The enrichment history of the intergalactic medium: O VI in Ly-alpha forest systems at redshift z ~ 2

A search for O VI at redshifts corresponding to Ly-alpha lines in the z_em ~ 2.4 QSOs HE1122-1648 and HE2217-2818 reveals that a substantial fraction of those with H I column densities log N(HI) > 14 (cm^{-2}) are highly ionized and show some heavy element enrichment. If these two sight lines are typical, then the O VI systems contain a cosmologically significant fraction of the baryons and the metals in the universe. For most systems the temperatures derived from the line widths are too low for collisional ionization to be responsible for the O VI lines. Photoionization models with a substantial hard ultraviolet flux can reproduce the observations for densities that are in good agreement with a model assuming local, hydrostatic equilibrium and heavy element abundances in the range ~ 10^{-3} - 10^{-2} solar. Photoionization by a UV flux much softer than that predicted by Haardt & Madau (1996) for a background dominated by quasars can be ruled out. Finally, we find one system with a very low H I column density for which both photoionization and collisional ionization models yield a metallicity close to solar and a density that is inconsistent with gravitational confinement, unless the gas fraction is negligible.Comment: 16 pages, 16 figures, 7 tables. Accepted for publication in the Astrophysical Journal. Minor change

A Survey of Weak MgII Absorbers at 0.4 < z < 2.4

We present results from a survey of weak MgII absorbers in the VLT/UVES spectra of 81 QSOs obtained from the ESO archive. In this survey, we identified 112 weak MgII systems within the redshift interval 0.4 < z < 2.4 with 86% completeness down to a rest-frame equivalent width of W_r(2796) = 0.02A, covering a cumulative redshift path length of deltaZ=77.3. From this sample, we estimate that the number of weak absorbers per unit redshift dN/dz increases from 1.06 +/- 0.04 at =1.9 to 1.76 +/- 0.08 at =1.2 and thereafter decreases to 1.51 +/- 0.09 at =0.9 and 1.06 +/- 0.10 at =0.6. Thus we find evidence for an evolution in the population of weak MgII absorbers, with their number density peaking at z=1.2. We also determine the equivalent width distribution of weak systems at =0.9 and =1.9. At 0.4 < z < 1.4, there is evidence for a turnover from a powerlaw of the form n(W_r) \propto W_r^{-1.04} at W_r(2796) < 0.1A. This turnover is more extreme at 1.4 < z < 2.4, where the equivalent width distribution is close to an extrapolation of the exponential distribution function found for strong MgII absorbers. Based on these results, we discuss the possibility that some fraction of weak MgII absorbers, particularly single cloud systems, are related to satellite clouds surrounding strong MgII systems. These structures could also be analogs to Milky Way high velocity clouds. In this context, the paucity of high redshift weak MgII absorbers is caused by a lack of isolated accreting clouds on to galaxies during that epoch.Comment: 14 pages, 11 figures, ApJ accepte

The evolution of HI and CIV quasar absorption line systems at 1.9 < z < 3.2

We have investigated the distribution and evolution of ~3100 intergalactic HI absorbers with HI column densities log N(HI) = [12.75, 17.0] at 1.9 < z < 3.2, using 18 high resolution, high S/N quasar spectra obtained from the ESO VLT/UVES archive. We used two sets of Voigt profile fitting analysis, one including all the available high-order Lyman lines to obtain reliable HI column densities of saturated lines, and another using only the Ly-alpha lines. There is no significant difference between the results from the two fits. Combining our results with literature data, the mean number density at 0 < z < 4 is not well described by a single power law and strongly suggests that its evolution slows down at z < 1.5 at the high and low column density ranges. We also divided our entire HI absorbers at 1.9 < z < 3.2 into two samples, the unenriched forest and the CIV-enriched forest, depending on whether HI lines are associated with CIV at log N(CIV) > 12.2 within a given velocity range. The entire HI column density distribution function (CDDF) can be described as the combination of these two well-characterised populations which overlap at log N(HI) ~ 15. At log N(HI) < 15, the unenriched forest dominates, showing a similar power-law distribution to the entire forest. The CIV-enriched forest dominates at log N(HI) > 15, with its distribution function proportional to N(HI)^(-1.45). However, it starts to flatten out at lower N(HI), since the enriched forest fraction decreases with decreasing N(HI). The deviation from the power law at log N(HI) = [14, 17] shown in the CDDF for the entire HI sample is a result of combining two different HI populations with a different CDDF shape. The total HI mass density relative to the critical density is Omega(HI) ~ 1.6 x 10^(-6) h^(-1), where the enriched forest accounts for ~40% of Omega(HI).Comment: 26 pages, 20 figures, accepted for AA, in pres

The Effect of Presence on Consumers\u27 Responses to Virtual Mirror Technology

Virtual mirror, an increasingly popular application of augmented reality (AR), allows consumers to view their visages overlaid with product images on digital displays. This study investigates the effect of presence on consumers\u27 responses to AR- (versus virtual reality (VR-) based virtual mirror technology. Results show that AR-based presentation is more likely to induce presence than VR-based presentation, thereby leading to greater mental imagery, favorable virtual mirror attitude, and stronger purchase intentions. Implications for digital retailers in the fashion industry are discussed

The Distribution of Column Densities and b Values in the Lyman-Alpha Forest

We describe the properties of the Ly$\alpha$ forest in the column density range \nhi \geq 2\times10^{12} cm$^{-2}$ based on 1056 lines in the wavelength range 4300--5100\AA 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 $2\times10^{12}$ 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\kms 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 $b_c$ 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\kms.Comment: 17 pages + 3 tables of text as 1 LaTeX file (uses aastex version 4 style macros: aaspp4.sty, flushrt) plus 1 uuencoded compressed tar file of 7 PostScript figures. Appendix tables and figures [complete spectra and line lists (~1.6 Mb uncompressed)] are available by anonymous ftp at ftp://hubble.ifa.hawaii.edu/pub/preprints. To appear in the October 1995 Astronomical Journa

A Homogeneous Sample of Sub-DLAs IV: Global Metallicity Evolution

An accurate method to measure the abundance of high-redshift galaxies consists in the observation of absorbers along the line of sight toward a background quasar. Here, we present abundance measurements of 13 z>3 sub-Damped Lyman-alpha Systems (quasar absorbers with HI column density 19 < log N(HI) < 20.3 cm^-2) based on the high resolution observations with VLT UVES spectrograph. These observations more than double the metallicity information for sub-DLAs previously available at z>3. This new data, combined with other sub-DLA measurements from the literature, confirm the stronger metallicity redshift evolution than for the classical Damped Lyman-alpha absorbers. Besides, these observations are used to compute for the first time the fraction of gas ionised from photo-ionisation modelling in a sample of sub-DLAs. Based on these results, we calculate that sub-DLAs contribute no more than 6% of the expected amount of metals at z~2.5. We therefore conclude that even if sub-DLAs are found to be more metal-rich than classical DLAs, they are insufficient to close the so-called ``missing metals problem''.Comment: 30 figures, 24 tables. Accepted for publication in MNRA