3,250 research outputs found
The impact of spatial fluctuations in the ultra-violet background on intergalactic carbon and silicon
Spatial inhomogeneities in the spectral shape of the ultra-violet background
(UVB) at the tail-end of HeII reionisation are thought to be the primary cause
of the large fluctuations observed in the HeII to HI Ly-a forest optical depth
ratio, tau_HeII/tau_HI, at z~2-3. These spectral hardness fluctuations will
also influence the ionisation balance of intergalactic metals; we extract
realistic quasar absorption spectra from a large hydrodynamical simulation to
examine their impact on intergalactic SiIV and CIV absorbers. Using a variety
of toy UVB models, we find that while the predicted spatial inhomogeneities in
spectral hardness have a significant impact on tau_HeII/tau_HI, the longer mean
free path for photons with frequencies above and below the HeII ionisation edge
means these fluctuations have less effect on the SiIV and CIV ionisation
balance. Furthermore, UVB models which produce the largest fluctuations in
specific intensity at the HeII ionisation edge also have the softest ionising
spectra, and thus result in photo-ionisation rates which are too low to produce
significant fluctuations in the observed tau_SiIV/tau_CIV. Instead, we find
spatial variations in the IGM metallicity will dominate any scatter in
tau_SiIV/tau_CIV. Our results suggest that observational evidence for
homogeneity in the observed tau_SiIV/tau_CIV distribution does not rule out the
possibility of significant fluctuations in the UVB spectral shape at z~2-3. On
the other hand, the scatter in metallicity inferred from observations of
intergalactic CIV and SiIV absorption at z~2-3 using spatially uniform
ionisation corrections is likely intrinsic, and therefore provides a valuable
constraint on intergalactic metal enrichment scenarios at these redshifts.Comment: 13 pages, 7 figures, accepted to MNRA
The nature and evolution of the highly ionized near-zones in the absorption spectra of z~6 quasars
We use state-of-the-art hydrodynamical simulations combined with a 1D
radiative transfer code to assess the extent to which the highly ionized
regions observed close to z~6 quasars, which we refer to as near-zones, can
constrain the ionization state of the surrounding IGM. We find the appearance
in Lya absorption of a quasar HII ionization front expanding into a neutral IGM
can be very similar to a classical proximity zone, produced by the enhancement
in ionizing flux close to a quasar embedded in a highly ionized IGM. The
observed sizes of these highly ionized near-zones and their redshift evolution
can be reproduced for a wide range of IGM neutral hydrogen fractions for
plausible values of the luminosity and lifetime of the quasars. The observed
near-zone sizes at the highest observed redshifts are equally consistent with a
significantly neutral and a highly ionized surrounding IGM. Stronger
constraints on the IGM neutral hydrogen fraction can be obtained by considering
the relative size of the near-zones in the Lya and Lyb regions of a quasar
spectrum. A large sample of high quality quasar absorption spectra with
accurate determinations of near-zone sizes and their redshift evolution in both
the Lya and Lyb regions should confirm or exclude the possibility that the
Universe is predominantly neutral at the highest observed redshifts. The width
of the discrete absorption features in these near-zones will contain important
additional information on the ionization state and the previous thermal history
of the IGM at these redshifts.Comment: 25 pages, 11 figures, accepted for publication in MNRA
On the rapid demise of Lyman-alpha emitters at z>7 due to the increasing incidence of optically thick absorption systems
A variety of independent observational studies have now reported a
significant decline in the fraction of Lyman-break galaxies which exhibit Ly-a
emission over the redshift interval z=6-7. In combination with the strong
damping wing extending redward of Ly-a in the spectrum of the bright z=7.085
quasar ULAS 1120+0641, this has strengthened suggestions that the hydrogen in
the intergalactic medium (IGM) is still substantially neutral at z~7. Current
theoretical models imply HI fractions as large as 40-90 per cent may be
required to explain these data assuming there is no intrinsic evolution in the
Ly-a emitter population. We propose that such large neutral fractions are not
necessary. Based on a hydrodynamical simulation which reproduces the absorption
spectra of high-redshift (z~6-7) quasars, we demonstrate that the opacity of
the intervening IGM redward of rest-frame Ly-a can rise rapidly in average
regions of the Universe simply because of the increasing incidence of
absorption systems which are optically thick to Lyman continuum photons as the
tail-end of reionisation is approached. Our simulations suggest these data do
not require a large change in the IGM neutral fraction by several tens of per
cent from z=6-7, but may instead be indicative of the rapid decrease in the
typical mean free path for ionising photons expected during the final stages of
reionisation.Comment: 11 pages, 6 figures, accepted to MNRA
Radiative transfer through the Intergalactic Medium
We use a probabilistic method to compute the propagation of an ionization
front corresponding to the re-ionization of the intergalactic medium in a LCDM
cosmology, including both hydrogen and helium. The effects of radiative
transfer substantially boost the temperature of the ionized gas over the case
of uniform re-ionization. The resulting temperature-density relation of the
ionized gas is both non-monotonic and multiple-valued, reflecting the non-local
character of radiative transfer and suggesting that a single polytropic
relation between local gas density and temperatue is a poor description of the
thermodynamic state of baryons in the post-reionization universe.Comment: 5 pages, 7 figures, letter accepted for publication in MNRA
New Measurements of the Ionizing Ultraviolet Background over 2 < z < 5 and Implications for Hydrogen Reionization
We present new measurements of the intensity of the ionizing ultraviolet
background and the global emissivity of ionizing photons over 2 < z < 5. Our
results are based on a suite of updated measurements of physical properties of
the high-redshift intergalactic medium (IGM), including gas temperatures and
the opacity of the IGM to Ly-alpha and ionizing photons. Consistent with
previous works, we find a relatively flat hydrogen photoionization rate over 2
< z < 5, although our measurements are roughly a factor of two higher than the
2008 values of Faucher-Giguere et al., due primarily to our lower gas
temperatures. The ionizing emissivity we derive is also generally higher than
other recent estimates due to a combination of lower gas temperatures, higher
ionizing opacity, and an accounting of cosmological radiative transfer effects.
We find evidence that the emissivity increases from z~3 to 5, reaching ~5
ionizing photons per atom per gigayear at z=4.75 for realistic galaxy spectra.
We further find that galaxies must dominate the emissivity near 1 Ryd at z > 4,
and possibly at all redshifts z > 2.4. Our results suggest that the
globally-averaged ionizing "efficiency" of star-forming galaxies increases
substantially with redshift over 3.2 < z < 4.75. This trend is consistent with
the conclusion often drawn from reionization models that the ionizing
efficiency of galaxies must be higher during reionization in order for galaxies
to reionize the IGM by z=6. Our emissivity values at z~5 suggest that ionizing
photons may have been a factor of two more abundant during the final stages of
reionization than previously indicated. The evolution of the ionizing
emissivity over 2 < z < 5 suggests, moreover, that the steep decline in the
photoionization rate from z~5 to 6 may indicate a rapid evolution in the mean
free path at z > 5.Comment: 19 pages, 14 figures, MNRAS, in pres
The observed ionization rate of the intergalactic medium and the ionizing emissivity at z >5: Evidence for a photon starved and extended epoch of reionization
We use a large set of hydrodynamical simulations, combined with measurements
of the Lyman alpha opacity of the IGM taken from the literature, to obtain
robust estimates for the photoionization rate per hydrogen atom at z=5 and 6.
We find the photoionization rate drops by a factor of two and four,
respectively, compared to our recent measurements at z = 2 - 4. The number of
ionizing photons emitted by known sources at z=5 and 6, based on an
extrapolation of source numbers below the detection limit and standard
assumptions for the relationship between the ionizing emissivity and observed
luminosity density at 1500 Angstroms, are in reasonable agreement with the
photoionization rates inferred from the Lyman alpha forest if the escape
fraction of ionizing photons from galaxies is large (>= 20 per cent). Claims to
the contrary may be attributed to the adoption of an unduly high value for the
clumping factor of ionized hydrogen. Using physically motivated assumptions for
the mean free path of ionizing photons our measurements of the photoionization
rate can be turned into an estimate of the ionizing emissivity. In comoving
units the inferred ionizing emissivity is nearly constant over the redshift
range 2-6 and corresponds to 1.5-3 photons emitted per hydrogen atom over a
time interval corresponding to the age of the Universe at z=6. This strongly
suggests that the epoch of reionization was photon-starved and extended.
[Abridged]Comment: 20 pages, 9 figures, accepted for publication in MNRA
A closer look at using quasar near-zones as a probe of neutral hydrogen in the intergalactic medium
We examine a large set of synthetic quasar spectra to realistically assess
the potential of using the relative sizes of highly ionized near-zones in the
Lya and Lyb forest as a probe of the neutral hydrogen content of the
intergalactic medium (IGM) at z>6. The scatter in the relative near-zone size
distribution, induced by underlying fluctuations in the baryonic density field
and the filtering of ionizing radiation, is considerable even for fixed
assumptions about the IGM neutral fraction. As a consequence, the current
observational data cannot distinguish between an IGM which is significantly
neutral or highly ionized just above z=6. Under standard assumptions for quasar
ages and ionizing luminosities, a future sample of several tens of high
resolution Lya and Lyb near-zone spectra should be capable of distinguishing
between a volume weighted neutral hydrogen fraction in the IGM which is greater
or less than 10 per cent.Comment: 6 pages, 3 figures, accepted for publication in MNRAS letter
Control of Blown Pack Spoilage in Vacuum Packaged Meat
End of project reportBlown pack spoilage (BPS) represents a significant commercial loss to Irish meat processors. This research discovered that the organisms causing BPS are ubiquitous in the abattoir environment, making eradication very difficult. The risk of BPS is best managed through a process of regular treatment of plant and equipment with a sporicidal agent such as peroxyacetic acid, good hygiene to minimise carcass contamination and removal of the heat shrinkage stage during vacuum packaging as this activates the spores and reduces the time to spoilage.National Development Plan 2007-201
Photo-heating and the fate of hard photons during the reionisation of HeII by quasars
We use a combination of analytic and numerical arguments to consider the
impact of quasar photo-heating during HeII reionisation on the thermal
evolution of the intergalactic medium (IGM). We demonstrate that rapid (\Delta
z 10^4 K) photo-heating is difficult to achieve
across the entire IGM unless quasar spectra are significantly harder than
implied by current observational constraints. Although filtering of intrinsic
quasar radiation through dense regions in the IGM does increase the mean excess
energy per HeII photo-ionisation, it also weakens the radiation intensity and
lowers the photo-ionisation rate, preventing rapid heating over time intervals
shorter than the local photo-ionisation timescale. Moreover, the hard photons
responsible for the strongest heating are more likely to deposit their energy
inside dense clumps. The abundance of such clumps is, however, uncertain and
model-dependent, leading to a fairly large uncertainty in the photo-heating
rates. Nevertheless, although some of the IGM may be exposed to a hardened and
weakened ionising background for long periods, most of the IGM must instead be
reionised by the more abundant, softer photons and with accordingly modest
heating rates (\Delta T < 10^4 K). The repeated ionisation of fossil quasar
HeIII regions does not increase the net heating because the recombination times
in these regions typically exceed the IGM cooling times and the average time
lag between successive rounds of quasar activity. Detailed line-of-sight
radiative transfer simulations confirm these expectations and predict a rich
thermal structure in the IGM during HeII reionisation. [Abridged]Comment: 20 pages, 6 figures, accepted by MNRA
Sensory and chemical evaluation of flavor in ground beef from grass- and grain-fed steers
A total of 48 yearling Angus, Hereford, and Charolais breeds were divided into two groups of 24 steers, and during the 1985 spring and summer, each group grazed a different pasture (PType): fescue- orchardgrass-clover or ryegrass. When each pasture no longer could sustain grazing in the summer, each group of steers was divided into 3 sub-groups of 8 steers each. One sub-group from each PType was killed directly off-pasture and represented the 0 days on feed (DOF) group. The other sub-groups were accustomed to a whole-shell com diet over a two-week period and were fed the com diet up to 112-DOF in a drylot at the University of Tennessee Plateau Experiment Station, Crossville, TN. At 0-, 56-, and 112-DOF, the sub-groups from each PType was transported to and slaughtered at Lay\u27s Packing Co. Knoxville, TN. Before slaughter, fat thickness at the 12th-13th rib and steer weight were recorded along with hot carcass weight at the plant for each animal. After the carcass of each steer was chilled at 6°C for a 48-hr period, a hindquarter from each was transported to The University of Tennessee Meats Laboratory. Ground beef containing approximately 20% fat was formulated from the biceps femoris muscle and external fat from the round of each carcass. Ground beef from each steer was analyzed for total lipid content. From each steer, cooked ground beef patties held at an internal temperature of 55 °C, were evaluated for aroma and flavor by Quantitative Descriptive Analysis (QDA). The headspace volatiles of cooked ground beef from 6 steers in each sub-group were also analyzed. Volatile compounds from cooked ground beef held at 55 °C were trapped on a Tenax GC trap, and then analyzed on a 30-m, 0.25 mm i.d. fuzed silica SPB-35 capillary column by gas chromotography and mass spectrometry. The relative percentage of abundance (RPA) of each volatile, intensity of each QDA flavor descriptor, total lipid content and animal measurements were statistically analyzed as a function of PType, DOF and their interaction. Factor analysis was also applied separately to the volatiles and the QDA descriptors to reduce these analyses to the lowest number of factors. The chemical volatiles were also correlated with QDA flavor analysis and the ground beef samples classified into sources by statistical procedures. Levels of the undesirable milky-oily aroma and flavor were significantly reduced and the desirable beef fat aroma and flavor intensities increased by feeding a com diet to beef cattle after they had grazed on pasture. Although the QDA panel used 6 different descriptors to evaluate beef flavor, factor analysis reduced all 6 to a single factor which was called Beefy . Differences found in the flavor descriptor intensities allowed the panel to correctly classify approximately 94% of the samples into their sources of PType and DOF groups. Forty volatile compounds, of which 29 were positively identified, were found in the headspace of the ground beef samples. Changes in the RPA of 22 volatile compounds of ground beef were found with increasing DOF. Levels of 11 volatiles (toluene, ethyl-benzene, propyl-benzene, l-ethyl-2-methyl-benzene, pentanal, nonane, 4-methyl-3-penten-2-one, 3-penten-2-one, and 3 unknown compounds) were found to be important in describing DOF affects for ground beef. Six of these compounds (toluene, pentanal, l-ethyl-2-methyl-benzene, 4-methyl-3-penten-2-one, and 2 unknowns) were able to correctly classify approximately 94% of the feeding groups into the correct days on feed classification, RPA of 16 volatile compounds were correlated with the intensities of QDA descriptors (P \u3c.05). Toluene was the only compound found to be positively correlated with milky-oily flavor in ground beef. Seven compounds were positively correlated with beef fat flavor with octane having the highest correlation coefficient (r=0.62). Five compounds (pentanal, ethyl-benzene, toluene, l-ethyl-2-methyl-benzene, and an unknown) were found to account for 51% of the variation for beef fat flavor thus indicating that any of these compounds may be suitable as indicators for Beefy flavor
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