion km, little more than the mean distance of Pluto to the sun and only about 1/1000 of a light year, while the Hubble Space Telescope could only resolve this object to a few tens of light years. So currently, any evidence is indirect at best. It is particularly exaggerated to take our current knowledge of these objects as a further evidence for the validity of Einstein's General Relativity, as was occasionally claimed.

The table above is partly based on STScI-PR97-01.

Hartmut Frommert (spider@seds.org)
Christine Kronberg (smil@lrz.uni-muenchen.de)

Last Modification: 14 Feb 1998, 17:30 MET