Paula Cleggett-Haleim
Headquarters, Washington, D.C.                                                        
July 19, 1993
(Phone:  202/358-0883)                                                
EMBARGOED Until July 20

Jim Elliott
Goddard Space Flight Center, Greenbelt, Md.
(Phone:  301/286-6256)

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

RELEASE:  93-133

HUBBLE FINDS A DOUBLE NUCLEUS IN THE ANDROMEDA GALAXY

 A team of astronomers using NASA's Hubble Space Telescope (HST) 
has discovered a "double nucleus" in the center of the neighboring 
spiral galaxy M31, located in the constellation Andromeda.

 A nucleus is a dense clustering of stars at the very center of a 
galaxy. 

 "Hubble shows that the M31 nucleus is much more complex than 
previously thought," said Dr. Tod R. Lauer of the National Optical 
Astronomy Observatories, Tuscon, Ariz. 

 The astronomers report that the brighter member of the double 
nucleus might be the remnants of another galaxy cannibalized by 
M31.  They say that an alternative possibility is that dust might 
dim the core to create the illusion of a pair of separate star 
clusters.  

 The Hubble images intensify the mystery of what's happening in 
the center of this galaxy," said Lauer.  "Neither interpretation 
offers a complete explanation of the M31 nuclear structure."

 The double nucleus discovery is based on image analysis conducted 
by Lauer, Dr. Sandra M. Faber of the University of California, 
Santa Cruz, and other members of the HST Wide Field/Planetary 
Camera Imaging Team.  

                             - more -
                                      
                              - 2 -
                                      
                                      
 The HST pictures show two bright spots at the heart of the M31 
galaxy.  The dimmer of the two "light-peaks" appears to mark the 
exact center of the galaxy.  The brighter peak is at least 5 light 
years away from the true center, but corresponds to what 
astronomers previously had thought was the nucleus of M31, based 
on ground-based observations. 

 Well-known as the Andromeda Galaxy, M31 (the 31st object in a 
catalog of non-stellar objects compiled by French astronomer 
Charles Messier in 1774) is located only 2.3 million light years 
away, making it the nearest major galaxy to the Earth's own Milky 
Way.  M31 dominates the small group of galaxies of which the Milky 
Way is a member and can be seen with the naked eye as a 
spindle-shaped "cloud" the width of the full Moon.  

 Like the Milky Way, M31 is a giant spiral-shaped disk of stars, 
with a bulbous central hub of older stars.  M31 has long been 
known to have a bright and extremely dense grouping of a few 
million stars clustered at the very center of its spherical hub.  
As seen from large ground-based telescopes, the starlight blends 
to resemble a single bright, almost point-like source.  Previous 
ground-based observations gave little hint of the true structure 
of the core, which is now revealed by Hubble. 

Pinpointing the M31 nucleus

 In the 1960's the first high resolution photographs of M31's core 
were obtained by Stratoscope II, a balloon-borne observatory.  The 
images were not as sensitive as Hubble's and only showed a single 
bright cluster of stars.

 An important clue came with observations obtained in 1986 by the 
late Jean-Luc Nieto, then at the Pic du Midi Observatory in 
France.  He found that the bright nucleus was offset by several 
light-years from the exact center of the galaxy's central bulge.  
The new HST images show that the dimmer peak instead is the true 
nucleus and that the bright point of light evident from 
ground-based telescopes corresponds to the brighter of the two 
peaks.

A cannibalized galaxy?

 One possible explanation for the second cluster being offset from 
the exact center is that it is the remnant of a smaller galaxy 
that fell into M31 perhaps a billion or so years ago.  The smaller 
galaxy's core is the only surviving fossil relic of the galactic 
collision.

 A problem with the collision scenario is that the remnant core 
should be torn apart by the massive black hole hypothesized to 
dwell at the exact center of M31.  The suspected black hole would 
be located in the middle of the dimmer peak uncovered by HST.  

                             - more -
                                      
                              - 3 -
                                      
                                      
 In 1988, the first evidence for a black hole at the exact center 
of M31 came from ground-based observations by Dr. John Kormendy 
(now at the University of Hawaii), and independently, by Dr. Alan 
Dressler (Observatories of the Carnegie Institution of 
Washington), and Dr. Douglas O. Richstone (University of 
Michigan).  Their data indicated an abrupt increase in the orbital 
velocities of stars in the center of the M31 nucleus. 

 This led the astronomers to conclude that M31 must have a strong 
but unseen concentration of mass at its center.  A black hole at 
least 10 million times the mass of the sun is the most likely type 
of object matching these characteristics.  A black hole is a 
theoretical object that is so dense that even light cannot escape 
its intense gravitational pull.

 If such a black hole really exists, than the remnant core from 
the cannibalized  galaxy would be torn apart in just a few 100,000 
years.  "This is very short in cosmic time," said Lauer.  "We 
would have to be looking at the galaxy at a very special time to 
see it now."  

 One way for the remnant to survive for a much longer time is if 
it has its own massive black hole.  Gravity from a black hole in 
the remnant would hold it together against destruction from the 
other black hole at the M31 center.

 "In retrospect, there may be evidence for this possibility in the 
spectra obtained by Kormendy, Dressler and Richstone, said Lauer.  
"One problem with this picture, however, is if the black hole in 
the remnant were too big, it would distort even the true nucleus 
of M31." 

A dust lane bisecting the nucleus?
 
 Another interpretation of the "twin peaks" is that the bright 
spot is just the outer portion of a large nuclear star cluster and 
that the central portions have been obscured by dust.  A thick 
ring of dust might even cut across the nucleus, creating the 
illusion of two separate objects rather than one elongated 
structure.  

 Lauer explains that the problem with this idea is that normal 
galactic dust would scatter the light such that it would appear 
reddened.  "But this is not the case, there are no color effects 
at all," he emphasized.  "This means that the dust grains would 
have to be much larger than average."  In the Milky Way galaxy, 
however, the interstellar dust grains are roughly the same size.  

 "We can only guess that earlier nuclear activity in M31 would 
have destroyed all the fine dust grains that would cause color 
effects," said Lauer. 

                             - more -
                                      
                                      
                                      
                              - 4 -
                                      
                                      
 M31 is an ideal target for Hubble once the telescopes's optics 
are improved during a Space Shuttle servicing mission in December.  
Spectrographs aboard Hubble will dissect the light from the two 
peaks of the double nucleus and determine if they are truly 
separate clusters.  Astronomers will be able to measure the 
velocity of stars to pin down whether there is a black hole in 
either or both.

 This is the latest in a series of Hubble observations that have 
uncovered unusual structures in the cores of galaxies.  Some of 
them might be fossil evidence of galactic collisions.  Hubble also 
has resolved very dense concentrations of stars in the cores of 
other galaxies that are circumstantial evidence that massive black 
holes are common among galaxies.

                             - end -

NOTE TO EDITORS:  Two photographs are available to news media by 
calling NASA's Broadcast and Imaging Branch on 202/358-1900.

                  Color                          B & W
               93-HC-316                  93-H-340
               93-HC-317                  93-H-341