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-..

We study the abundance of silicon in the intergalactic medium by analyzing the statistics of Si IV, C IV, and H I pixel optical depths in a sample of 19 high-quality quasar absorption spectra, which we compare with realistic spectra drawn from a hydrodynamical simulation. Simulations with a constant and uniform Si/C ratio, a C distribution as derived in Paper II of this series, and a UV background (UVB) model from Haardt & Madau reproduce the observed trends in the ratio of Si IV and C IV optical depths, tau(Si IV)/tau(C IV). The ratio tau(Si Iv)/tau(C IV) depends strongly on tau(C IV), but it is nearly independent of redshift for fixed tau(C IV) and is inconsistent with a sharp change in the hardness of the UVB at zapproximate to3. Scaling the simulated optical depth ratios gives a measurement of the global Si/C ratio (using our fiducial UVB, which includes both galaxy and quasar contributions) of [Si/C]=0.77+/-0.05, with a possible systematic error of similar to0.1 dex. The inferred [Si/C] depends on the shape of the UVB (harder backgrounds leading to higher [Si/C]), ranging from [Si/C]similar or equal to1.5 for a quasar-only UVB to [Si/C]similar to0.25 for a UVB including both galaxies and artificial softening; this provides the dominant uncertainty in the overall [Si/C]. Examination of the full tau(Si IV)/tau(C IV) distribution yields no evidence for inhomogeneity in [Si/C] and constrains the width of a lognormal probability distribution in [Si/C] to be much smaller than that of [C/H]; this implies a common origin for Si and C. Since the inferred [Si/C] depends on the UVB shape, this also suggests that inhomogeneities in the hardness of the UVB are small. There is no evidence for evolution in [Si/C]. Variation in the inferred [Si/C] with density depends on the UVB and rules out the quasar-only model unless [Si/C] increases sharply at low density. Comparisons with low-metallicity halo stars and nucleosynthetic yields suggest either that our fiducial UVB is too hard or that supermassive Population Ill stars might have to be included. The inferred [Si/C], if extrapolated to low density, corresponds to a contribution to the cosmic Si abundance of [Si/H]=-2.0, or Omega(Si)similar or equal to3.2x10(-7), a significant fraction of all Si production expected by zapproximate to3.

We present a study of the spatial coherence of the intergalactic medium toward two pairs of high-redshift quasars with moderate angular separations observed with Keck ESI, Q1422+2309A/Q1424+2255 (z(em) approximate to 3.63, Deltatheta 39") and Q1439-0034A/B (z(em) approximate to 4.25, Deltatheta = 33"). The cross-correlation of transmitted flux in the Lyalpha forest shows a 5-7 sigma peak at zero velocity lag for both pairs. This strongly suggests that at least some of the absorbing structures span the 230 300 h(70)(-1) 70 proper kpc transverse separation between sight lines. We also statistically examine the similarity between paired spectra as a function of transmitted flux, a measure that may be useful for comparison with numerical simulations. In investigating the dependence of the correlation functions on spectral characteristics, we find that photon noise has little effect for a signal-to-noise ratio of greater than or similar to10 per resolution element. However, the agreement between the autocorrelation along the line sight and the cross-correlation between sight lines, a potential test of cosmological geometry, depends significantly on instrumental resolution. Finally, we present an inventory of metal lines. These include a pair of strong C IV systems at z approximate to 3.4, appearing only toward Q1439B, and an Mg II + Fe II system present toward Q1439A/B at z approximate to 1.68.

We present the results from a Hubble Space Telescope ACS search for supernovae associated with X-ray flashes 020903, 040701, 040812, and 040916. We find strong evidence that XRF 020903 (z = 0.25) was associated with a SN 1998bw-like supernova and confirm this using optical spectroscopy at t similar to 25 days. We find no evidence, however, for SN1998bw-like supernovae associated with the other three events. In the case of XRF 040701 (z = 0.21), we rule out even a faint supernova similar to SN 2002ap, using template light curves for several local Type Ic supernovae. For the two cases in which the redshift is not known, XRFs 040812 and 040916, we derive robust redshift limits, assuming that they were accompanied by supernovae similar to SN 1998bw, and compare these limits with photometric redshift constraints provided by their host galaxies. We supplement this analysis with results for three additional events ( XRFs 011030, 020427, and 030723) and discuss the observed diversity of supernovae associated with X-ray flashes and gamma-ray bursts. We conclude that XRF-SNe exist but can be significantly fainter than SN 1998bw, possibly consistent with the observed spread in local Type Ibc supernovae.

We present the results from a Hubble Space Telescope ACS study of the supernovae (SNe) associated with gamma-ray bursts (GRBs) 040924 (z = 0.86) and 041006 (z = 0.71). We find evidence that both GRBs were associated with an SN 1998bw -like supernova dimmed by similar to 1.5 and similar to 0.3 mag, respectively, making GRB 040924 the faintest GRB-associated SN ever detected. We study the luminosity dispersion in GRB/XRF-associated SNe and compare to local Type Ibc SNe from the literature. We find significant overlap between the two samples, suggesting that GRB/XRF-associated SNe are not necessarily more luminous and do not necessarily produce more Ni-56 than local SNe. Based on the current (limited) data sets, we find that the two samples may share a similar Ni-56 production mechanism.