Three main types:
- Radio Galaxies
- usually elliptical galaxies
- strong radio emission (millions of times more than
normal galaxies)
- radio comes from core and from lobes extending beyond visible galaxy
- Figure 16.28 shows this as well [Link to Figure 16.18]
- most have strong redshifts
- most are far away
- jets of gas ejected from core
- (BL Lac objects are probably radio galaxies with one lobe pointed toward us)
- Seyfert Galaxies
- spiral galaxies
- very luminous core at all wavelengths
- luminosity of entire Milky Way from a region a few parsecs across
- most have strong redshifts
- most are far away
- a few are relatively close
- luminosity of core changes rapidly (minutes)
- core must be relatively small
- contain rapidly moving jets of hot gas ejected from center (see bright emission spectra)
- Quasars
- QUAsi Stellar Radio Sources
- look somewhat like stars in a telescope
- newer images suggest they are the centers of galaxies
- very strong emission lines
- very strong redshifts (very far away - up to 10 billion
light years)
- none are detected close to us
- only see quasars when we look back in time
- Observe some jets of gas ejected from core
- very luminous
- 10 - 10,000 x brighter than the entire Milky Way
- luminosity changes rapidly (hours)
- core must be relatively small
Source of activity:
lots of energy in a very small region
- might be accretion disk surrounding a large black hole
(hundred million solar masses!)
- Figure 16.24 shows a picture of one [Link to Figure 16.24]
- magnetic field around black hole could channel some gas into jets shot out along the poles (like a neutron star/pulsar)
- while the black hole is growing, it has a significant accretion disk which produces lots of luminosity.
- eventually the black hole uses up the available gas and stars - activity dies off
- so we only see active galaxies if we look back in time => large distances
Life stories:
Perhaps ???
- quasars are common early stages in the life of galaxies
- quasars develop into
- Seyfert galaxies (spirals)
- Radio galaxies (ellipticals)
- which develop into normal galaxies
Using Quasars:
- very bright and very distant
- easy to see, but their light must pass through or near lots of stuff
- look at absorption lines in quasars to see cool dust clouds between galaxies
- observe gravitational lens effects if quasar is behind
another galaxy (see multiple images or rings)
- Figure 16.26 shows this [Link to Figure 16.26a], [Link to Figure 16.26b]
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