Massive Central Objects Detected in Galaxies

**Massive Central Objects Detected in Galaxies**
Galaxy	Const	Type	Distance	Luminosity	Mass	DT	Comment	Sources
Milky Way	[Sgr]	Sbc	0.028	1.9	2	S	Sgr A, Galactic Center	Genzel et al. 1997; Ghez et al. 1998
M31	And	Sb	2.9	5.2	30	S	Andromeda Galaxy	Dressler/Richstone 1988; Kormendy 1988b
M32	And	E2	2.9	0.25	3	S	Satellite of M31	Tonry 1984, 1987; Dressler/Richstone 1988
NGC 3115	Sex	SO	27	14.2	2	S		Kormendy & Richstone 1992
M106	CVn	Sbc	24	1.3	40	M		Miyoshi et al. 1995
NGC 4261	Vir	E2	90	33	400	G		Ferrarese et al. 1996
M87	Vir	E0	60	56	3000	G		Harms et al. 1994
M104	Vir	Sa	50	47	1000	S	Sombrero Galaxy	Kormendy 1988c
NGC 3377	Leo	E5	38	5.2	100	S		Kormendy et al. 1998; Richstone et al. 1999
M105	Leo	E1	38	13	100	S		Gebhardt et al. 1999
NGC 4486B	Vir	E0	60	0.82	500	S	Satellite of M87	Kormendy et al. 1997
M77	Cet	Sb	60		10	M	Caltech IR detection	Greenhill et al. 1996, NRAO PR Jan 15/2000
M84	Vir	S0	60	56	300	G	HST STIS (May 1997)	Bower et al. 1998
NGC 4342	Vir	S0	60		50	S		Cretton/van den Bosch 1999
NGC 7052	Vul	E4	191		300	G		van der Marel/van den Bosch 1998
NGC 6251	UMi	E2	346		600	G		Ferrarese & Ford 1999
NGC 4945	Cen	Scd	15		1	M		Greenhill et al. 1997

Key:

Galaxy: Identification/Catalog number of galaxy
Const: Constellation
Type: Hubble type
Distance: Distance in million light years (of massive central object)
Luminosity: Galaxy luminosity in billion solar luminosities; bulge only for spirals
Mass: Central Object's Mass in million solar masses
DT: Detection Type (S: Stellar dynamics, G: Gas dynamics, M: Maser sources)
Comments: Common name of galaxy or other comment
Source: Discovery papers

This table summarizes data for some of the well-established massive central 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 central 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 proportional 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 and John Kormendy's CENSUS of Black Hole Candidates in galactic nuclei (more technical version).

Hartmut Frommert
Christine Kronberg
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Last Modification: February 3, 2000