Chapter_8 - Chapter 8 Active Galaxies As briefly mentioned...

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Chapter 8 Active Galaxies As briefly mentioned in section 7.2, an active galaxy is a galaxy in which much of the light is nonstellar, and much of the light comes from a small central nucleus. Active galaxies have many distinctive attributes. Active galaxies are high in luminosity ( L > 3 × 10 10 L fl ). Dwarf galaxies are never active galaxies. Active galaxies produce large amounts of nonthermal emission . That is, they have more radio and x-ray emission than you’d expect if all their light came from the hot photospheres of stars. Active galaxies have much of their light concentrated in a small central nucleus . Active galactic nuclei are variable on short time scales ( t 1 month). Active galactic nuclei have jets leading away from them, seen at both radio and visible wavelengths. Active galactic nuclei have strong emission lines in their spectra. These emission lines can be either extremely broad ( v 10 , 000 km s - 1 ) or relatively narrow ( v 300 km s - 1 ). In order to be labeled as an active galaxy, a galaxy need not have every attribute listed above; it’s more like a “choose any 4 out of 6” proposition. Moreover, even seemingly normal galaxies, like our own, have central nuclei 181
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182 CHAPTER 8. ACTIVE GALAXIES where interesting and highly energetic phenomena are taking place. To un- derstand active galactic nuclei, therefore, let’s start close to home, by taking a journey to the center of our galaxy. 8.1 The Nucleus of Our Galaxy The center of our galaxy is 8000 parsecs away from us, in the direction of Sagittarius. At a distance of 8000 parsecs, an angle of 1 arcsecond corre- sponds to a distance of d = 8000 AU = 0 . 039 pc. In the V band, there are A V = 28 magnitudes of extinction between us and the Galactic center, which pretty well rules out observations at visible wavelengths. However, at in- frared wavelengths of λ 2 μ m, there are only 2 magnitudes of extinction. Adaptive optics (mentioned in section 7.7 of BA ) permits viewing the Galac- tic center at infrared wavelengths with a resolution of 0 . 1 arcsec, as shown in Figure 8.1. This permits us to resolve structures as small as d 800 AU at Figure 8.1: Stars in a region 0.5 parsecs on a side, near the Galactic center. Left image is without adaptive optics; right image is with adaptive optics. the Galactic center. The stars in the infrared image of Figure 8.1 are mostly cool giants. If we assume that the ratio of giants to main sequence stars is the same at the Galactic center as in our neighborhood, we deduce that the number density of stars within a parsec of the center is n ? 10 7 pc - 3 .
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8.1. NUCLEUS OF OUR GALAXY 183 For comparison, the number density of stars in the solar neighborhood is n 0 . 1 pc - 3 . If the Sun were half a parsec from the Galactic center, The nearest star would be 1000 AU away.
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