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.

We present detailed optical, near-infrared, and radio observations of the X-ray flash XRF 050416a obtained with Palomar and Siding Springs Observatories, as well as the HST and VLA, placing this event among the best-studied X-ray flashes to date. In addition, we present an optical spectrum from Keck LRIS from which we measure the redshift of the burst, z = 0.6528. At this redshift the isotropic-equivalent prompt energy release was about 10(51) ergs, and using a standard afterglow synchrotron model, we find that the blast wave kinetic energy is a factor of 10 larger, EK,(iso) approximate to 10(52) ergs. The lack of an observed jet break to t similar to 20 days indicates that the opening angle is theta(j) greater than or similar to 7 degrees and the total beaming-corrected relativistic energy is greater than or similar to 10(50) ergs. We further show that the burst produced a strong radio flare at t similar to 40 days accompanied by an observed flattening in the X-ray band, which we attribute to an abrupt circumburst density jump or an episode of energy injection (from either a refreshed shock or off-axis ejecta). Late-time observations with HST show evidence for an associated supernova with peak optical luminosity roughly comparable to that of SN 1998bw. Next, we show that the host galaxy of XRF 050416a is actively forming stars at a rate of at least 2 M-circle dot yr(-1) with a luminosity of L-B approximate to 0.5L* and metallicity of Z similar to 0.2-0.8 Z(circle dot). Finally, we discuss the nature of XRF 050416a in the context of short-hard GRBs and under the framework of off-axis and dirty fireball models for X-ray flashes.

We carry out cosmological chemodynamical simulations with different strengths of supernova ( SN) feedback and study how galactic winds from star-forming galaxies affect the features of hydrogen ( Hi) and metal ( C IV and O VI) absorption systems in the intergalactic medium at high redshift. We find that the outflows tend to escape to low-density regions, and hardly affect the dense filaments visible in H I absorption. As a result, the strength of H I absorption near galaxies is not reduced by galactic winds, but even slightly increases. We also find that a lack of H I absorption for lines of sight ( LOSs) close to galaxies, as found by Adelberger et al., can be created by hot gas around the galaxies induced by accretion shock heating. In contrast to HI, metal absorption systems are sensitive to the presence of winds. The models without feedback can produce the strong C IV and O VI absorption lines in LOSs within 50 kpc from galaxies, while strong SN feedback is capable of creating strong C IV and O vi lines out to about twice that distance. We also analyze the mean transmissivity of H I, C IV, and O VI within 1 h(-1) Mpc from star-forming galaxies. The probability distribution of the transmissivity of H I is independent of the strength of SN feedback, but strong feedback produces LOSs with lower transmissivity of metal lines. In addition, strong feedback can produce strong O VI lines even in cases where H I absorption is weak. We conclude that O VI is probably the best tracer for galactic winds at high redshift.

We have conducted a long-slit search for low surface brightness Ly alpha emitters at redshift 2: 67 < z < 3.75. A 92 hr long exposure with the ESO VLT FORS2 instrument down to a 1 sigma surface brightness detection limit of 8 x 10(-20) erg cm(-2) s(-1) arcsec(-2) per arcsec(2) aperture yielded a sample of 27 single line emitters with fluxes of a few x 10(-18) erg s(-1) cm(-2). We present arguments that most objects are indeed Ly alpha. The large comoving number density, 3 x 10(-2) h(70)(3) Mpc(-3), the large covering factor, dN/dz similar to 0.2-1, and the often extended Ly alpha emission suggest that the emitters can be identified with the elusive host population of damped Ly alpha systems (DLAS) and high column density Lyman limit systems (LLS). A small inferred star formation rate, perhaps supplemented by cooling radiation, appears to energetically dominate the Ly alpha emission, and is consistent with the low metallicity, low dust content, and theoretically inferred low masses of DLAS, and with the relative lack of success of earlier searches for their optical counterparts. Some of the line profiles show evidence for radiative transfer in galactic outflows. Stacking surface brightness profiles, we find emission out to at least 4 ''. The centrally concentrated emission of most objects appears to light up the outskirts of the emitters (where LLS arise) down to a column density where the conversion from UV to Ly alpha photon becomes inefficient. DLAS, high column density LLS, and the emitter population discovered in this survey appear to be different observational manifestations of the same low-mass, protogalactic building blocks of present-day L-* galaxies.