22-6 Infrared, radio, X-ray, and gamma-ray observations are used to probe the galactic center

At the very center of the Milky Way Galaxy is a maelstrom of activity centered on a supermassive black hole

The innermost region of our Galaxy is an active, crowded place. If you lived on a planet near the galactic center, you could see a million stars as bright as Sirius, the brightest single star in our own night sky. The total intensity of starlight from all those nearby stars would be equivalent to 200 of our full moons. In effect, night would never really fall on a planet near the center of the Milky Way. At the center of this empire of light, however, lies the darkest of all objects in the universe—a black hole millions of times more massive than the Sun.

Sagittarius A*: Heart of Darkness

Because of the severe interstellar absorption at visual wavelengths, most of our information about the galactic center comes from infrared and radio observations. Figure 22-27 shows three infrared views of the center of our Galaxy. Figure 22-27a is a wide-angle view covering a 50° segment of the Milky Way from Sagittarius through Scorpius. (The top photograph that opens this chapter shows this same region at visible wavelengths, viewed at a different angle.) The prominent reddish band through the center of this false-color infrared image is a layer of dust in the plane of the Galaxy. Figure 22-27b is an IRAS view of the galactic center. It is surrounded by numerous streamers of dust (shown in blue). The strongest infrared emission (shown in white) comes from sources that also emit radio waves. One of these sources, Sagittarius A (say “A star”), lies at the very center of the Galaxy. (Its position, pinpointed with simultaneous observations by radio telescopes scattered around the world, seems to be very near the gravitational center of the Galaxy.) The high-resolution infrared view in Figure 22-27c, made using adaptive optics (see Section 6-3), shows hundreds of stars crowded within 1 ly (0.3 pc) of Sagittarius A*. Compare this region to our region of the Galaxy, where the average distance between stars is more than a light-year.

Figure 22-27: R I V U X G
The Galactic Center (a) In this false-color infrared image, the reddish band is dust in the plane of the Galaxy and the fainter bluish blobs are interstellar clouds heated by young O and B stars. (b) This close-up infrared view covers the area outlined by the white rectangle in (a). (c) Adaptive optics reveals stars densely packed around the galactic center.
(a, b: NASA; c: European Southern Observatory)

Sagittarius A* itself does not appear in infrared images. Nonetheless, astronomers have used infrared observations to make truly startling discoveries about this object. Since the 1990s, two research groups—one headed by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and another led by Andrea Ghez at the University of California, Los Angeles—have been using infrared detectors to monitor the motions of stars in the immediate vicinity of Sagittarius A*. They have found a number of stars orbiting Sagittarius A* at speeds in excess of 1500 km/s (Figure 22-28). (By comparison, Earth orbits the Sun at a lackadaisical 30 km/s.) In 2000 the UCLA group observed one such star, called S0-16, as its elliptical orbit brought it within a mere 45 AU from Sagittarius A* (1½ times the distance from the Sun to Neptune). At its closest approach, S0-16 was traveling at a breathtaking speed of 12,000 km/s, or 4% of the speed of light!

Figure 22-28: R I V U X G
Stars Orbiting Sagittarius A* The colored dots superimposed on this infrared image show the motion of seven stars in the vicinity of the unseen massive object (denoted by the yellow five-pointed star) at the position of the radio source Sagittarius A*. The orbits were measured over a 15-year period. Analysis of the orbits indicates that the stars are held in orbit by a black hole of 4.1 million solar masses. The blue dots for S0-16 show this star reached 4% the speed of light at its closest approach to the black hole.
(Keck/UCLA Galactic Center Group)

658

In order to keep stars like S0-16 in such small, rapid orbits, Sagittarius A* must exert a powerful gravitational force and hence must be very massive. By applying Newton’s form of Kepler’s third law to the motions of these stars around Sagittarius A*, the UCLA group calculates the mass of Sagittarius A* to be a remarkable 4.1 million solar masses (4.1 × 106 M). Furthermore, the small separation between S0-16 and Sagittarius A* at closest approach shows that Sagittarius A* can be no more than 45 AU in radius. An object this massive and this compact can only be one thing—a supermassive black hole (see Section 22-4).

CONCEPT CHECK 22-13

How would stars around Sagittarius A* be moving if a supermassive black hole were not found at the galactic center?

X-rays from Around a Supermassive Black Hole

Evidence in favor of this picture comes from the Chandra X-ray Observatory, which has observed X-ray flares coming from Sagittarius A*. The flares brighten dramatically over the space of just 10 minutes, which shows that the size of the flare’s source can be no larger than the distance that light travels in 10 minutes. (We used a similar argument in Section 21-3 to show that the flickering X-ray source Cygnus X-1 must be very small.) In 10 minutes light travels a distance of 1.8 × 108 km or 1.2 AU, and only a black hole could pack a mass of 4.1 × 106 M into a volume that size or smaller. The X-ray flares were presumably emitted by blobs of material that were compressed and heated as they fell into the black hole (see Section 21-3).

659

The X-ray flares from Sagittarius A* are relatively feeble, which suggests that the supermassive black hole is swallowing only relatively small amounts of material. But the region around Sagittarius A* is nonetheless an active and dynamic place. Figure 22-29a is a wide-angle radio image of the galactic center covering an area more than 60 pc (200 ly) across. Huge filaments of gas stretch for 20 pc (65 ly) northward of the galactic center (to the right and upward in Figure 22-29a), then abruptly arch southward (down and to the left in the figure). The orderly arrangement of these filaments is reminiscent of prominences on the Sun (see Section 18-10, especially Figure 18-28). This suggests that, as on the Sun, there is ionized gas at the galactic center that is being controlled by a powerful magnetic field. Indeed, much of the radio emission from the galactic center is synchrotron radiation: As we saw in Section 22-4, such radiation is produced by high-energy electrons spiraling in a magnetic field.

Figure 22-29: The Energetic Center of the Galaxy (a) The area shown in this radio image has the same angular size as the full moon. Sagittarius A*, at the very center of the Galaxy, is one of the brightest radio sources in the sky. Magnetic fields shape nearby interstellar gas into immense, graceful arches. (b) This composite of images at X-ray wavelengths from 0.16 to 0.62 nm shows lobes of gas on either side of Sagittarius A*. The character of the X-ray emission shows that the gas temperature is as high as 2 × 107 K.
(VLA, F. Sadeh et al./NRAO; b: F. K. Baganoff et al./CXC/MIT/NASA)

The false-color X-ray image in Figure 22-29b shows the immediate vicinity of Sagittarius A*. The black hole is flanked by lobes of hot, ionized, X-ray–emitting gas that extend for dozens of light-years. These lobes are thought to be the relics of immense explosions that may have taken place over the past several thousand years. Perhaps these past explosions cleared away much of the material around Sagittarius A*, leaving only small amounts to fall into the black hole. This could explain why the X-ray flares from Sagittarius A* are so weak.

There is nonetheless evidence that Sagittarius A* has been more active in the recent past. Between 2002 and 2005, Michael Muno and his colleagues at the California Institute of Technology observed that certain nebulae within about 50 ly of Sagittarius A* suddenly became very bright at X-ray wavelengths. The character of the X-ray light was such that it could not have originated from the nebulae themselves. Muno and colleagues concluded that X-rays emitted from Sagittarius A* about 50 years earlier had struck and excited the nebulae, causing the nebulae to glow intensely at X-ray wavelengths. To produce such an intense X-ray glow, an object the size of the planet Mercury must have fallen into the supermassive black hole.

CONCEPT CHECK 22-14

Why would the sudden brightening of a nebula near the galactic center suggest the presence of a supermassive black hole?

Gamma Rays Offer Clues to Possible Black Hole Activity

Figure 22-30: Galactic Gamma-Ray Bubbles This is an artist’s illustration of giant gamma-ray “bubbles” seen above and below our galactic plane, possibly due to the black hole at the galactic center. Each of the bubbles extends about 25,000 light-years away from the plane. The bubbles are estimated to be a few million years old. The gamma ray data also contain hints of linear jetlike features, which are sometimes found around black holes, but the existence of the jets is far from clear. If the jets are real, producing them might have required the black hole to consume a giant hydrogen cloud of about 10,000 solar masses, with only a small portion escaping into the jets.
(David A. Aguilar [CfA])

In 2010, astronomers with the Fermi Gamma-ray Space Telescope (FGST) reported seeing enormous gamma-ray “bubbles” above and below the plane of our Galaxy (Figure 22-30). The gamma rays are produced when high-speed electrons collide with infrared or visible photons and bump the photon energies up to gamma-ray levels. What remains unclear is what created the high-speed electrons. One idea involves a hot “wind” of electrons and other particles blown away from an accretion disk of matter (see Figure 21-11) around the black hole at our Galaxy’s center.

It is too early to say if the gamma-ray bubbles are caused by a galactic black hole. However, if these interpretations hold up, they certainly point to interesting behavior by the black hole at the center of our Galaxy. As intriguing as it is, the supermassive black hole at the center of our Galaxy is not unique. Observations show that such titanic black holes are a feature of most large galaxies. In Chapter 24 we will see how black holes of this kind power quasars, the most luminous sustained light sources in the cosmos.