The Absorption Lines in Quasi-Stellar Objects

Abstract

The gas producing the absorption lines seen in the spectra of some high-redshift QSOs can be physically related to the QSO or it can be located by chance on the line of sight to the quasar. If it is physically adjacent to the quasar, it will be photoionized and the observed pattern of absorption lines can be produced. However, an ejection model has great difficulty explaining the sharpness of the observed absorption features compared with the large difference between absorption and emission redshift. A model in which the gas is part of the intergalactic medium or the extreme outer parts of galaxies is discussed. In this case, the gas may be collisionally ionized. To produce the observed ions C iv, N v, and O vi in absorption, the temperature must be about 10^5 °K. For solar metal abundance, it is then surprising that La is the strongest line in absorption. An attempt to enhance the La absorption by adding a cool region in which hydrogen is predominantly neutral fails as this region is photoionized by radiation from the hot region. Therefore, the metal abundance in the absorbing gas must be less than solar if the gas is not photoionized.