Hubble Space Telescope Images the Dense Nucleus of Galaxy M32
8 April 1992Astronomers 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, Dr. Sandra M. Faber of the University of California, Santa Cruz, and other members of the HST Wide Field/Planetary Camera Imaging Team.
M32 is quite small and compact as elliptical galaxies go, containing roughly 400 million stars within a diameter of only 1000 light-years. At a distance of only 2.3 million light-years, M32 (the thirty-second object in a catalog of non-stellar objects compiled by French astronomer Charles Messier in 1774) is one of the closest neighbors to our own Milky Way galaxy. M32 is a satellite of the great spiral galaxy in Andromeda M31, which dominates the small group of galaxies of which our own 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, and independently by Dr. Alan Dressler of the Observatories of the Carnegie Institution of Washington, and Dr. Douglas O. Richstone of the University of Michigan. 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 Hubble Space Telescope 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 of 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 our own 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 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 one thousand times smaller than the black hole that might exist at the heart of M87.
Although M32 is about 20 times closer to us than M87, its much smaller black hole means that the brightness cusp is also much smaller and is close to HST's resolution limits. Unlike M87, M32 also 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 Space Telescope. 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 move slowly enough to gravitationally attract each other. 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.
Which scenario is correct? 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 supermassive 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.