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Galaxies and the Universe - Evolution of quasars

Galaxies and the Universe - Evolution of quasars - Gala ie...

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1/15/12 Gala[ieV and Whe UniYeUVe - EYolXWion of TXaVaUV 1/2 ZZZ.aVWU.Xa.edX/keel/gala[ieV/TVoeYolYe.hWml Evolution of QSOs and other Active Nuclei QSOs, some BL Lac objects, and the most powerful radio galaxies (at their redshift distances, which is an issue we've dealt with already) are luminous enough to be detected at large lookback times (beyond 80% of the Hubble time these days) and with path lengths long enough to offer attractive cosmological probes. Since (as far as we can see) they are active nuclei ( of galaxies ), we might also hope to use them as indirect indicators of galaxy evolution, and as background light sources for absorption-line studies of galaxy disks and intergalactic clouds. Major tests that have been applied include Number counts (the log N - log S test) The Schmidt V/V m test for evolution Evolution of the luminosity function with redshift Redshift evolution of the population of QSO absorption systems We deal here with space densities in comoving coordinates, so that the (1+z) ñ due to the Hubble expansion is factored out. NXmbeU coXnWV: For any set of sources with the same luminosity L and constant space density U , in the approximation of flat spacetime the observed distribution of fluxes S will have the number of sources brighter than S given by N(> S) a S -3/2 or log N(> S) = const - 1.5 log S which is why one reads of a log N - log S relation. Since this holds for any individual value of L
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