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.

We present a high S/N ratio optical spectrum of the bright quasar HS 1700+6416. These data usefully complement the UV HST data from Vogel & Reimers (1995). We analyse the metal line systems using photo-ionization models allowing for inhomogeneities in the gas. The models are able to reproduce within a factor of two the large N(He I)/N(H I) ratio together with the mean column densities of the heavy element species observed in the z similar to 2 systems. The density contrast between low and high density regions is of the order of 30. Although the [O/C] abundance ratio seems slighly larger than solar, firm conclusion should await higher spectral resolution data. A break at the He II ionization limit of a factor of ten is acceptable in the ionizing spectrum. Abundances are found to be about 0.08 Z..

Measurements of absorption pattern differences produced by high redshift galactic and intergalactic gas on multiple lines of sight to gravitationally-lensed QSOs can be used to investigate the nature and time scales of hydrodynamic disturbances in early galaxies and the intergalactic medium (IGM). The relative differences between the absorption systems as a function of projected separation on the sky constrains the rate of energy input into the IGM and the frequency of recurrent star forming events at high redshift. Both the amplitude of the turbulence and the coherence length of the clouds are consistent with the clouds being produced, stirred, or destroyed by star formation or merger-triggered gasdynamics on a time scale of 10(7)-10(8) yr. The results are based on a survey of absorption systems in lensed z similar to 3 QSOs with Keck HIRES.

Keck HIRES spectra with a resolution of 6.6 or 4.3 km s(-1) were obtained of the separate images of three gravitationally lensed QSOs, namely, Q0142-0959 A,B (UM 673 A,B; z(em) = 2.72), HE 1104-1805 A,B (z(em) = 2.32) and Q1422-231 A,C (z(em) = 3.62). The typical separation of the images on the sky is similar to1". The corresponding transverse distances between the lines of sight range from a few tens of kiloparsecs at the redshift of the lens to a few parsecs at the source. We studied the velocity differences and column density differences in C IV doublets in each QSO, including single isolated doublets, complex clumps of doublets, and subclumps. Unlike the low-ionization gas clouds typical of the interstellar gas in the Galaxy or damped Ly alpha galaxies, the spatial density distribution of C IV absorbing gas clouds turns out to be mostly featureless on scales up to a few hundred parsecs, with column density differences rising to 50% or more over separations beyond a few kiloparsecs. Similarly, velocity shear becomes detectable only over distances larger than a few hundred parsecs, rising to similar to 70 km s(-1) at a few kiloparsecs. The absorption systems become more coherent with decreasing redshift distance to the background QSO; this finding confirms that all three QSOs used are indeed lensed, as opposed to being genuine QSO pairs. The amount of turbulence in C IV gas along and across each line of sight was measured and a crude estimate of the energy input rate obtained. The energy transmitted to the gas is substantially less than in present-day star-forming regions, and the gas is less turbulent on a given spatial scale than, e.g., local H II regions. The quiescence of C IV clouds, taken with their probable low density, implies that these objects are not internal to galaxies. The C IV absorbers could be gas expelled recently to large radii and raining back onto its parent galaxy, or pre-enriched gas from an earlier (Population III?) episode of star formation, falling into the nearest mass concentration. However, while the metals in the gas may have been formed at higher redshifts (z > 5?), the residual turbulence in the clouds and the minimum coherence length measured here imply that the gas was stirred more recently, possibly by star formation events recurring on a timescale on the order of 10(7)-10(8) yr.

Spectra obtained with the Keck HIRES instrument of the Ly alpha forests in the lines of sight to the A and C components of the gravitationally lensed quasi-stellar object Q1422+231 were used to investigate the structure of the intergalactic medium at mean redshift [z] similar to 3.3 on subkiloparsec scales. We measured the cross-correlation amplitude between the two Ly alpha forests for a mean transverse separation of 120 h(50)(-1) pc and computed the rms column density and velocity differences between individual absorption systems seen in both lines of sight. The rms differences between the velocity centroids of the Ly alpha forest lines were found to be less than about 400 ms(-1) for unsaturated H I absorption lines with column densities in the range 12 < log N(H I) < 14.13. The rate of energy transfer into the low-density intergalactic medium on a typical scale of 100 pc seems to be lower by 3-4 orders of magnitude than the rate measured earlier for strong C IV metal absorption systems. The tight correlation between H I column density and baryonic density in the intergalactic medium was used to obtain a conservative upper limit on the rms fluctuations of the baryonic density field at [z] = 3.26, namely, [[(Delta log rho)(2)]](1/2) less than or equal to 3.1 x 10(-2) on a scale of 110 h(50)(-1) pc. The fraction of the absorption lines that are different across the lines of sight was used to determine the filling factor of the universe for gas that has suffered recent hydrodynamic disturbances. We thereby derived upper limits on the filling factor of galactic outflows at high redshift. Short-lived, short-range ancient winds are essentially unconstrained by this method, but strong winds blowing for a substantial fraction of a Hubble time (at z = 3.3) appear to fill less than 20% of the volume of the universe.

We have obtained Keck HIRES spectra of three images of the quadruply gravitationally lensed quasar Q2237+0305 to study low-ionization absorption systems and their differences in terms of projected velocity and column density across the lines of sight. We detect Ca II absorption from our Galaxy and a system of high-velocity clouds from the lensing galaxy (z = 0 : 039) with multiple Ca II components in all three sight lines. Unlike the situation in our Galaxy, there is no prominent Ca II absorption component ( with an equivalent width exceeding 60 70 mAngstrom) close to the velocity centroid of the lensing galaxy Q2237+0305. Instead, Ca II components with total equivalent widths similar to those of Galactic intermediate- and high-velocity clouds are spread out over several hundred kilometers per second in projection along the sight lines at impact parameters of less than 1 kpc through the bulge of the galaxy. A Ca II absorbing thick disk as in our Galaxy does not seem to extend into the bulge region of the 2237+0305 galaxy, whereas high-velocity clouds seem to be a more universal feature. We have also studied three low-ionization Mg II-Fe II systems in detail. All three Mg II systems cover all three lines of sight, suggesting that the gaseous structures giving rise to Mg II complexes are larger than similar to0.5 kpc. However, in most cases it is difficult to trace individual Mg II "cloudlets" over distances larger than 200-300 h(50)(-1) pc, indicating that typical sizes of the Mg II cloudlets are smaller than the sizes inferred earlier for the individual clouds of high-ionization gas seen in C IV absorption. We tentatively interpret the absorption pattern of the strongest Mg II system in terms of an expanding bubble or galactic wind and show that the possible loci occupied by the model bubble in radius-velocity space overlap with the observed characteristics of Galactic supershells.

The Lynx arc, with a redshift of 3.357, was discovered during spectroscopic follow-up of the z =0.70 cluster RX J0848+ 4456 from the ROSAT Deep Cluster Survey. The arc is characterized by a very red R - K color and strong, narrow emission lines. Analysis of HST WFPC2 imaging and Keck optical and infrared spectroscopy shows that the arc is an H II galaxy magnified by a factor of similar to 10 by a complex cluster environment. The high intrinsic luminosity, the emission-line spectrum, the absorption components seen in Lyalpha and C IV, and the rest-frame ultraviolet continuum are all consistent with a simple H II region model containing similar to 10(6) hot O stars. The best-fit parameters for this model imply a very hot ionizing continuum (T-BB similar or equal to 80, 000 K), a high ionization parameter (log U similar or equal to - 1), and a low nebular metallicity (Z/Z(.) similar or equal to 0: 05). The narrowness of the emission lines requires a low mass-to-light ratio for the ionizing stars, suggestive of an extremely low metallicity stellar cluster. The apparent overabundance of silicon in the nebula could indicate enrichment by past pair-instability supernovae, requiring stars more massive than similar to140 M-..