The halo of the galaxy

When Shapley investigated the distribution of globular clusters in space, he found that they had a nearly spherical distribution around the Galactic Center. The globular clusters form a spherical halo around the Galaxy. Little else is known to exist in the halo for sure. Stray stars are seen. The halo contains some gas that is hot and ionized. Cosmic rays whirl around, trapped by the Galaxy's magnetic field. The halo may also contain as yet undetected objects, such as very faint, low-mass stars. It extends to at least 100,000 pc trom the center of the Galaxy.

Globular Clusters You've already read about the physical characteristics and stellar content of globular clusters. To review briefly: globular clusters have a spherical shapes (some 10 to 100 parsecs in diameter) and contain 104 to 105 Population II stars, each a little less than one solar mass. The globular clusters fill a somewhat flattened sphere around the Galaxy's center. A typical cluster moves on an elliptical orbit, which brings it out to an extreme distance of 10 to 12 kpc from the Galaxy's nucleus. The clusters orbit at speeds of about 100 to 150 km/s, diving into and shooting out of the disk. These passages have helped to wipe globular clusters clear of any gas and dust they once had. The outer halo of the Galaxy has no exact that are gravitationally bound to the Galaxy.

Other Material in the Halo The Galaxy's halo is of low density and far from us, so it's hard to observe. Some indirect evidence and theoretical arguments indicate that the halo may actually contain considerable material, perhaps more mass than is in the visible part of the Galaxy. What might it be?

Astronomers have spotted a few RR Lyrae variables above and below the galactic plane, typically 5 kpc away, that do not belong to globular clusters. In addition to these stars, the halo may contain a large number of low mass, faint red stars that are difficult to observe directly. Recent observations of a few other nearby galaxies similar to our own Galaxy imply that they may also have extensive, massive halos of faint red stars.

The halo also contains gas, but much less than the disk of the Galaxy. Observations at 21 cm show hydrogen clouds traveling with high speeds above and below the galactic plane, so the halo has some HI in it. Most of the halo's gas, however, is probably ionized hydrogen, but so sparse that boundary. Observations of the placement of the farthest globular clusters indicate that the halo extends out to at least 100 kpc, far beyond the limits of the Galaxy's disk. It probably even includes the Magellanic Clouds, the two companion galaxies it has so far eluded detection. This gas could come from the disk, blown out by supernova explosions, expanding H II regions, and stellar winds. The halo may be fairly hot and expanding into inter-galactic space, a coronal gas in analogy to the sun's.

This gas may mark the source of the puzzling high-velocity clouds, which 21-cm observations show to have speeds of greater than 70 km/s relative to the sun. They appear as discrete entities in almost all directions in the sky, visible above the plane of the Galaxy by 10° or more. Doppler shift observations indicate that most clouds are blueshifted, moving toward us.

Consider the coronal gas at a temperature of about 1 x 106 К and a density of some 103 particles per cubic meter. This gas expands upward and outward from the disk at speeds of a few hundred kilometers per second. As it rises, the gas cools and becomes clumpy at distances of 5 to 10 kpc above the Galaxy's plane. The cool, denser clouds fall down into the the plane, arriving close to the place of its origin. The effect is like that of a water fountain: the gas gets recycled and circulates every 107 years or so in the upward and downward flow.

The halo may contain high-speed electrons and protons-cosmic rays trapped by the Galaxy's magnetic field. There is no solid, unambiguous evidence for a cosmic ray halo, but some observations-especially the fact that energetic cosmic rays striking the earth come from all directions- suggest that it may indeed exist. But the halo may also contain other objects, currently unobservable, except possibly by their gravitational effects. Low-mass, main-sequence stars would be very hard to detect. Smaller objects, similar to planets and asteroids may also exist. Some astronomers have even suggested a large number of black holes. We just don't know what else might be there.