Galaxies and the Universe - Galaxy Formation

Galaxies and the Universe - Galaxy Formation - 1/15/12 Gala...

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1/15/12 Gala[ieV and Whe UniYeuVe - Gala[\ FoumaWion 1/3 ZZZ.aVWu.Xa.edX/keel/gala[ieV/galfoum.hWml Galaxy Formation Like anything else in the universe, we would like to know how galaxies got that way. There are starting to be observational limits on the redshift of galaxy formation from both directions - stellar evolution and homogeneity of the microwave background. Note again that an observer's and theorist's times of galaxy formation may differ. As Peebles (1989, in The Epoch of Gala[\ Formation , Kluwer, p. 1) remarks, an observer would likely date galaxy formation when most of a galaxy's stars are born, while a theorist would time the process from the collapse of a substantial portion of a galaxy's present-day mass out of the Hubble flow. Thus a galaxy might be young in stellar terms but dynamically well-evolved. As just discussed under galaxy evolution , there are several galactic chronometers that may run at different rates for various kinds of galaxy. The main things we need to account for are the ages of galaxies, their masses, and the distinction between bulges and disks. In a sense, galaxy formation is like star formation (which we don't understand either) on a larger scale, with the added complication of a superimposed Hubble flow. Given the existence of a density enhancement (we'll come back to this), a mass will collapse if it exceeds the equivalent of the Jeans mass. In fact, as Zel'dovich and Novikov show ( Relativistic Astroph\sics , vol. 2, U. Chicago, p. 244), the Jeans theory is a remarkably good approximation of the relativistic result. In this case, a perturbation will lead to collapse if it has wavelength sufficiently large: Λ > Λ J = 27 ( T /μ ) 1/2 ȡ 0 1/2 parsecs, and consequently mass greater than the Jeans mass M > M J = 33 ( T / μ 3/2 ȡ 0 -1/2 in solar masses. In these expressions, μ is the mean molecular weight (1 for neutral hydrogen, 1/2 for ionized hydrogen, and so on), the temperature is in K, and ȡ 0 is the unperturbed density in units of 10 -24 g/cm³. In more useful units, Longair gives the conversion M J = 3.75 × 10 18 / Ω B h²) in solar masses, for the case dominated by ordinary matter. In dealing with density perturbations, it is usual to do the equivalent of Fourier decomposition and treat the
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Galaxies and the Universe - Galaxy Formation - 1/15/12 Gala...

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