## Average gas distribution

To understand star formation on a galactic scale, we must know how the interstellar gas, out of which the stars will be formed, is distributed in the galaxy. We are interested in the average distributions of various constituents. By "average" we mean that we are interested only in the large-scale structure. We would like to know the radial distribution of interstellar gas. (Remember, this is not the same as M(R), which includes mass in all forms.) We would also like to know the degree to which the gas is confined to the disk. We can express this as a thickness of the disk, as determined from various constituents. We would also like to know whether the thickness is constant, or whether it varies with position in the galaxy. Finally, we would like to know if the plane of the galaxy is truly flat, or if it has some large-scale bumps and wiggles.

We first look at HI. The amount of HI doesn't fall off very quickly as one goes to larger R. For example, the mass of HI interior to R0 is about 1 X109 M0, and the mass exterior to R0 is about

It is assumed that we know R0 and Q0. Once we know Q we can use the known rotation curve to find the value of R to which the Q corresponds.

There are a few limitations to this technique. It does not work for material whose radial velocity due to galactic rotation is less than that due to the random motions of the clouds. This rules out material near longitudes of 0° and 180°, as well as material close to the Sun.

Another problem arises for material inside the Sun's orbit. There are two points along the same line of sight that produce the same radial velocity. (The one exception is the subcentral point.) Both of these points are the same distance from the galactic center, but they are different distances from us. This problem is called the distance ambiguity. We can use the rotation curve to say that the object is in one of two places, and we must then use other information to resolve the

Fig 16.10.

Radial distribution of H2, with the H2 deduced from CO observations, assuming a constant conversion from CO luminosity to mass.There is growing evidence that this conversion factor actually changes with environment, and this curve may underestimate the mass in the outer galaxy by a factor of three.Also, remember that there is a larger volume, so even a lower density of material can still translate into a significant mass. [Thomas Dame, CFA]

### Fig 16.10.

Radial distribution of H2, with the H2 deduced from CO observations, assuming a constant conversion from CO luminosity to mass.There is growing evidence that this conversion factor actually changes with environment, and this curve may underestimate the mass in the outer galaxy by a factor of three.Also, remember that there is a larger volume, so even a lower density of material can still translate into a significant mass. [Thomas Dame, CFA]

2 X 10 M0. (Of course, this larger mass is spread out over a larger volume. See Problem 16.5.) Note that the mass of HI is only about 1% of the total mass interior to a given radius. This means that the gas does not provide most of the large-scale gravitational force in the galaxy. It just responds to the gravitational effects of the stars, and whatever dark matter there is in the halo.

The abundance of H2 (Fig. 16.10) falls off more rapidly with R than does that of HI. Inside the Sun's orbit, the mass of H2 is approximately equal to that of HI, about 1 X 109 M0. Outside the Sun's orbit, the mass of H2 is about 5 X 108 M0, about one-quarter that of HI. There appears to be a peak in the H2 distribution about 6 kpc from the galactic center. This is sometimes called the molecular ring. It appears that most of the H2 seems to be concentrated into a few thousand giant molecular clouds, rather than a large number of small clouds. The distribution of molecular hydrogen H2 is generally deduced indirectly from observations of CO. There are still disagreements over how to derive the H2 abundance from the intensity of the CO emission. There is growing evidence that the conversion factor changes with galactic environment, and that the mass of the outer galaxy (like the derived mass distribution in Fig. 16.10) are underestimated by as much as a factor of three.

We generally express the thickness of the disk by finding the separation between the two points, one above and one below, at which the HI density falls to half of its value in the middle of the plane. This is called the full-width at halfmaximum or FWHM. At the orbit of the Sun, the thickness of the HI layer is about 300 pc. At R = 15 kpc, the thickness is about 1 kpc. This trend is shown in Fig. 16.11. This means that the plane becomes thicker as one goes farther out from the center of the galaxy (as depicted schematically in Fig. 16.3c). This is called the "flaring" of the galactic disk. In addition, we find that the disk isn't flat. It has a warp to it, like the brim of a hat (Fig. 16.12). This is also shown schematically in Fig. 16.3(c). The warp is most prominent outside the Sun's orbit. The bend is upward in the first and second quadrants and downward in the third and fourth quadrants.

It is also interesting to see the degree to which various constitents (atomic, molecular, ionized) are

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