The H I column density distribution function of QSO absorption line systems is investigated using recent data with high spectral resolution, and extensive surveys of the Lyman limit systems and damped Ly alpha systems.

We present high spectral resolution (R similar to 15 000) and high signal to noise ratio (20-50) of two z(em) similar to 2 quasars PKS 0424-131 and Q 0450-131 in the wavelength range 370-510 nm.

We discuss a method to detect or put upper limits on our motion relative to a rest frame provided by point sources at high redshift, and we apply the technique to real and artificial redshift surveys of intergalactic gas clouds (Lyman alpha absorption systems). The redshift regime that can be sampled this way extends from z approximately 0 (UV observations) right up to the redshifts of the most distant QSO (z approximately 5 at present). The detection of our motion with respect to this high-z frame is based on the direction-dependent changes in the number of objects per redshift interval, as induced by the Doppler effect. The presently available data samples (not optimal for our purpose) enable us to establish an upper limit upsilon/c < 0.05 for the motion of the Solar system relative to the Ly alpha forest at mean redshift = 2.9.

If gravitational clustering is a hierarchical process, the present large-scale structure of the galaxy distribution implies that structures on smaller scales must have formed at high redshift. We simulate the formation of small-scale structure (average cell mass: Delta $($) over bar$$ m(b) = 104.2 M.) and the evolution of photoionized gas, in the specific case of the CDM + Lambda model. The photoionized gas has a natural minimal scale of collapse, the Jeans scale (m(b,J) similar or equal to 10(9) M.). We find that low column density (N-HI less than or equal to 10(14) cm(-2)) lines originate in regions resembling Zel'dovich pancakes, where gas with overdensities in the range 3-30 is enclosed by two shocks but is typically reexpanding at approximately the Hubble velocity. However, higher column density lines stem from more overdense regions where the shocked gas is cooling. We show that this model can probably account for the observed number of lines, their distribution in column density and b-parameters, as well as the cloud physical sizes as observed in gravitationally lensed quasars. We find a redshift evolution that is too steep; however, this may be due to insufficient dynamical range in the simulation or because the specific model is incorrect. The model predicts that high signal-to-noise observations should find systematic deviations from Voigt profiles, mainly in the form of broad wings in the line profiles, and that a fluctuating Gunn-Peterson effect will be detected, which can be modeled as a superposition of weak lines with a wide range of b-parameters.

We present spectra of 3C 273 between 1216 and 1250 Angstrom obtained in the (pre-COSTAR [Corrective Optics Space Telescope Axial Replacement instrument]) configuration of the GHRS, taken with the G160M grating, with a resolution of approximate to 20 km s(-1). The two strong Ly alpha lines at velocities of similar to 1000 and similar to 1600 km s(-1) are well fitted with Voigt profiles and yield column densities, Doppler parameters and redshifts of log N(H I) = 14.19 +/- 0.04, V-Dop = 40.7 +/- 3.0 km s(-1), V = 1012.4 +/- 2.0 km s(-1), and log N(H I) = 14.22 +/- 0.07, V-Dop 34.2 +/- 3.3 km s(-1) and V = 1582.0 +/- 2.0 km s(-1), respectively. Motivated by the initial announcement by Williams and Schommer of detectable Ha emission associated with the similar to 1600 km s(-1) cloud, we discuss the difficulty of finding models which can account for emission of that magnitude given the observed neutral hydrogen column density, though a recent reobservation by these authors has shown the initial detection to be spurious. The C/H abundance ratio is probably less than about one-fourth of the solar abundance in these clouds, although this result is very uncertain and model dependent.

We have obtained Halpha observations with the Maryland-Caltech Fabry-Perot Spectrometer attached to the Cassegrain focus of the 1.5 m telescope at Palomar Observatory in order to set limits on the number of ionizing photons from the local metagalactic radiation field. We have observed the SW component of the Haynes-Giovanelli cloud H I 1225 + 01, an intergalactic cloud which should be optimum for measuring the metagalactic flux because it is nearly opaque to ionizing photons, it does not appear to be significantly shielded from the metagalactic radiation field, and the limits on embedded or nearby ionizing sources are unusually low. For the area of the cloud with an H I column density greater than 10(19) cm-2 we set a 2 sigma limit of 1.1 x 10(-19) ergs cm-2 s-1 arcsec-2 (20 mR) for the surface brightness of diffuse Halpha. This implies a 2 sigma upper limit on the incident one-sided ionizing flux of PHI(ex) < 3 x 10(4) cm-2 s-1. For a radiation field of the form J(nu) approximately nu-1.4, this yields a firm 2 sigma upper limit on the local metagalactic photoionization rate of GAMMA < 2 x 10(-13) s-1, and an upper limit for the radiation field J(nu) at the Lyman limit of J(nu0) < 8 x 10(-23) ergs cm-2 s-1 Hz-1 sr-1. We discuss previous efforts to constrain the metagalactic ionizing flux using Halpha surface brightness observations and also other methods, and conclude that our result places the firmest upper limit on this flux.

We describe the properties of the Ly alpha forest in the column density range N(H1)greater than or equal to 2 X 10(12) cm(-2) based on 1056 lines in the wavelength range 4300-5100 Angstrom measured in extremely high S/N, R=36 000 spectra of four quasars. The column density distribution is well described by a -1.5 power law to 2 X 10(12) cm(-2) below which limit confusion becomes too severe to measure a spectrum of individual clouds. The distribution of b values shows a well defined lower envelope with a cutoff at b=20 km s(-1) corresponding to a cloud temperature of 24 000 K, There is only a very small fraction (less than 1%) of narrow line clouds which cannot be identified with metal lines, From modeling the Ly alpha absorption lines as complexes of clouds each with thermal broadening corresponding to b, we find the b distribution can be understood if there is a mean of 3.25 clouds per absorption line with a spread in velocity centroids characterized by a dispersion of 10.75 km s(-1). (C) 1995 American Astronomical Society.

The redshift z(abs) = 3.08 absorption complex towards the z(em) = 3.12 quasar 0420 - 388 has high H I column density components with heavy element abundances similar to 1/10 solar. A low-redshift component of the complex has H I column density similar to 10(18) cm(-2), and some of the higher order Lyman lines are clean enough to permit a measurement of the deuterium column density for this component. However, while the putative deuterium column density is fairly well-determined, the H I column density is very uncertain so, if the deuterium identification is correct, the D/H ratio for this component could have any value > 2 x 10(-5). The D I/O I ratio is much better constrained, and has a value similar to 2. If the O I/H I ratio is constant throughout the complex, then D/H similar to 2 x 10(-4).

We have searched for low surface brightness (LSB)galaxies near the line of sight to QSO 3C 273 in two mosaic images covering a maximum area of 53' x 52' on the sky down to a limiting central surface brightness of 26.4 (in Gunn r). From trials to detect simulated LSB galaxies of various sizes and appearances, we conclude that known types of such objects cannot be causing the two absorption lines at the lowest redshifts (cz = 1012 km s(-1) and cz = 1582 km s(-1)) if within 106 h(75)(-1) kpc and 164 h(75)(-1) kpc, respectively, of the line of 75 sight. If galaxies are responsible for these two clouds, they must be either fainter than our detection limits or lie beyond the edges of our images. Galaxies close in velocity to one or the other of the two absorption systems occurring out to at least 1.1 h(75)(-1) Mpc (projected transverse separation) are found from the literature. 75 We find mounting evidence that many Ly alpha absorption systems are tracing the same large-scale structures as galaxies although it is not clear whether in the majority of cases there is a physical connection between absorbers and galaxies other than association in redshift. Coherent absorption on Mpc scales is unlikely to be associated with a single galaxy.