Returning to the molecular gas, the CO line emission commonly used to trace this component has a shorter wavelength than the 21 cm HI line and so provides higher spatial resolution. Numerous surveys have established that the molecular gas is also spread throughout the Galaxy and mostly contained in discrete clouds. A rotationally excited CO molecule decays much more quickly than an HI atom in the F = 1 state. Hence CO is intrinsically a stronger emitter. So much radiation is produced, however, that most molecular clouds are optically thick to the 2.6 mm line, i. e., a photon is absorbed and reradiated many times before reaching the surface. Under these circumstances, the received intensity from a single cloud is not proportional to the column density. Nevertheless, the line is still useful for tracing the total H2 contained in a gravitationally bound ensemble of clouds, as we will discuss in Chapter 6.
The global distribution of H2 is quite different from HI (see Figure 2.3). There is little molecular gas outside 10 kpc, but the surface density inside climbs quickly, reaching a broad maximum centered at 6 kpc. The origin of this massive molecular ring is uncertain, but is presumably related to Galactic spiral structure. The falloff in density inside the ring is broadly consistent with the decline in the measured star formation rate. Thus, the sharp rise in molecular density within 1 kpc of the Galactic center is especially intriguing. Whatever the origin of this gas, observations show that it is producing large numbers of massive stars. Note finally that the inward rise in star formation activity near the Sun's position is consistent with the fact that the observed metallicity in both stars and gas has a measured radial gradient in the same sense.
Molecular gas is more closely confined to the Galactic plane than the atomic component, with a scale height of 60 pc at the solar position, or about half the HI value. While the atomic gas fills much of the disk up to its nominal scale height, the molecular component occupies only about one percent of the available volume. The difference in scale heights reflects the lower random velocity of the molecular cloud ensemble. This velocity has a median value of 4kms~1 and seems to vary little with either cloud mass or Galactocentric radius. The dominant gravitational force toward the plane is that from the stars, whose surface density falls steeply with radius. Hence the H2 thickness should increase outward, a conclusion confirmed by the CO surveys.
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