Galaxies and the Universe - Emission -Line Spectra

Galaxies and the Universe - Emission -Line Spectra -...

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1/15/12 Gala[ieV and Whe UniYeuVe - EmiVVion -Line SpecWua 1/8 ZZZ.aVWu.Xa.edX/keel/gala[ieV/emiVVion.hWml Interpreting Emission-Line Spectra Because various emission lines sample different regimes of temperature, density, and ionization, emission spectra are uniquely powerful probes of conditions around active nuclei. Remember, though, that they do sample conditions around the central engine, which we can study only as it affects the surrounding gas. Important references on emission-line processes are Osterbrock, Astrophysics of Gaseous Nebulae and Active Galactic Nuclei (University Science Books); Spitzer, Physical Processes in the Interstellar Medium (Wiley-Interscience); Aller, Physics of Thermal Gaseous Nebulae , and for specifics of AGN applications, Davidson and Netzer 1979 (Rev. Mod. Phys. 51, 715) and Peterson's AGN textbook . Review of the recombination theory in the star-formation lecture will be helpful. Every state of an atom or ion has quantum numbers of importance here as follows: Each ( l,s ) pair has multiplicity g = 2s+1 - the number of energy levels with distinct j . For historical reasons, different values of l=0,1,2,3 are denoted in spectroscopic notation by S,P,D,F (strong, principal, diffuse, faint), then G,H,. .. A spectroscopic term (atomic state) is denoted by n g L J . For any possible transition between two states m,n there exist the Einstein transition coefficients A mn , B mn , B nm . These give the probabilities of transition between these two states. A mn is the spontaneous transition rate per unit time per atom, formally given by in the first-order (electric dipole) expansion. The B values - the stimulated emission coefficient and the absorption coefficient - are related as may be shown by considering thermodynamic equilibrium and detailed balance: Here, for incident radiation intensity at the frequency Ȟ mn equal to U Ȟ , the probability of an induced downward transition, or stimulated emission, is B mn U Ȟ . Transitions with Δ l = ± 1 or Δ s = ± 1 are dipole (permitted) transitions. Other kinds, via electric quadrupole or magnetic dipole moment, are possible but have generally much lower A mn ; hence the term forbidden transitions. The upper levels in these cases are said to be metastable
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1/15/12 Gala[ieV and Whe UniYeuVe - EmiVVion -Line SpecWua 2/8 ZZZ.aVWu.Xa.edX/keel/gala[ieV/emiVVion.hWml because of their relatively long lifetimes. Application of these may be seen in a schematic few-level atom; these are all part of the "ground state" n=0 . Only 4-5 of the possible transitions have high enough probabilities to be observed in a realistic case, satisfying the appropriate selection rules. . An electron may reach an upper level for any of a number of reasons: photoionization from a lower level collisional excitation cascade from a higher level (as in masers) recombination to an excited state two-electron processes Various of these are important for different ions. Having already discussed pure recombination, we still need to
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This note was uploaded on 01/15/2012 for the course AY 620 taught by Professor Williamkeel during the Fall '09 term at Alabama.

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Galaxies and the Universe - Emission -Line Spectra -...

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