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Number density - dwarfs and the clouds aren't detectable...

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Number density: The distribution with equivalent with at fixed redshift is close to a power law in column density. If one counts lines to some fixed limiting equivalent width in spectra of several QSOs and converts to density per cubic Mpc in comoving coordinates (factoring out the (1+z) 3 due simply to Hubble expanion), this shows that the density drops toward recent times. The clouds are disappearing. However, they aren't all gone. HST spectra of 3C 273 show seven Lyman α systems at z < 0.15, some perhaps associated with the Virgo complex. From archival HST FOS data and Keck spectra, this plot compares the Lyman α forest at low redshift, for 3C 273, and high, showing the strong evolution of the absorbing population: Seeing these systems at low redshift allows a reasonable test for matches with galaxies near the QSO line of sight. While the Lyman α clouds vaguely follow the galaxy distribution in redshift, there are not at this point exact matches to material associated with particular galaxies, even
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Unformatted text preview: dwarfs, and the clouds aren't detectable emitters of Hα. There is strong evidence that the Lyman α lines are underabundant at redshifts close to that of the background quasar, the proximity effect , usually taken to mean that the QSO ionizes all the material within some megaparsecs too highly to see the absorption. This provides a way to estimate the ambient ionizing-radiation intensity as a function of redshift, using the better-known QSO radiation as a point of comparison and seeing how far away it makes a significant difference. Sizes: (or more precisely correlation scales): look at pairs of QSOs along similar lines of sight, appearing within a few arcseconds as we see them, and count the coincident absorption systems and those that don't match. Gravitational lenses are excellent for this. The few cases for which this is possible suggest a typical dimension of some tens of kiloparsecs....
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