osmology with Active Nuclei as Probes It has long been seductive to try probing the world model of cosmology by trying to use AGN as standard candles or standard measuring rods. Radio galaxies seen at 2.2 microns have a very well-behaved Hubble diagram, and for a while it appeared that radio QSOs with flat-spectrum core sources showed a neat relation between luminosity and equivalent width of C IV λ 1549 emission (the Baldwin effect , as in Nature 273, 431, 1978, which seems to have been the result of sheer bad luck in object selection and has just about gone away with recent data collections). We can also use AGNs to probe the properties of galaxies along our line of sight to them, galaxies that are in many cases individually impossible to detect directly, by studying their influence on the propagation of light from the distant AGN. This may be in the form of gravitational lensing (which we've already treated) or absorption-line systems . Aside from the Lyman α forest, these include metal-line systems, damped Lyman
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This note was uploaded on 11/10/2011 for the course AST AST1002 taught by Professor Emilyhoward during the Fall '10 term at Broward College.