The Origin of Dust Grains and of Giant Molecular Clouds

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In order to better understand the distribution of dust in our masks, some key clues are provided by studying galaxies which are orientated "edge-on." An outstanding example is the Sombrero Hat Galaxy. Optical images show its huge central bulge cut by a dramatic lane of cosmic dust which lies in the disk of this galaxy (see Figure 169).

In the infrared, the dust becomes transparent and the dust lane disappears: astronomers see only the bulge and the bright disk of stars. In spiral galaxies that are seen perfectly edge-on, one cannot see the spiral arms. They lie forever obscured in the disk, seen at ninety degrees from our vantage point. The dust, which lies in a thin flat layer in the plane of the disk, absorbs much of the light from the disk. All we therefore see in optical images is the thin dust lane neatly bisecting the bright stellar disk. As we image spiral galaxies at longer and longer wavelengths, the dust becomes less and less opaque as we penetrate their masks.

When astronomers actually penetrate the mask of edge-on spiral galaxies, the dust is almost transparent, so that the dust lane disappears and a bright disk of stars emerges from behind the interstellar dust.

In spirals like our Milky Way which are actively forming stars, we know that the dust layer is very thin (only 500 light years in thickness) and is closely tied to the thin layer of gas in these spirals. So Shrouds of the Night in such examples are very thin indeed. The gas simply appears to have settled down into a thin layer and has taken the dust with it.

What about the dust in galaxies in which there is essentially no gas? Some of the edge-on galaxies that we have studied are the lenticular class. These galaxies are technically known as S0 galaxies. Lenticular galaxies have flat disks (like spiral galaxies) but their disks have no real spiral structure or gas layer, and their star formation has ceased. In these galaxies, we observe that the dust lane is much thicker, about 1000 light years in thickness, similar to the thickness of the disk of old stars. It is evident that some Shrouds of the Night Shrouds of the Night can be spatially much thicker than in our Milky Way, especially in galaxies devoid of star

312 formation.

In optical images, the dust in these edge-on lenticular galaxies makes the disk of stars disappear completely. As we image these lenticular galaxies at longer wavelengths and the dust becomes transparent, one suddenly sees a bright disk of stars in place of the dark dust lane. What does this say about the origin of the dust in such systems which are not forming stars?

The thickness of our dust Shrouds in such examples is clearly inextricably linked to the thickness of the disk of old stars, and not of the gas, in these relatively gas-free galaxies. This is quite plausible, because, as discussed in an earlier chapter, cosmic dust grains are believed to be produced by cool old stars, such as carbon stars and other highly evolved stars. The thickness of the dust and of the disk of old stars is very similar, because the dust grains actually originate in the atmospheres of the old stars themselves. The stars are producing their own smoke-screen as they die.

While we now know that cosmic masks come in a variety of thicknesses, how do the grains of cosmic dust from the atmospheres of cool stars actually get organized into giant clouds of molecules, from which young stars are birthed?

The whole story of the production and of the time development of dust grains still requires a great deal of research. Astronomer George Herbig stresses this very point, when we asked him to cast his eye forward to some of the outstanding issues still to be solved. He asks a simple, but profound, question: "Where does the material that we see in molecular clouds come from, and how does it become organized in that fashion?"

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