The center of the galaxy

Dust largely obscures the center of the Galaxy from us; the extinction in its direction is almost 30 magnitudes. Since each 5 magnitudes amounts to a reduction in flux by a factor of 100, the flux from the Galactic Center is 1012 times fainter than it would be in the absence of dust. If the dust were not there, the nucleus would appear brighter than the full moon!

The extinction from dust prevents our direct optical view of the Galactic Center, but there are a few regions of lesser absorption where optical astronomers have glimpsed at least a few parts of the nuclear bulge. In addition, we can surmise the nature of the stars in the central region from observations of the centers of other galaxies, such as M31. From these two kinds of observation, we picture the center of the Galaxy as containing mostly old Population I stars densely packed together. So jammed are the stars in this region that if you lived there, the nighttime sky would be as bright as the twilight on the earth.

Recall that dust scatters red light much less than blue light, so infrared observations can penetrate farther into the center. Radio waves, which are longer than infrared, are even less affected by the dust. On the other hand, x-rays are so energetic that they go right through the dust. Radio, infrared, and x-ray astronomers can probe the Galactic Center more easily than can optical astronomers.

These observations show that the heart of the Galaxy is a bizarre place: not only does it contain many very old Population I stars, but also some very young supergiant M stars and О stars. Motions of gas here suggest a high concentration of mass at the very center-perhaps a black hole. In the very center of the Galaxy lies an ultrasmall (by astronomical standards) radio source less the 140 AU (about 10 light hours) in size.

Radio Observations Let's look first at the continuous emission of the Galactic Center (Fig. 23.31). An intense radio source lies smack in the direction to the center. It is called Sagittarius A, or Sgr A for short. Clustered around Sgr A-and all lying more or less along the galactic plane (or equator)-is a string of radio sources.

Now let's check out the emission in the form of spectral lines. The hydrogen recombination lines, from the ionized hydrogen in the Galactic Center, are very broad. Now one physical process that commonly broadens spectral lines is the Doppler shift caused by the rotation of a mass of gas. The regions of the gas approaching us give blueshifted lines; those receding, redshifted ones. When the entire rotating mass is observed within the field of view of a radio telescope, all the various Doppler shifts get smeared together, resulting in a broad line. If interpreted in this way, the broad lines imply rotational speeds of 150 km/s at a distance

X-rays from the Galactic Center Astronomers have known for a number of years that the Galactic Center emits x-rays. Observations in the 1970s by the Uhuru x-ray satellite found an extended x-ray source about 2° in size. Later observations, including those made with the Einstein X-Ray Observatory, found several pointlike sources. Most of these are either x-ray bursters or binary x-ray sources, in which matter falling onto neutron stars produces the x-rays. One coincides with the presumed center of the Galaxy within the Sgr A complex; it has an x-ray luminosity of 1028 W. The other sources lie along the ridge in the same location as the cluster of infrared sources.

Does a Black Hole Lurk in the Core? A puzzle generated by the radio and infrared line observations arises from the rapid rotational motions in the Galaxy's core. Here it appears that the rotational speeds increase rapidly toward the center. Why is that a problem? First, if the Galaxy's core consisted simply of stars spread out throughout its volume, you would expect the rotational velocities to decrease toward the core's center, rather than increase, because as you get closer in you have less and less mass to bind the moving materials gravitationally. Second, the rotational speeds are so high that a huge mass is needed to hold all that speedy gas together. To account for the rapid and increasing rotation requires a mass at the center of several million solar masses-all lumped together in a region less than 0.04 pc in diameter! An approximate calculation shows this point. The infrared line observations show a rotational speed of about 300 km/s at a distance of 0.4 pc from the center.

What form might this mass have? One possibility-and it's very hard to come up with another- is that the mass is locked up in a black hole. If it were in the form of, say, solar-mass stars, these stars would be separated on the average only 1 to 2 AU from each other. That seems unlikely, because stars so close, especially if many of them were red giants, would collide rather frequently. The idea that a supermassive black hole lurks in the heart of the Galaxy has yet to be confirmed, but indirect support for the idea comes from observations that a few other galaxies may have a similar mass concentration in their nuclear region.

Jack Hills has proposed a possible test for the existence of a 106-solar-mass black hole in the core that involves its interaction with binary stars in the Galactic Center. If a binary star system comes within a few AU of a supermassive black hole, one star becomes bound in an orbit around it, and the other is ejected outward at a speed up to 4000 km/s. The detection of a star moving outward from the Galactic Center with this hypervelocity, which is greater than the escape velocity from the Galaxy, would be nearly definitive proof of a supermassive black hole. Meanwhile, the trapped companion, if an ordinary star, is disrupted by tidal forces and falls into the accretion disk around the supermassive black hole. If the companion is a degenerate star, however, it remains orbiting at 50 to 500 AU for a long time. Such stellar remnants could accumulate in large numbers and act as grinders, tearing apart normal stars that pass near them and helping to provide fuel for the central black hole.