Structural Components of the Milky

We have seen how the structure of the Milky Way can be globally described by means of an almost spherical halo of old stars and and a disc of gas and young and middle-aged stars. In a more detailed picture more small-scale features can also be distinguished.

The Thick Disc. In the traditional scheme dividing the stars of the Milky Way into a series of populations it was left undecided whether the populations should be considered as qualitatively different classes or merely steps along a continuous sequence. As the quality and quantity of observations have improved, it has become clear that what was defined as the intermediate population II really represents a separate component of the Milky Way with a pattern of element abundances stellar motions that separate it clearly from the old (thin) disc. This population is now referred to as the thick galactic disc. A thick disc has also been detected in some other galaxies, but it does not appear to be a universal feature of all disc galaxies.

The Galactic Bar. As will be seen in the next chapter, Sect. 18.1, a large fraction of all disc galaxies are barred, with an elongated light distribution at the centre. The first indication that this might also be the case for the Milky Way was found in velocity measurements of neutral hydrogen, which were incompatible with gas moving along circular orbits. In 1971 W. W. Shane showed that the motions of the gas could be explained, if there is a central bar pointing about 20° away from the direction of the Galatic centre.

It is more difficult to detect a bar in observations of the stars. This was first done using the COBE satellite, which (apart from mapping the cosmic microwave background, see Sect. 19.7) also made a map of the sky at the infrared wavelengths dominated by the light of old stars. Because of perspective, the nearer end of the bar at positive galactic longitude will look slightly different than the farther end. Such an asymmetry was present in the infrared map, consistent with a bar with an axial ratio of 0.6.

Later confirmation of the existence of the bar has come from mapping of the central distribution of old stars using near-infrared photometric distances, see Fig. 17.20.

Spiral Structure. As mentioned earlier, the Milky Way appears to be a spiral galaxy, but there is no general agreement on the detailed form of the spiral pattern. For example, in 1976 Y.M. Georgelin and Y.P. Georgelin determined the distances of HII regions by radio and optical observations. In the optical region their method is independent of assumptions about the galactic rotation law. They then fitted four spiral arms through the HII regions.

Fig. 17.20. The Milky Way bar viewed from the North Galactic pole. Solid symbols indicate the mean positions of red clump giants in given directions, and the thick grey lines represent the range of their distances. The line through the mean distances illustrates a bar of 3 kpc length inclined 22.5° to the direction to the Galactic centre. The contour map shows a model of the bar derived from infrared observations. (C.Babusiaux, G.Gilmore 2005, MNRAS 358, 1309, Fig.6)

Fig. 17.20. The Milky Way bar viewed from the North Galactic pole. Solid symbols indicate the mean positions of red clump giants in given directions, and the thick grey lines represent the range of their distances. The line through the mean distances illustrates a bar of 3 kpc length inclined 22.5° to the direction to the Galactic centre. The contour map shows a model of the bar derived from infrared observations. (C.Babusiaux, G.Gilmore 2005, MNRAS 358, 1309, Fig.6)

Later investigations using a variety of methods, both optical and radio have confirmed that a four-armed pattern gives the best description of the spiral structure in the Sun's vicinity (Fig. 17.22). The pitch angle of the spiral in this model is about 11.3°. Three of the arms start at the position of the galactic bar.

The cause of the spiral structure is a long-standing problem. A small perturbation in the disc will quickly be stretched into a spiral shape by differential rotation. However, such a spiral would disappear in a few galactic revolutions, a few hundred million years.

An important step forward in the study of the spiral structure was the density wave theory developed by Chia-Chiao Lin and Frank H. Shu in the 1960's. The spiral structure is taken to be a wavelike variation of the density of the disc. The spiral pattern rotates as a solid body with an angular velocity smaller than that of the galactic rotation, while the stars and gas in the disc pass through the wave.

Wide-Field Radio Image of the Galactic Center

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Fig. 17.22. General view of the spiral pattern of the Milky Way. Different tracers of spiral arms lead to somewhat different patterns, but they tend to agree that a four-armed pattern like the the one indicated here gives the best overall representation. The names of the arms are those most commonly used. See also Fig. 17.11. (Y.Xu et al. 2006, Science 311,54)

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Fig. 17.22. General view of the spiral pattern of the Milky Way. Different tracers of spiral arms lead to somewhat different patterns, but they tend to agree that a four-armed pattern like the the one indicated here gives the best overall representation. The names of the arms are those most commonly used. See also Fig. 17.11. (Y.Xu et al. 2006, Science 311,54)

The density wave theory explains in a natural way why young objects, like molecular clouds, HII regions and bright young stars are found in the spiral arms. As gas passes through the wave, it is strongly compressed. The internal gravity of the gas clouds then becomes more important and causes them to collapse and form stars.

It takes about 107 years for the material to pass through a spiral arm. By that time, the hot, bright stars have finished their evolution, their ultraviolet radiation has ceased and the H II regions have disappeared. The less massive stars formed in the spiral arms are spread out in the disc by their peculiar velocities.

It is not yet clear what gives rise to the spiral wave. For some further discussion of spiral structure, see Sect. 18.4.

Tornado (SNR?)

Fig. 17.21. The central parts of the Milky Way in a radio picture. The observations were made with the VLA telescope. (Image Kassim et. al., Naval Research Laboratory)

The Galactic Centre. Our knowledge of the centre of the Milky Way is mostly based on radio and infrared observations. In the optical region the view to the centre is blocked by the dark clouds in the Sagittarius spiral arm about 2 kpc from us. The galactic centre is inter-

17.5 The Formation and Evolution of the Milky Way esting because it may be a small-scale version of the much more violently active nuclei of some external galaxies (see Sect. 18.7). It therefore provides opportunities to study at close hand phenomena related to active galaxies. Since active galactic nuclei are thought to contain black holes with masses larger than 107 Me, there may also be a large black hole at the galactic centre.

As one approaches the galactic centre the stellar density continues to rise towards a sharp central peak. In contrast, the galactic gas disc has a central hole of radius about 3 kpc. This may be due to the galactic bar, which will channel gas into the galactic nucleus leaving a gas-free zone at larger radii.

Inside the central hole is a dense nuclear gas disc. Its radius is about 1.5 kpc in neutral hydrogen, but most of its mass is molecular and concentrated within 300 pc of the nucleus. In this region the mass of molecular gas is about 108 Me, or 5% of the total molecular mass of the Milky Way. The molecular clouds are probably confined by the pressure from surrounding very hot (T « 108 K) gas. This hot gas may then expand vertically, forming a galactic wind. Gas lost to a wind or to star formation is replenished with infalling gas from larger radii.

The central 10 pc are dominated by the radio continuum source Sgr A and a dense star cluster observed in the infrared. There is also molecular gas with complex motions and signs of star formation activity. Within Sgr A there is a unique point-like radio continuum source known as Sgr A*. The position of Sgr A* agrees to within 1" with the centre of a cluster of stars that is much denser than anything observed in the galactic disc. If the galactic centre contains a large black hole, Sgr A* is the natural candidate.

The luminosity of the galactic centre could be provided by the central star cluster, but there are still reasons to expect that a large black hole may be present. The central mass distribution can be estimated by modelling the observed motions of stars and gas (cf. Sect. 18.2). In particular it has recently become possible to measure the proper motion of stars in the central cluster, which has allowed the determination of their orbits. The best fit with the observations is obtained with models with an extended stellar mass distribution together with a pointmass of about 3 x 106 Me. The size of Sgr A* measured by very long baseline interferometry is less than 10 AU.

The most plausible explanation of this very compact structure is that Sgr A* is indeed a black hole with mass of a few million solar masses.

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