The Heart of the Milky Way

Fulvio Melia. American Scientist. Volume 88, Issue 4. Jul/Aug 2000.

A glance at the central hub of any galaxy will tell you that there must be something very special going on deep inside. Galactic nuclei are inevitably the brightest part of a galaxy, and just about everything else in the galaxy orbits the central axis. What could be attracting all that attention? For decades now, astronomers have been trying to determine what might lie at the center of our own Milky Way. It’s been an exceedingly difficult task because most of the visible light the center emits is absorbed by thick clouds of dust in the galactic plane, where our Sun resides. Despite the intrinsic brightness of the galactic center, optical telescopes are nearly useless when it comes to viewing it.

All is not lost in the dust, however. Photons of other wavelengths-radio, infrared, gamma ray and x ray-can wiggle their way through the dust clouds with little interference. Fortunately for galactic astronomers, the Milky Way’s inner sanctum shines through gloriously at these wavelengths.

The picture that is being uncovered is proving to be more complex and bizarre than anyone might have supposed. It has become apparent that there are several components in the central region, each more exotic than the next. To make things even more interesting, they also happen to be interacting with each other-making for a dynamic astrophysical soup. And in the center of it all sits an especially peculiar concentration of mass-the equivalent of nearly three million suns-that is packed into an area no more than one-tenth of a light-year across.

Zeroing in on the identities of this mass and its companions has occupied a good part of many astronomical careers, including my own. Our efforts have been rewarded with a fascinating story about the galactic center, one that continues to unfold in finer detail nearly every day. Here we will take a closer look at what the most recent explorations reveal about the heart that beats in the center of our Galaxy.

Now Playing on the radio

Tune in to the galactic center at just the right radio wavelength (about 90 centimeters) and you’ll see phenomena swirling to a dance seen nowhere else in the Milky Way. All of the radiation visible here is produced by various forms of gas and particles moving at hundreds of kilometers per second, some of it at nearly the speed of light. Although there are stars near the galactic center, none of them are seen here because they are relatively dim at radio wavelengths.

Near the center of this field (which is about 1,500 light-years across) sits the bright blob known as the Sagittarius A (Sgr A, pronounced “saj-ay’) Complex. This structure encompasses the innermost 100 light-years of our Galaxy and, as the name suggests, contains detail within that is not evident in this image.

Although stars are not visible, the drama of their collective births and deaths is manifest throughout the galactic center. Regions of star formation (such as Sgr B1 , Sgr B2 and Sgr D HII) are discernible as newborn stars heat the surrounding gas to the point that it shines in the radio. At the end of their lives, the most massive stars collapse in supernova explosions, leaving behind debris in the form of supernova remnants (SNR) that will glow for millions of years. Other elements here include enigmatic filamentlike structures-such as the Arc, the Snake and the Threads-and various diffuse patches consisting of highly ionized hydrogen gas.

The image on the facing page was made by the Very Large Array (VLA), a collection of 27 radio telescopes in Socorro, New Mexico. Here the plane of the galaxy runs diagonally from the upper left to the lower right (as is true for all the images in this article). (image processed by Namir Kassim and collaborators at the Naval Research Laboratory, using Department of Defense High Performance Computing resources.)

The Sgr A Complex and the Arc

As one zooms in toward the center of the Galaxy (the inner 200 light-years), detail becomes evident within Sgr A as a diffuse ovoid region called Sgr A East (aqua blob, lower right) and a spiral-like pattern called Sgr A West (pink). The shelllike structure of Sgr A East (about 30 light-years across) encompasses Sgr A West, but much of the shell lies behind this spiral pattern as we view it from the Earth.

The Arc becomes defined as a set of radio-emitting streamers that appear to be interacting with the Sgr A complex. The Arc and the other filamentary structures within a few hundred light-years of the center are believed to trace the large-scale structure of the magnetic field in the region. These filaments are thought to emit synchrotron radiation (high-speed electrons spiraling around a magnetic field). The image was made by the VLA at a wavelength of 20 centimeters. (Image courtesy of Farhad Yusef Zadeh, Northwestern University.)

What’s Bigger than a Supernova?

The appearance of Sgr A East (see “The Sgr A Complex and the Arc’s is reminiscent of a supernova remnant but may have actually been produced by an explosion 50 to 100 times more energetic than a supernova. What could produce such a colossal blast? In one scenario, a wayward star wanders too close to the steep

gravitational potential well of a central black hole, which pulls the star into a long, thin spike during its inward trajectory. As the intruding star recedes from the center, the gravitational energy is quickly dissipated into heat-pausing the star to blow up like a supernova shell, except with much greater force.

Some recent images of the galactic center at energies of 511 kilo-electron-volts (left, top) and hundreds of mega-electron-volts (left, bottom) support the idea that a massive explosion produced Sgr A East within the past 100,000 years. The upper image (about 15,000 lightyears across) shows that large amounts of antimatter (in the form of positrons) are being annihilated at the galactic center (yellow), with moderate emissions (orange) along the galactic plane and above the galactic bulge. The lower image (about 13,000 light-years across) indicates that the center of the Galaxy is a very bright source (white) of high-energy photons.

Such high-energy emissions would be produced if the expanding shell of a large explosion were to collide with a relatively cold molecular cloud surrounding it. Protons that bounce back and forth across the resulting shock fronts would be pumped up to ultra-relativistic energies (much like a basketball that speeds up as a dribbler’s hand gets closer to the floor). The high-energy protons would then collide with the ambient hydrogen gas, producing gamma rays and also positrons that are almost immediately destroyed in collisions with electrons (producing a characteristic gamma-ray emission at 511 kilo-electron-volts).

Both images were made by instruments aboard the Compton Gamma Ray Observatory. (Top image courtesy of D. D. Dixon, University of California, Riverside, and William R. Purcell, Ball Aerospace & Technologies Corporation. Bottom image courtesy of John Mattox, Boston University.)

Three Arms Hug the Middle

At a radio wavelength of 6 centimeters, Sgr A West reveals itself to be a three-armed spiral consisting of highly ionized gas whirling around the galactic center. Each arm in the spiral is about three light-years long, but it is not clear whether they are part of a physical spiral or merely a superposition of independent gas streams flowing into the middle. At a distance of three light-years from the center, the gas moves at a velocity of about 105 kilometers per second, which indicates a mass concentration of over 3.5 million suns inside this radius. The hub of the gas spiral corresponds to a very bright radio source (orange, ovoid shape) known as Sgr A* (“saj-ay-star”*—the dynamic center of our Galaxy.

Astronomers still lack a complete grasp of the gas motions in the region, however, because the three-dimensional shape of Sgr A West is not fully understood and because fierce stellar winds from hot stars in the area collide with the gas, pushing it around in complex ways (see below). The image was made by the VLA. (Image courtesy of Kwok-Yung Lo, Academia Sinica, Taiwan, and Farhad Yusef Zadeh, Northwestern University.)

When Stellar Winds Collide

Massive stars in the central part of our Galaxy are constantly losing mass in the form of hot winds (mostly protons and electrons) that blow in all directions, colliding with each other and the ambient gas. The collisions result in gas condensations of various densities, producing the tesselated pattern shown in this hydrodynamic simulation (red is high density; black, low density). The density generally increases toward the center as the gas is pulled in by gravity. Here Sgr A* lies in the middle and a few stars are visible on the right-hand side of the field, which is about 0.5 lightyears across. (Image courtesy of the author.)

A Molecular Ring

A lumpy doughnut-shaped molecular cloud (violet), called the circumnuclear disk, encircles the threearmed spiral of Sgr A West (orange). More than 10 light-years across, the circumnuclear disk is made of fragile molecules and rotates at a velocity of about 110 kilometers per second. Ionized streamers extend from the northwestern part of the ring (upper right) and may indicate the general orientation of the powerful magnetic field in this part of the Milky Way.

Here a radio-wavelength image of ionized gas at 1.2 centimeters (orange) is superimposed on the distribution of hydrogen cyanide molecules (violet) to reveal the relation between these structures. (Image courtesy of Farhad Yusef-Zadeh, Northwestern University.)

A Galactic Doughnut with a Heavy Filling

A cartoon of the inner 15 light-years of our Galaxy shows the exotic collection of astrophysical phenomena that lie within the cavity of the circumnuclear disk. The central attraction is the strong radio source Sgr A*, which gives all indications of being a supermassive black hole-almost certainly the only one in our Galaxy (see “The Dark Monster, facing page). Nearby, a concentrated cluster of at least two dozen hot blue stars (IRS 16) bathes the entire cavity with ultraviolet radiation and seems to be the source of a powerful wind that sweeps through the galactic center with a velocity of nearly 700 kilometers per second. The wind may be the cause of the shocked molecular gas in the ring, and the snakelike form of Sgr A West’s arms (especially

noticeable in the Northern Arm). A red supergiant star (IRS 7) is also feeling the effects of the wind and radiation from IRS 16, as the star’s surface is stripped off to form an ionized comet-like tail that points away from the galactic center.

The existence of a peculiar “Mini-cavity,” which forms a hole in the radio emission from Sgr A West (below), could also be explained by the effects of the winds emanating from IRS 16. The strong gravitational potential of Sgr A* may focus the winds toward the Mini-cavity, effectively excavating a hole in the ionized gas.

The Dark Monster

What darkness lurks in the heart of the Milky Way? Calculations of the mass (white circles) enclosed within a given radius of the galactic center indicate that the local distribution of luminous stars (yellow line) cannot account for the orbital movement of the gas and stars in the region. The bulk of the mass must not be visible. Several possibilities have been proposed, including a collection of underluminous objects such as brown dwarfs and dead stars. Aside from the fact that a highly compact cluster of dead stars is not physically tenable, such models (pink line) are not consistent with the mass distribution in the center. Instead the density of dark matter in the galactic center indicates a single pointlike object: a supermassive black hole of about 2.6 million solar masses (orange line).

Darkness Visible

The bright radio source Sgr A*-sitting prominently as an ovoid shape (cross) nestled in the crotch of Sgr A West-is a prime candidate for the giant black hole that is thought to lie in the center of our Galaxy. This of course raises the question, if Sgr A* is actually a black hole, why does it shine so brightly? A lone black hole should indeed be black, with no light escaping it whatsoever. And there lies the answer, for Sgr A* is hardly alone, surrounded as it is by a dense cluster of stars, dust and gas. In fact, the density of the interstellar medium near Sgr A* is 10,000 times greater than it is in our solar neighborhood.

Being so close to the gravitational potential of Sgr A* has an effect on the gas that is analogous to what happens to air compressed inside a bicycle pump: it heats up. Of course, the physics is a little more exotic. As the gas plummets toward Sgr A*, it accelerates rapidly, at speeds topping 1,000 kilometers per second, and since it is fully ionized it produces an intense magnetic field. The gravitational stress on the infalling gas is so severe that it may reach temperatures exceeding 10 billion degrees. The net result is that before the matter crosses the black hole’s event horizon, it emits a shower of radiowaves through a variety of mechanisms, including particle-particle collisions (Bremsstrahlung), collisions between energetic particles and the ambient radiation (Compton scattering), and by charged particles spiraling around the magnetic field lines (cyclotron and synchroton processes). The bright spot known as Sgr A* is the radiation produced by this gas in its death throes.

Here a 1.2-centimeter radio image (orange) is superimposed on a 2-micrometer infrared image (blue) of the stars in the region. (Image courtesy of the author.)

The Shadow of a Black Hole

Although a black hole is itself invisible, its “shadow” may betray its presence. Because of the strong gravitational field of a black hole, light from the infalling gas behind Sgr A* does not make it directly to us but is bent away or simply swallowed into the darkness. This should be manifested as the absence of radiation-a shadow-at the edge of the black hole’s event horizon.

The image shows a general relativistic raytracing calculation of the black-hole event horizon at Sgr A* as it would appear if there were no intervening medium between us and the galactic center. This shadow is predicted to be about 30 microarcseconds across-a diameter that should be resolved within the next 10 years by currently evolving technology (very long baseline interferometry at sub-millimeter wavelengths). Such an image would not only provide another test of Einstein’s theory of general relativity, it would conclusively establish that a black hole does indeed exist at the center of the Milky Way. (Image courtesy of the author.)

Most astronomers are convinced that the galactic center is home to a supermassive black hole of about 2.6 million solar masses. This is indeed large for a black hole, but quite small compared to the bloated monsters in other galaxies. The center of the giant elliptical galaxy M87, for example, is believed to harbor a black hole of nearly 3 billion solar masses. Nevertheless, the special beauty of our supermassive black hole is that it is merely 28,000 light-years away (unlike M87, which is 50 million light-years away). Our relative proximity to such a beast provides an unprecedented opportunity to understand how the central engines of galactic nuclei stir up all the energy that they do.

One of the more interesting questions that must be answered, for example, is why the Milky Way’s central black hole radiates energy the way it does. Although Sgr A* appears to be devouring matter at a fairly healthy rate (about one-5,000th of a solar mass per year), for some unknown reason it produces neither x rays nor gamma rays-high-energy photons that would be expected of a black hole of its size. It thus appears to be quite inefficient at radiating the energy from the matter it swallows-but why?

Other phenomena in the galactic center also beg for an explanation. The filamentary loops that rise hundreds of light-years above the galactic plane are something of a mystery. Some physicists have suggested that these threads may be related to “cosmic strings,” hypothesized remnants of the early universe that are presumed to help nucleate galaxy formation (see “Superconducting Cosmic Strings,” May June 2000), but this is far from certain. And the physical relationship between the various components in the galactic center is still not entirely understood, nor is their evolutionary history. As we’ve seen in this article, the galactic center is a complicated place-it should fulfill the careers of many astronomers for years to come.