6,308 research outputs found
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 forest in the column density
range \nhi \geq 2\times10^{12} cm based on 1056 lines in the
wavelength range 4300--5100\AA measured in extremely high S/N,
spectra of four quasars. The column density distribution is well described by a
-1.5 power law to cm, below which limit confusion
becomes too severe to measure a spectrum of individual clouds. The distribution
of 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 absorption lines as complexes of
clouds each with thermal broadening corresponding to we find the
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
- …