lecture notes9

Lecture notes9

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Unformatted text preview: ± Δν D ln τ Lyα . This implies a double ­peaked line profile. This is observed in the spectrum of some extragalactic objects2. € An ionized gas that is undergoing expansion (e.g. a stellar wind, the early Universe, …) can undergo additional processes due to the redshifting of the Lyman ­α line, which opens up another escape channel. For an optically thick medium, expanding with a velocity field v = Hx, a photon escapes after scattering ~τS times, where τS is the Sobolev optical depth: 3n (H 0 ) λ3 α ALyα Ly . τS = 8πH (This can be derived by integrating the cross section times H0 density over distance travelled, where “distance” s is now related to frequency by dν/ds = −H/λ.) € 2 See e.g. the z=2.8 Lyman ­α emission object that is proposed to be due to recombination radiation from a nearby quasar (Adelberger et al 2006, ApJ 637, 74). 8 € A key feature of expanding models is that the radiation is trapped by multiple scatterings until it is on the red side of the line, at which point it is released and propagates freely. The geometry and velocity structure of the system are clearly of critical importance in modeling the shape of the Lyman ­α line. B. HELIUM PROCESSES Helium undergoes similar radiative transfer processes in its main resonance lines (He II 304 Å; He I 584 Å). These are not directly observable because of H I absorption in the intervening neutral ISM. A more interesting case is that the metastable triplet helium level (lifetime 2 hours) can develop a large population, thereby enabling radiative transfer physics in the lines that o...
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This document was uploaded on 03/08/2014 for the course AY 102 at Caltech.

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