Halo of the Milky Way

The halo of our Galaxy is the large roughly spherical distribution of stars and ??? of our Galaxy. The stars (and globular clusters) are thought to have formed in the early stages of the formation of the Milky Way (and are referred to as Population II stars) and typically will therefore be stellar remnants, low mass stars, or other low mass objects (e.g., planets or brown dwarfs). Halo stars are not the first generation of stars, however, because they contain elements heavier than hydrogen and helium, that is, material which has been processed through stars at least once. According to the measurements of the mass of the Milky Way, it is inferred that the bulk of the mass of the Milky Way is Dark Matter contained in the halo of the Galaxy. It is not clear in what form the mass resides, however. Over the years, the halo material has been suggested to be in the form of normal stars, planet-sized objets, brown dwarfs, black holes, neutron stars, white dwarfs, and fundamental particles (e.g., exotic particles such as Weakly Interacting Massive Particles [WIMPs, that may include neutrinos], axions, ...). It is on this aspect of the halo (and, is an interesting question for the Universe at large) that I will spend some time.

Massive Compact Halo Objects (MACHOs)

An interesting experiment to study the possible types of compact objects in the halo of the Milky Way galaxy has been in progress for the last few years. In this work, one takes advantage of a prediction of the General Theory of Relativity (GTR) to probe the structure of the halo. GTR predicts that the path of a light ray will bend as it passes by a massive object,

In this sense, massive objects act like lenses (which gives rise to the name for the phenomenon, gravitational lensing). Like an ordinary lens, a gravitational lens concentrates the light from the lensed star thereby amplifying it or creating multiple images of it. This actually occurs in the Universe as demonstrated by the multiple images seen around this distant elliptical galaxy.

How do we Probe the Structure of the Halo?

In the MACHO project, the idea is as follows. Astronomers point their telescopes in the direction of our nearest neighbor galaxy, the Large Magellanic Cloud (LMC), and the Bulge of our own Galaxy. The project team regularly take pictures of the LMC and the Bulge. The LMC and Bulge contains many billions of stars of which roughly twenty million are monitored by the MACHO team.

What is expected to be seen?

Since the objects in our halo orbit about the center of our Galaxy there is a chance that a halo object will pass in front of an LMC or Bulge star. If a halo object passes in front of an LMC or Bulge star, the light from the star will be amplified so that,

(For further results, see MACHO team home page.)

• The number of LMC and Bulge stars which show this effect ===> the number of objects in the halo of our Galaxy.
• The way in which the light amplifies ===> the properties of the lensing object.

What has been found?

Based on the ongoing MACHO experiment, we infer that there is roughly 2x10¹¹ M(Sun) of MACHO material in the halo. Each MACHO has a mass of roughly 0.1 to 1 M(Sun), that is, the observed MACHOs are of stellar-mass. The MACHOs may be of low mass but they are not planets, brown dwarfs, or comet-like objects; they are still too massive. The MACHOs make up a large chunk of the halo mass, but still leave much material unaccounted for. Much of our halo may still be made up of more exotic types of matter or more exotic objects.

A recent interesting result is that gravitational lensing may be used to discover planets. An Earth-like planet with 5x the mass of the Earth and an orbital pereiod of ~10 years has been detected,