From: yee@trident.arc.nasa.gov (Peter E. Yee)
Subject: NASA's Hubble finds new evidence for massive black holes (Forwarded)
Apparently-To: sci-space-news@ames.arc.nasa.gov
Followup-To: sci.space
Organization: NASA Ames Research Center, Moffett Field, CA
Date: Wed, 8 Apr 1992 18:08:34 GMT

Paula Cleggett-Haleim
Headquarters, Washington, D.C.                  April 8, 1992
(Phone:  202/453-1547)

Ray Villard
Space Telescope Science Institute, Baltimore, Md.
(Phone:  410/338-4514)

Dr. Tod Lauer
National Optical Astronomy Observatories, Tuscon, Ariz.
(Phone:  602/325-9290)

Dr. Sandra Faber
University of California, Santa Cruz
(Phone:  408/459-2944)

RELEASE:  92-46

NASA'S HUBBLE FINDS NEW EVIDENCE FOR MASSIVE BLACK HOLES


	Astronomers report that they have found new evidence 
that a black hole, weighing 3 million times the mass of the 
Sun, exists at the center of the nearby elliptical galaxy 
M32, based on images obtained with NASA's Hubble Space 
Telescope (HST).  The images show that the stars in M32 
become extremely concentrated toward the nucleus.

	This central structure resembles the gravitational 
"signature" of a massive black hole.  The presence of a black 
hole in an ordinary galaxy like M32 may mean that inactive 
black holes are common to the centers of galaxies.  The new 
HST images show that M32 is an interesting "laboratory" for 
testing theories of the formation of massive black holes.

	This result is based on image analysis conducted by Dr. 
Tod R. Lauer of the National Optical Astronomy Observatories, 
Tuscon, Ariz., Dr. Sandra M. Faber of the University of 
California, Santa Cruz, and other members of the  HST Wide 
Field/Planetary Camera (WFPC) Imaging Team.

	M32 is quite small and compact as elliptical galaxies 
go, containing about 400 million stars within a diameter of 
only 1,000 light-years.  At a distance of 2.3 million light-
years, M32 (the 32nd object in a catalog of non-stellar 
objects compiled by French astronomer Charles Messier in 
1774) is one of the closest neighbors to the Milky Way 
galaxy.

	M32 is a satellite of the great spiral galaxy in 
Andromeda M31, which dominates the small group of galaxies of 
which the Milky Way is a member.  M31 can be seen with the 
naked eye as a spindle-shaped "cloud" the width of the full 
moon, and its small companion M32 can be seen with a small 
telescope.

	M32 has been among the best candidates for a galaxy 
with a massive central black hole.  This was first proposed 
in 1987 by Dr. John L. Tonry of the Massachusetts Institute 
of Technology, Cambridge, and independently by Dr. Alan 
Dressler of the Observatories of the Carnegie Institution, 
Wash., and by Dr. Douglas O. Richstone of the University of 
Michigan, Ann Arbor.

	Their observations, made with ground-based telescopes, 
showed an abrupt increase in the orbital velocities of stars 
towards the center of  M32.  This data led the astronomers to 
conclude that M32 must have a strong but unseen concentration 
of mass at its center.  A black hole at least several million 
times the mass of the Sun is the most likely type of object 
matching these characteristics.

	Ground-based images, however, do not have enough 
resolution to detect the effects of a massive black hole on 
the structure of M32.  The HST images analyzed by Lauer, 
Faber and co-investigators on the WFPC imaging team now show 
the nucleus of M32 in clear detail. 

	They find that the density of stars in the nucleus of 
M32 appears to increase steadily towards the center, with no 
sign of leveling off.  These results are very similar to the 
predictions for what a massive black hole should do to the 
central structure of a galaxy.

	"This is the densest stellar system known to 
astronomers," says Lauer.  "The density of stars at the 
center of M32 may be over 100 million times greater than the 
distribution of stars in the neighborhood of the Sun.  A 
visitor to a planet at the center of M32 would see a starry 
night sky so saturated with stars that their combined light 
would be brighter than 100 full moons.  The night would never 
get darker than mid-twilight on the Earth, and one could even 
read a newspaper by starlight."

	To date, HST has uncovered the gravitational signature 
of a black hole in one other galaxy, the giant elliptical 
galaxy called M87.  Both M32 and M87 have a distinctive 
central concentration of starlight called a "cusp."  These 
two galaxies are quite different, though, in that the black 
hole proposed for M32 is roughly 1,000 times smaller than the 
black hole that might exist at the heart of M87.

	Although M32 is about 20 times closer to the Earth than 
M87, its much smaller black hole means that the brightness 
cusp also is much smaller and is close to HST's resolution 
limits.  Unlike M87, M32 lacks any form of nuclear activity, 
which means that at present the black hole would not be 
accreting significant amounts of matter.  This result also 
raises the possibility that small inactive black holes are 
common to the centers of galaxies.

	Because the region dominated by the black hole is so 
small, Lauer and Faber also considered the possibility that 
no black hole is present.  Instead, the star density might 
level off just beyond the resolution limits of HST.  If this 
were the case for M32, it would force the researchers to 
conclude that the center of M32 is unstable and vulnerable to 
collapse.

	A black hole at the center of M32 would have the 
paradoxical effect of stabilizing the galaxy's nucleus.  
That's because the stars orbit so rapidly around the black 
hole, they move past each other too quickly to 
gravitationally capture each other or collide.  The black 
hole thus keeps the center of a galaxy "stirred up."

	In the absence of a black hole, however, the stars 
would move slowly enough to attract each other 
gravitationally.  Collisions between stars become much more 
frequent, and heavier slower moving stars sink to the center 
of the galaxy causing it to collapse.  

	The fate of the collapsing core is uncertain.  One 
possibility is that binary stars formed during the collapse 
would provide enough kinetic energy to halt the collapse by 
transferring momentum to single stars.  This would make the 
core rebound, like a rubber ball that has been squeezed and 
then relaxed. 

	An alternative possibility is that runaway merging of 
stars would occur during core collapse, leading to the 
formation of a black hole in any case.  If so, this would 
rule out alternative explanations that don't require a black 
hole.

	If the core is really unstable, the researchers would 
expect to find evidence of merged and captured stars called 
"blue stragglers'' (HST has in fact uncovered such stars at 
the core of a globular cluster, a much smaller aggregate of 
stars than M32).  The shape of the starlight distribution at 
the core would also be different from that which HST detects.

	The Hubble images instead show that the population of 
stars in the nucleus is the same as that further out in the 
galaxy, and that the shape of M32 remains constant into the 
center.  This means that a core collapse has not recently 
occurred.

	At the present time, astrophysical theories are not 
sophisticated enough to say whether or not M32 would have to 
evolve to make a central black hole, but do raise this as an 
intriguing possibility.  The new HST observations thus 
identify M32 as an interesting "laboratory" where astronomers 
can test theories of massive black hole formation.

	The search for super massive black holes in the cores 
of galaxies is one of the primary missions of NASA's Hubble 
Space Telescope.  By investigating both active and quiescent 
galaxies, astronomers will have a better idea of the 
conditions and events which lead to the formation and growth 
of super-massive black holes.

	The Space Telescope Science Institute is operated by 
the Association of Universities for Research in Astronomy, 
Inc., for NASA, under contract with the Goddard Space Flight 
Center, Greenbelt, Md.  The Hubble Space Telescope is a 
project of international cooperation between NASA and the 
European Space Agency.
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