No abstract

The decrease in number density of Lyman-α clouds near the background quasar is an observational result which is often called the 'proximity' or 'inverse' effect. It is thought that, for nearby clouds, the quasar's flux dominates the background radiation field, increasing the ionization state of the clouds and reducing the (observed) H I column density.In this paper we analyse a sample of 11 quasars from the literature for which accurate column density estimates of the Lyman-α lines exist. We confirm, to a significance level of more than 3 standard deviations, that the proximity effect exists. If it is related to the background flux then the intensity and evolution of the background have been constrained.Using a maximum likelihood method, we determine the strength of the extragalactic ionizing background for 2.0 < z < 4.5, taking account of possible systematic errors in our determination and estimating the effect of biases inherent in the data. If the background is constant we find that it has an intensity of 100 +50 −30 J 23 , where J 23 is defined as 10 −23 ergs cm −2 Hz −1 sr −1 . There is no significant evidence for a change in this value with redshift.

The narrow-line region (NLR) of the Seyfert 2 galaxy NGC 3393 is dominated by a symmetric structure which appears as S-shaped arms in Hubble Space T elescope (HST ) images. These arms, which occupy the central few arcseconds of the nucleus, border a linear, triple-lobed radio source. We use HST imaging and spectra, ground-based optical images, long-slit spectra, Fabry-Perot imaging spectroscopy, and VLA radio data to perform a detailed investigation of the kinematics and ionization of the lineemitting gas in NGC 3393 and of its relationship with the relativistic gas responsible for the radio emis-, 6584) shows a biconical structure, consistent with the anisotropic nuclear ionizing radiation expected in the uniÐed scheme. Extrapolation to ionizing frequencies of our upper limit to the 2100 Ñux of the nuclear source provides a factor º3 ] 104 too A few ionizing photons to account for the recombination line emission, which also suggests that the nuclear ionizing source radiates anisotropically. However, the kinetic energy of the outÑow is sufficient to power the line emission via photoionizing shocks, and a tentative detection of extended UV emission is consistent with this model. Furthermore, the broad component of the emission lines has a similar orientation and spatial extent as the triple radio source. Nevertheless, other tests are inconsistent with the photoionizing shock modelÈthere is no correlation between local velocity dispersion, surface brightness, and excitation, and the gaseous abundances of [Ca II], Al II], and Mg II are much lower than expected if these species have been liberated into the gas phase through grain destruction by shocks. We conclude that the radio lobes appear to have created denser regions of gas on their leading edges, thus forming the S-shaped arms, but that the ionization is most likely due to photoionization by an obscured central source.