Structure of the Milky

The most conspicuous feature of the Galaxy is its highly flattened disk of stars. Our Sun orbits at a radius of 8.5 kpc, or about a third of the distance to the outer edge. The local rotation speed is 220 km s~\ corresponding to a period of 2.4 x 108 yr within this differentially rotating structure. The local thickness of the disk i. e., the average vertical excursion of the stars within it, varies systematically with spectral type. Thus, O and B stars have a characteristic half-thickness of 100 pc, while the figure increases to 350 pc for G stars like the Sun.5 These observations refer to the solar neighborhood, i. e., to objects closer than about 0.5 kpc.

The densest concentration of stars is in the central bulge. This nearly spherical configuration extends out of the disk plane and has a radius of 3 kpc. Even farther from the plane is an extended stellar halo (or spheroid) consisting of both globular clusters and a large population of field stars. Each cluster is a compact group with roughly 105 members. Halo stars, known collectively as Population II, are the oldest in the Galaxy, with only 1 percent or less of the heavy element content of those in the disk (Population I). The total mass of the stellar halo is at most comparable to that of the disk, about 6 x 1010 Me. Finally, there is evidence for another nonplanar component, the unseen dark halo. The composition and spatial distribution of this (probably nonstellar) matter are still not known, but its total mass exceeds that of the disk and spheroid, perhaps by an order of magnitude.

What does the Galaxy look like? The solar system is embedded within the disk, so it is difficult to obtain a global view by direct observation, as opposed to theoretical reconstruction. While huge numbers of stars are visible at optical wavelengths, interstellar dust dims the light from the more distant ones. Hence, the effective viewing distance is limited to several kpc. However, red giant stars are relatively luminous and have such low surface temperatures that they emit copiously in the near infrared. Their radiation penetrates the dust and can thus be detected over much greater distances. The top panel of Figure 1.19 is a near-infrared portrait of the Galaxy obtained with the COBE satellite. Clearly evident in this remarkable image are the very thin disk and the central bulge. The bottom panel of the figure shows the more familiar, optical image.

5 We define the half-thickness Az of a planar distribution of matter as one half the ratio of total surface density to the volume density at the midplane. Alternatively, one may specify the scale height h, defined as the location where the volume density falls to 1 of its midplane value. For a Gaussian distribution of density, Az = (y/if/2) h = 0.89 h.

Figure 1.19 Two views of the Milky Way. The upper panel depicts the near-infrared emission, as seen by the COBE satellite. Beneath this is the optical image, shown at the same angular scale.

1.4.2 Spiral Arms

A true face-on view of our Galaxy is, of course, impossible to obtain, but would resemble the external galaxy M51 (NGC 5194), shown in the two panels of Figure 1.20. Here the most conspicuous feature is the presence of well-delineated spiral arms. Morphologically, M51 is a slightly later-type galaxy than the Milky Way, which has a somewhat larger nucleus and less extended arms. Other galaxies exhibit no spiral structure at all. Extreme early-type systems, the ellipticals, resemble three-dimensional spheroids rather than flattened disks.6 In addition, there is a motley crew of irregulars, typified by the small companion to M51 seen in Figure 1.20.

According to density wave theory, the arms in spiral galaxies are not composed of a fixed group of stars, but represent a wave-like enhancement of density and luminosity rotating at a characteristic pattern speed. Both stars and gas in the underlying disk periodically overtake the arms and pass through them. Notice that, in Figure 1.20, the arms are more evident in the left image, taken in blue light, whereas the central bulge shows up strongly in the righthand,

6 The designations "early" and "late," as applied to galaxies, refer to position along the Hubble morphological sequence. The same terms are used for stars, "early" ones being those with Teff higher than solar. In neither case is there an implied temporal ordering.


Near Infrared

Figure 1.20 The galaxy M51 seen in blue light (left panel) and through a red filter (rightpanel).

Note the prominence of the spiral arms in the blue image.

near-infrared photograph. The color of a galaxy is determined by the relative abundance and distribution in spectral type of its component stars. O and B stars, which have the highest surface temperature, dominate the blue image, while red giants, i. e., dying stars of relatively low mass, are better seen through the infrared filter. Apparently, the most massive stars are concentrated heavily in spiral arms.

Recall that the main-sequence lifetime of a typical O star is of order 106 yr, only about 1 percent of the rotation period of a spiral arm. Thus spiral arms must produce, at each location in the disk, a temporary increase in gas density leading to a rise in the local star formation rate. Although stars of all mass are formed, it is the exceptionally bright O and B stars that are most conspicuous in optical photographs. Once the spiral wave has passed, the rate of forming new stars drops back down to its former low level.

This picture is reinforced by looking at the gas content of spiral galaxies. As we have seen, the molecular clouds that form stars can be detected most readily through their emission in CO. Figure 1.21 is yet another view of the galaxy M51. Here, an intensity map in the 2.6 mm CO line has been superposed on an optical image in the red Ha line, produced by atomic hydrogen heated to temperatures near 104 K. It is clear that the molecular gas is indeed concentrated along the spiral arms. Note that the radio contours seen here represent only 30 percent of the total CO emission. The remainder emanates from the interarm region, but is too smoothly distributed to be detected by the interferometer used for this observation. Closer inspection reveals that the Ha arms are displaced about 300 pc downstream from those seen in CO. Since Ha mainly stems from O and B stars, the implication is that cold gas entering the arms first condenses to form large cloud structures that later produce the massive stars. For a representative velocity of 100 km s"1 for material entering the arms, the corresponding time lag is 3 x 106 yr.

Figure 1.21 Map ofM51 in the 2.6 mm line of12C16O (solid contours) andtheHa line at 6563 A (dark patches).

Right Ascension Offset Aa (arcsec)

Figure 1.21 Map ofM51 in the 2.6 mm line of12C16O (solid contours) andtheHa line at 6563 A (dark patches).

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