Interacting galaxies and QSO absorption lines

Abstract

THE spectra of quasi-stellar objects are observed to have absorption lines with redshifts mostly below the redshift of the emission lines¹. The absorbing clouds are thought to be either near the QSO^1–5 or at cosmological distances from it^6–8. On the first interpretation, the clouds are supposed to be accelerated to high velocities (≲ 0.5c away from the QSO) by radiation pressure. However, the small internal velocity dispersions inferred for the clouds from the absorption line widths (∼ 50 km s⁻¹) pose serious problems to this model⁹ and we consider here the second interpretation. The interception cross section of intervening galaxies for QSO photons has hitherto been calculated¹⁰ on the assumption that the galaxies do not evolve. The data so far available (Fig. 1), although probably contaminated by selection effects¹, seem to indicate that the number of absorption redshift systems increases more rapidly as a function of the emission line redshift than can be accounted for without such evolution. The possibility has been considered^7,8 that the intervening clouds are collapsing protogalaxies, but we think it likely that most galaxies form¹¹ before z = 5 and, instead, attribute the rapid increase in the number of redshift systems to tidal interaction of galaxies at earlier epochs. Because galaxies were closer together in the past, chance collisions were more frequent and the effects of these interactions were more pronounced.