The Minimum Universal Metal Density Between Redshifts of 1.5 and 5.5

It appears that the Lyman alpha forest is becoming thick at a redshift of about 5.5, cutting off the higher redshift intergalactic medium from view in neutral hydrogen. However, the effects of star formation at higher redshift are still readable in the intergalactic metal lines. In this paper I use observations of 32 quasars with emission redshifts in the range 2.31 to 5.86 to study the evolution of the intergalactic metal density from z = 1.5 to z = 5.5. The C IV column density distribution function is consistent with being invariant throughout this redshift range. From direct integration, I determine Omega_CIV to be in the range (2.5 - 7) \times 10^{-8} and Omega_SiIV in the range (0.9 - 3) \times 10^{-8} between z = 1.5 and z = 5. The metallicity at z = 5 exceeds 3.5 \times 10^{-4}, which in turn implies that this fraction of the universal massive star formation took place beyond this redshift. This is sufficient to have ionized the intergalactic medium.Comment: This posting (v3) adds a factor of (1/c) in equation (1), and also the definition of Delta X, both omitted from v2. This is a typographical error and there is no impact on the published values of Omega_ion in the text and figures. v2: Revised to correct an error in equation (1) in the published version. This was a typographical error only and all values of Omega_ion in the text and figures of the published version were correctly computed with H_0 = 65 km/s/Mpc and Omega_m = 1. (5 pages including 4 figures.

The Properties of Intergalactic C IV and Si IV Absorption, I: Optimal Analysis of an Extremely High S/N Quasar Sample

We have analyzed the properties of metals in the high redshift intergalactic medium using a novel objective pixel optical depth technique on a sample of extremely high signal-to-noise Keck HIRES and ESI spectra of 26 quasars between redshifts 2.1 and 6.4. The technique relies on using the doublet nature of the common ions CIV and SiIV that are the principal metal tracers in the intergalactic medium outside of the Ly alpha forest. Optical depths are statistically corrected for contamination by other lines, telluric absorption, bad pixels, continuum fitting, etc. and for incompleteness, and we achieve in this way an increased sensitivity of approximately 0.5 dex over previous analyses. Unlike existing POD techniques, we do not compare the ion optical depths with HI optical depths; we therefore avoid problems arising from different velocity widths in the ion and HI. We have shown how the conventional analysis can be reproduced using a percolation method to generate pseudo-clouds from ion optical depths. We find that for the higher resolution HIRES data there is a tight relation, tau ~ N^{0.7}, between the peak optical depth and the column density. From the optical depth vectors themselves we show that there is little evolution in the total amount of CIV from z = 2 to z = 5, though there is a turndown of at least a factor of two in Omega(CIV) above z = 5. We do, however, see substantial evolution in the ratio, SiIV/CIV. Two subsequent papers will investigate what fraction of the absorbers lie in galatic wind outflows (Paper II) and what metallicity is associated with regions of tau(Ly alpha) < 1 (Paper III).Comment: 38 pages, 23 figures. Accepted for publication in AJ. Spectral data will be available at http://www.ifa.hawaii.edu/~acowie/spectra.html on publication of the pape

The Evolution of the Intergalactic Medium Transmission to Redshift Six

We have measured the transmission of the Lyman alpha forest produced by neutral hydrogen scattering in the intergalactic medium between redshifts 2 and 6.3 using high signal to noise, high resolution (R > 5000) observations of 50 quasars spread over the redshift range. We use a uniform set of 15 Angstrom intervals covering Lyman alpha, Lyman beta, and Lyman gamma absorption regions to tabulate the forest transmission as a function of redshift. The transmitted fractions show a relatively smooth evolution over the entire range of redshifts, which can be modelled with a smoothly decreasing ionization rate. Previous claims of an abrupt change at a redshift of approximately 6 appear in part to be a consequence of an incorrect conversion of Lyman beta to Lyman alpha optical depths. The tabulated transmissions can be used to calculate the colors of objects with a specified input spectrum as a function of redshift. We calculate the colors of a flat f_nu galaxy with a large intrinsic continuum break, as an important example.Comment: 32 pages, 6 figures, 6 table

A Lower Limit to the Universal Density of Metals at z \sim 3

Column density distribution functions of CIV with 12.05 < log (N) < 14.35 and SiIV with 11.70 < log (N) < 13.93 have been obtained using 81 CIV absorbers and 35 SiIV absorbers redward of the Ly alpha forest in the lines of sight to seven quasars with 2.518 < z(em) < 3.78. These distribution functions have been directly integrated to yield ion densities at z = 3 to 3.5 of Omega(CIV) = (2.0 \pm 0.5) x 10(-8) and Omega(SiIV) = (7.0 \pm 2.6) x 10(-9) with H0 = 65 km/s/Mpc and q0 = 0.02 (1 sigma errors). A larger sample of 11 quasar lines of sight was used to measure CII/CIV, SiIII/SiIV, and NV/CIV ratios, which suggest that CIV and SiIV are the dominant ionization stages and that corrections to Omega(Carbon) and Omega(Silicon) are no more than a factor of two. Normalizing the alpha-process elements to silicon and the Fe-coproduction elements to carbon gives a density of heavy elements in these forest clouds of Omega(metals) = (3.3 \pm 0.8) x 10(-7) (H0 = 65, q0 = 0.02). The implications for the amount of star formation and for the ionization of the IGM prior to z = 3 are discussed.Comment: 12 pages LaTeX (aaspp4.sty) with 3 encapsulated postscript figures. To be published in ApJ Letters (accepted September 3, 1997

Faintest Galaxy Morphologies from HSTHST WFPC2 Imaging of the Hawaii Survey Fields

We present very deep $HST$ WFPC2 images in the F814W filter of two Hawaii Survey fields, SSA13 and SSA22. Using these data with previous ground-based imaging and spectroscopy, we compare the colors, star-forming properties and morphologies of the faintest galaxies with a reference sample of bright nearby galaxies and analyze the changes in field galaxy morphology with magnitude. Our principal result is the identification of a new morphological class of ``chain'' galaxies at the faintest magnitudes. Based on limited spectroscopy, we tentatively conclude that these are linearly organized giant star-forming regions at $z = 0.5-3$ and, if this is correct, that these are large galaxies in the process of formation.Comment: 18 pages + 1 table of text as 1 LaTeX file (uses aastex style macros: aaspp.sty, flushrt.sty) plus 1 uuencoded compressed tar file of 12 PostScript figures (Figs. 3-9, 16-17, and 21-23). The remaining gray-scale plots are available by anonymous ftp at ftp://hubble.ifa.hawaii.edu/pub/preprints/plates To appear in the October 1995 Astronomical Journa

Metal Enrichment and Ionization Balance in the Lyman α\alpha Forest at z=3z = 3

The recent discovery of carbon in close to half of the low neutral hydrogen column density [N({\rm H~I}) > 3\ten{14}\cm2] Lyman forest clouds toward $z \sim 3$ quasars has challenged the widely held view of this forest as a chemically pristine population uniformly distributed in the intergalactic medium, but has not eliminated the possibility that a primordial population might be present as well. Using extremely high signal-to-noise observations of a sample of quasars we now show that \ion{C}{4} can be found in 75% of clouds with N({\rm H~I}) > 3\ten{14}\cm2 and more than 90% of those with N({\rm H~I}) > 1.6\ten{15}\cm2. Clouds with N({\rm H~I}) > 10^{15}\cm2 show a narrow range of ionization ratios, spanning less than an order of magnitude in \ion{C}{4}/\ion{H}{1}, \ion{C}{2}/\ion{C}{4}, \ion{Si}{4}/\ion{C}{4} and \ion{N}{5}/\ion{C}{4}, and their line widths require that they be photoionized rather than collisionally ionized. This in turn implies that the systems have a spread of less than an order of magnitude in both volume density and metallicity. Carbon is seen to have a typical abundance of very approximately $10^{-2}$ of solar and Si/C about three times solar, so that the chemical abundances of these clouds are very similar to those of Galactic halo stars. \ion{Si}{4}/\ion{C}{4} decreases rapidly with redshift from high values ($> 0.1$) at $z > 3.1$, a circumstance which we interpret as a change in the ionizing spectrum as the intergalactic medium becomes optically thin to He$^+$\ ionizing photons. Weak clustering is seen in the \ion{C}{4} systems for \Delta v < 250\kms, which we argue provides an upper limit to the clustering of \ion{H}{1} clouds. If the clouds are associated with galaxies, this requires a rapid evolution in galaxy clustering between $z = 3$ and $z = 0$.Comment: 31 pages plus 5 tables, 21 Postscript figures, Figures 1 and 2 available at http://www.ifa.hawaii.edu/faculty/acowie/igm_aj.html . To be published in Astronomical Journa