90,885 research outputs found
Failure properties of loaded fiber bundles having a lower cutoff in fiber threshold distribution
Presence of lower cutoff in fiber threshold distribution may affect the
failure properties of a bundle of fibers subjected to external load. We
investigate this possibility both in a equal load sharing (ELS) fiber bundle
model and in local load sharing (LLS) one. We show analytically that in ELS
model, the critical strength gets modified due to the presence of lower cutoff
and it becomes bounded by an upper limit. Although the dynamic exponents for
the susceptibility and relaxation time remain unchanged, the avalanche size
distribution shows a permanent deviation from the mean-fiels power law. In the
LLS model, we analytically estimate the upper limit of the lower cutoff above
which the bundle fails at one instant. Also the system size variation of
bundle's strength and the avalanche statistics show strong dependence on the
lower cutoff level.Comment: 7 pages and 7 figure
The evolution of Omega(HI) and the epoch of formation of damped Lyman-alpha absorbers
We present a study of the evolution of the column density distribution,
f(N,z), and total neutral hydrogen mass in high-column density quasar absorbers
using candidates from a recent high-redshift survey for damped Lyman-alpha
(DLA) and Lyman limit system (LLS) absorbers. The observed number of LLS
(N(HI)> 1.6 * 10^{17} atom/cm^2) is used to constrain f(N,z) below the
classical DLA Wolfe et al. (1986) definition of 2 * 10^{20} atom/cm^2. The
joint LLS-DLA analysis shows unambiguously that f(N,z) deviates significantly
from a single power law and that a Gamma-law distribution of the form
f(N,z)=(f_*/N_*)(N/N_*)^{-Beta} exp(-N/N_*) provides a better description of
the observations. These results are used to determine the amount of neutral gas
contained in DLAs and in systems with lower column density. Whilst in the
redshift range 2 to 3.5, ~90% of the neutral HI mass is in DLAs, we find that
at z>3.5 this fraction drops to only 55% and that the remaining 'missing' mass
fraction of the neutral gas lies in sub-DLAs with N(HI) 10^{19} - 2 * 10^{20}
atom/cm^2. The characteristic column density, N_*, changes from 1.6 * 10^{21}
atom/cm^2 at z3.5, supporting a picture
where at z>3.5, we are directly observing the formation of high column density
neutral hydrogen DLA systems from lower column density units. Moreover since
current metallicity studies of DLA systems focus on the higher column density
systems they may be giving a biased or incomplete view of global galactic
chemical evolution at z>3. After correcting the observed mass in HI for the
``missing'' neutral gas the comoving mass density now shows no evidence for a
decrease above z=2. (abridged)Comment: Replaced to match version published in MNRAS. One figure and appendix
added, analysis and conclusions unchange
The Population of Weak Mg II Absorbers I. A Survey of 26 QSO HIRES/Keck Spectra
We present a search for "weak" MgII absorbers [those with W_r(2796) < 0.3 A
in the HIRES/Keck spectra of 26 QSOs. We found 30, of which 23 are newly
discovered. The spectra are 80% complete to W_r(2796) = 0.02 A and have a
cumulative redshift path of ~17.2 for the redshift range 0.4 < z < 1.4. The
number of absorbers per unit redshift, dN/dz, is seen to increase as the
equivalent width threshold is decreased; we obtained dN/dz = 1.74+/-0.10 for
our 0.02 <= W_r(2796) < 0.3 A sample. The equivalent width distribution follows
a power law with slope -1.0; there is no turnover down to W_r(2796) = 0.02 A at
= 0.9. Weak absorbers comprise at least 65% of the total MgII absorption
population, which outnumbers Lyman limit systems (LLS) by a factor of
3.8+/-1.1; the majority of weak MgII absorbers must arise in sub-LLS
environments. Tentatively, we predict that ~5% of the Lyman-alpha forest clouds
with W_r(1215) > 0.1 A will have detectable MgII absorption to W_r,min(2796) =
0.02 A and that this is primarily a high-metallicity selection effect (Z/Z_sun]
> -1). This implies that MgII absorbing structures figure prominently as
tracers of sub-LLS environments where gas has been processed by stars. We
compare the number density of W_r(2796) > 0.02 A absorbers with that of both
high and low surface brightness galaxies and find a fiducial absorber size of
35h^-1 to 63h^-1 kpc, depending upon the assumed galaxy population and their
absorption properties. The individual absorbing "clouds" have W_r(2796) <= 0.15
A and their narrow (often unresolved) line widths imply temperatures of ~25,000
K. We measured W_r(1548) from CIV in FOS/HST archival spectra and, based upon
comparisons with FeII, found a range of ionization conditions (low, high, and
multi-phase) in absorbers selected by weak MgII.Comment: Accepted Version: 43 pages, PostScript figures embedded; accepted to
ApJ; updated version includes analysis of CIV absorptio
The evolution of Ω_(Hi) and the epoch of formation of damped Lyman α absorbers
We present a study of the evolution of the column density distribution, f(N, z), and total neutral hydrogen mass in high column density quasar absorbers using candidates from a recent high-redshift survey for damped Lyman α (DLA) and Lyman-limit system (LLS) absorbers. The observed number of LLS [N(H_i) >1.6 × 10^(17) atom cm^(−2)] is used to constrain f(N, z) below the classical DLA definition of 2 × 10^(20) atom cm^(−2). The evolution of the number density of LLS is consistent with our previous work but steeper than previously published work of other authors. At z= 5, the number density of Lyman-limit systems per unit redshift is ∼5, implying that these systems are a major source of ultraviolet (UV) opacity in the high-redshift Universe. The joint LLS–DLA analysis shows unambiguously that f(N, z) deviates significantly from a single power law and that a Γ-law distribution of the form f(N,z) = (f_*/N_*)(N/N_*)^(−β)exp(−N/N_*) provides a better description of the observations. These results are used to determine the amount of neutral gas contained in DLAs and in systems with lower column density. Whilst in the redshift range 2–3.5, ∼90 per cent of the neutral H i mass is in DLAs, we find that at z > 3.5 this fraction drops to only 55 per cent and that the remaining ‘missing’ mass fraction of the neutral gas lies in sub-DLAs with N(H i) 10^(19)–2 × 10^(20) atom cm^(−2). The characteristic column density, N_*, changes from 1.6 × 10^(21) atom cm^(−2) at z 3.5, supporting a picture where at z > 3.5, we are directly observing the formation of high column density neutral hydrogen DLA systems from lower column density units. Moreover, since current metallicity studies of DLA systems focus on the higher column density systems they may be giving a biased or incomplete view of global galactic chemical evolution at z > 3. After correcting the observed mass in H i for the ‘missing’ neutral gas the comoving mass density now shows no evidence for a decrease above z= 2
The HST/ACS+WFC3 Survey for Lyman Limit Systems II: Science
We present the first science results from our Hubble Space Telescope Survey
for Lyman limit absorption systems (LLS) using the low dispersion spectroscopic
modes of the Advanced Camera for Surveys and the Wide Field Camera 3. Through
an analysis of 71 quasars, we determine the incidence frequency of LLS per unit
redshift and per unit path length, l(z) and l(x) respectively, over the
redshift range 1 < z< 2.6, and find a weighted mean of l(x)=0.29 +/-0.05 for
2.0 < z < 2.5 through a joint analysis of our sample and that of Ribaudo et al.
(2011). Through stacked spectrum analysis, we determine a median (mean) value
of the mean free path to ionizing radiation at z=2.4 of lambda_mfp =
243(252)h^(-1) Mpc, with an error on the mean value of +/- 43h^(-1) Mpc. We
also re-evaluate the estimates of lambda_mfp from Prochaska et al. (2009) and
place constraints on the evolution of lambda_mfp with redshift, including an
estimate of the "breakthrough" redshift of z = 1.6. Consistent with results at
higher z, we find that a significant fraction of the opacity for absorption of
ionizing photons comes from systems with N_HI <= 10^{17.5} cm^(-2) with a value
for the total Lyman opacity of tau_lyman = 0.40 +/- 0.15. Finally, we determine
that at minimum, a 5-parameter (4 power-law) model is needed to describe the
column density distribution function f(N_HI, X) at z \sim 2.4, find that
f(N_HI,X) undergoes no significant change in shape between z \sim 2.4 and z
\sim 3.7, and provide our best fit model for f(N_HI,X).Comment: 36 pages, 20 figures, 10 tables, revision to match accepted ApJ
versio
A Definitive Survey for Lyman Limit Systems at z~3.5 with the Sloan Digital Sky Survey
We perform a semi-automated survey for tau>=2 Lyman Limit systems (LLSs) in
quasar spectra from the Sloan Digital Sky Survey, Data Release 7. From a
starting sample of 2473 quasars with zem=3.6-4.4, we analyze 469 spectra
meeting strict seletion criteria for a total redshift path Dz=93.8 and identify
192 intervening systems at z>3.3. The incidence of tau>=2 LLSs per unit
redshift, l(z), is well described by a single-power law at these redshifts:
l(z) = C_LLS [(1+z)/(1+z_*)]^gamma, with z_*=3.7, C_LLS = 1.9+/-0.2, and gamma
= 5.2+/-1.5 (68% c.l.). These values are systematically lower than previous
estimates (especially at z<4) but are consistent with recent measurements of
the mean free path to ionizing radiation. Extrapolations of this power-law to
z=0 are inconsistent with previous estimations of l(z) at z<1 and suggest a
break at z~2, similar to that observed for the Lya forest. Our results also
indicate that the systems giving rise to LLS absorption decrease by ~50% in
comoving number density and/or physical size from z=4 to 3.3, perhaps due to an
enhanced extragalactic ultraviolet background. The observations place an
integral constraint on the HI frequency distribution f(N_HI,X) and indicate
that the power-law slope beta= dln[f(N,X)]/dln[N] is likely shallower than beta
= -1 at N_HI=10^18 cm^-2. Including other constraints on f(N_HI,X) from the
literature, we infer that beta is steeper than beta = -1.7 at N_HI~10^15 cm^-2,
implying at least two inflections in f(N_HI,X). We also perform a survey for
proximate LLSs (PLLSs) and find that l(z)_PLLS is systematically lower ~25%
than intervening systems. Finally, we estimate that systematic effects impose
an uncertainty of 10-20% in the l(z) measurements; these effects may limit the
precision of all future surveys.Comment: 26 pages, 17 figures (most in color). Submitted to Ap
The Keck+Magellan Survey for Lyman Limit Absorption I: The Frequency Distribution of Super Lyman Limit Systems
We present the results of a survey for super Lyman limit systems (SLLS;
defined to be absorbers with 19.0 <= log(NHI) <= 20.3 cm^-2) from a large
sample of high resolution spectra acquired using the Keck and Magellan
telescopes. Specifically, we present 47 new SLLS from 113 QSO sightlines. We
focus on the neutral hydrogen frequency distribution f(N,X) of the SLLS and its
moments, and compare these results with the Lyman-alpha forest and the damped
Lyman alpha systems (DLA; absorbers with log(NHI) >= 20.3 cm^-2). We find that
that f(N,X) of the SLLS can be reasonably described with a power-law of index
alpha = -1.43^{+0.15}_{-0.16} or alpha = -1.19^{+0.20}_{-0.21} depending on
whether we set the lower N(HI) bound for the analysis at 10^{19.0} cm^-2 or
10^{19.3}$ cm^-2, respectively. The results indicate a flattening in the slope
of f(N,X) between the SLLS and DLA. We find little evidence for redshift
evolution in the shape of f(N,X) for the SLLS over the redshift range of the
sample 1.68 < z < 4.47 and only tentative evidence for evolution in the zeroth
moment of f(N,X), the line density l_lls(X). We introduce the observable
distribution function O(N,X) and its moment, which elucidates comparisons of HI
absorbers from the Lyman-alpha through to the DLA. We find that a simple three
parameter function can fit O(N,X) over the range 17.0 <= log(NHI) <=22.0. We
use these results to predict that f(N,X) must show two additional inflections
below the SLLS regime to match the observed f(N,X) distribution of the
Lyman-alpha forest. Finally, we demonstrate that SLLS contribute a minor
fraction (~15%) of the universe's hydrogen atoms and, therefore, an even small
fraction of the mass in predominantly neutral gas.Comment: 15 pages, 10 figures, accepted to the Astrophysical Journal. Revision
includes updated reference
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