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Emission Lines and Models for AGN

Emission Lines and Models for AGN - Emission Lines and...

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Emission Lines and Models for AGN The very existence of strong line emission tells us that there must be strong UV radiation from the AGN, and the line ratios tell us that its spectral shape is quite different from a stellar distribution. The line profiles give an obvious distinction between the narrow-line region (NLR) and the broad-line region (BLR) (though there is of course a transition zone, and these are extremes of a continuous gas distribution). The kinematic distinction is mirrored in the densities of these regions; only transitions emitted at high densities (up to 10 9 cm -3 ) come from the NLR. We have a general picture of a compact, dense BLR huddled around the core source, and a much more extensive NLR surrounding this and extending in some cases for kiloparsecs. The NLR is spatially resolved in many AGN, with interesting spatial and kinematic structure. Even though it is more distant from the seat of the action, the low density makes classical H II region physics applicable and the interpretation of conditions there less model-dependent than for the BLR.
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