Some galaxies appear to be giving out excessive amounts of radiation in the infrared. When we studied star formation in Chapter 15, we saw that regions with recent star formation give off a lot of infrared radiation. The energy comes from the newly formed stars, and heats the dust (from the parent cloud) surrounding the stars. The dust then glows in the infrared. The more energy the young stars put into the cloud, the more infrared radiation is released. The excess infrared radiation from some galaxies suggests that those galaxies have very high rates of star formation. The rate is so high that it cannot be sustained for very long, or it would use up all of the interstellar material. This leads to the idea that this excessive star formation is a short-lived phenomenon. We therefore call such galaxies starburst galaxies.
Fig. 19.1 shows a typical starburst galaxy. In Fig. 19.1 (a), we see an optical image of the whole galaxy. Notice that it doesn't look very unusual in this optical image. There does appear to be a lot of obscuration from dust near the center. In Fig. 19.1 (b), we have a combined image, which shows various tracers of massive star formation (including UV, IR and Ha). We see a small ring of bright young stars, close to the center. The dust lanes that we see correspond to giant molecular clouds.
In Fig. 19.2, we see a spectrum of a starburst galaxy M82. Notice the large number of emission lines. These generally signify the presence of hot gas.
In Chapter 15, we saw that star formation takes place in molecular clouds. We detect molecular clouds by emission from carbon monoxide (CO) in the millimeter part of the spectrum. If the strong infrared emission is really telling us that there is a lot of star formation taking place, then we should be able to 'see' the molecular clouds in which the stars are forming. We 'see' them by looking for the CO emission from those clouds. Indeed, this CO emission is observed. By comparing the CO maps with far infrared maps, we see that the CO and far infrared emissions are strongest in the same place. This is basically what we see in our galaxy, when we have star formation in a molecular cloud (as depicted in Fig. 15.2). This suggests that starburst galaxies really do have a lot of star formation in complexes of molecular clouds.
We can form a better picture of the individual star forming regions by looking at near infrared images. The near infrared image has two advantages: (1) since the wavelength is shorter than the far infrared, it corresponds to hotter temperatures, and allows us to isolate hotter objects; (2) because of the shorter wavelength, we
Images of the starbust galaxy NGC 4314, at a distance of 13 Mpc. (a) A normal image of the whole galaxy, a barred spiral. (b) An HST image of the central region.This is a composite of images taken in through ultraviolet, blue, visible, infrared and Ha filters. It shows that most of the recent star formation is in a small ring about the center of the galaxy.We also see dust lanes inside the ring, showing the locations of the giant molecular clouds. [(a) McDonald Observatory; (b) STScl/NASA]
can produce images with better angular resolution than at longer wavelengths. These near infrared images show clusters of recently formed massive stars.
How do we know that the stars that have formed are truly massive? That is, how do we know that the emission comes from some number of massive stars, rather than a larger number of less massive stars? We know that after a relatively short time, massive stars must undergo supernova explosions (Chapter 11). This means that some number of the stars that have formed should have had time to go supernova. These should no longer be visible as stars, but as supernova remnants. As we saw in Chapter 11, super-
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