Galaxies and the Universe - Star Formation in Galaxies

Galaxies and the Universe - Star Formation in Galaxies -...

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Unformatted text preview: 1/15/12 Gala[ieV and Whe UniYeuVe - SWau FoumaWion in Gala[ieV 1/7 ZZZ.aVWu.Xa.edX/keel/gala[ieV/Vfu.hWml Star Formation in Galaxies Star formation is galaxy evolution caught in the act. Measurements of the star-formation rate (SFR) usually apply strictly only to OB stars and may be extrapolated to all masses, since less massive ones can't be distinguished from the older background population. Some observational indicators of star formation include: (1) Balmer and other recombination emission lines (2) Ultraviolet continuum emission from hot stars (3) Infrared thermal emission (reradiated starlight) (4) Radio continuum emission (free-free and nonthermal) (5) CO emission from molecular clouds (6) Integrated constraints from total optical luminosity. RecombinaWion lineV (especially H): Line emission is characteristic of H II regions, zones of ionized gas around young star clusters that still contain OB stars (spectral types B0.5 and hotter). These stars are special in that they are hot enough to produce significant fluxes of ionizing radiation, shortward of the Lyman limit. H II regions may be found wholesale by narrow-band imaging at H if internal extinction is not too large, and also by radio surveys for recombination-line emission (though this works best in our own galaxy). To turn measures of emission-line intensity into star-formation rate, first consider the physics of photoionization and recombination. I use the notation from Osterbrock in Astroph\sics of Gaseous Nebulae and Active Galactic Nuclei , which will be no great surprise to anyone looking at my graduate institution. Hydrogen is ionized by absorption of Lyman continuum photons ( 912 , energy above 13.6 eV). The line radiation we detect arises from recombination of the electrons so released with another proton, and a cascade toward the ground state. This occurs with a recombination rate , such that in terms of the electron velocity distribution f(v) and cross-section for capture to quantum state n, 2 L given by {nL . We deal only with the state 2 L since cascades to here occur very rapidly. The velocity distribution may be taken as a thermal (Maxwellian) form in the electron temperature T (in fact defining this temperature): because, even though the electrons are released with a different f(v) , Coulomb interactions with ions (predominantly protons) thermalize their velocities on a timescale short compared to the recombination time. In the so-called nebular approximation, the atom is undisturbed after recombination, so that the subsequent radiative cascade is governed only by the radiative decay coefficients (Einstein A values); that is, the cascade is rapid compared to the photoionization timescale. Then the total capture coefficient (to all quantum states) is A = n n and the recombination time is 1/15/12 Gala[ieV and Whe UniYeuVe - SWau FoumaWion in Gala[ieV 2/7 ZZZ.aVWu.Xa.edX/keel/gala[ieV/Vfu.hWml for N e in cm-3 . At high Lyman optical depth (in most H II regions, for example), the cascade stops at ....
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Galaxies and the Universe - Star Formation in Galaxies -...

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