12,048 research outputs found
Thermal constraints on the reionisation of hydrogen by population-II stellar sources
Measurements of the intergalactic medium (IGM) temperature provide a
potentially powerful constraint on the reionisation history due to the thermal
imprint left by the photo-ionisation of neutral hydrogen. However, until
recently IGM temperature measurements were limited to redshifts 2 < z < 4.8,
restricting the ability of these data to probe the reionisation history at z >
6. In this work, we use recent measurements of the IGM temperature in the
near-zones of seven quasars at z ~ 5.8 - 6.4, combined with a semi-numerical
model for inhomogeneous reionisation, to establish new constraints on the
redshift at which hydrogen reionisation completed. We calibrate the model to
reproduce observational constraints on the electron scattering optical depth
and the HI photo-ionisation rate, and compute the resulting spatially
inhomogeneous temperature distribution at z ~ 6 for a variety of reionisation
scenarios. Under standard assumptions for the ionising spectra of population-II
sources, the near-zone temperature measurements constrain the redshift by which
hydrogen reionisation was complete to be z > 7.9 (6.5) at 68 (95) per cent
confidence. We conclude that future temperature measurements around other high
redshift quasars will significantly increase the power of this technique,
enabling these results to be tightened and generalised.Comment: 15 pages, 8 figures, accepted for publication in MNRA
In Remembrance of Dr. Samantha Rachel Gorse Roberts
Dr. Samantha Roberts, a professor at Texas A&M University-Commerce, was a significant member of the association’s leadership team. Dr. Roberts had a strong passion for student development and mentorship and was heavily involved with the undergraduate and graduate case study competitions. Dr. Clay Bolton provides a touching tribute to our dear colleague and friend
Cosmic Variance In the Transparency of the Intergalactic Medium After Reionization
Following the completion of cosmic reionization, the mean-free-path of
ionizing photons was set by a population of Ly-limit absorbers. As the
mean-free-path steadily grew, the intensity of the ionizing background also
grew, thus lowering the residual neutral fraction of hydrogen in ionization
equilibrium throughout the diffuse intergalactic medium (IGM). Ly-alpha photons
provide a sensitive probe for tracing the distribution of this residual
hydrogen at the end of reionization. Here we calculate the cosmic variance
among different lines-of-sight in the distribution of the mean Ly-alpha optical
depths. We find fractional variations in the effective post-reionization
optical depth that are of order unity on a scale of ~100 co-moving Mpc, in
agreement with observations towards high-redshift quasars. Significant
contributions to these variations are provided by the cosmic variance in the
density contrast on the scale of the mean-free-path for ionizing photons, and
by fluctuations in the ionizing background induced by delayed or enhanced
structure formation. Cosmic variance results in a highly asymmetric
distribution of transmission through the IGM, with fractional fluctuations in
Ly-alpha transmission that ar larger than in Ly-beta transmission.Comment: 7 pages 3 figures. Replaced with version accepted for publication in
Ap
Near-zone sizes and the rest frame extreme ultra-violet spectral index of the highest redshift quasars
The discovery of quasars with redshifts higher than six has prompted a great
deal of discussion in the literature regarding the role of quasars, both as
sources of reionization, and as probes of the ionization state of the IGM.
However the extreme ultra-violet (EUV) spectral index cannot be measured
directly for high redshift quasars owing to absorption at frequencies above the
Lyman limit, and as a result, studies of the impact of quasars on the
intergalactic medium during reionization must assume a spectral energy
distribution in the extreme ultra-violet based on observations at low redshift,
z<1. In this paper we use regions of high Ly-alpha transmission (near-zones)
around the highest redshift quasars to measure the quasar EUV spectral index at
z~6. We jointly fit the available observations for variation of near-zone size
with both redshift and luminosity, and propose that the observed relation
provides evidence for an EUV spectral index that varies with absolute magnitude
in the high redshift quasar sample, becoming softer at higher luminosity. Using
a large suite of detailed numerical simulations we find that the typical value
of spectral index for a luminous quasar at z~6 is constrained to be
alpha=1.3+/-0.4 for a specific luminosity of the form L\propto\nu^{-alpha}. We
find the scatter in spectral index among individual quasars to be in the range
~0.75-1.25. These values are in agreement with direct observations at low
redshift, and indicate that there has been no significant evolution in the EUV
spectral index of quasars over 90% of cosmic time.Comment: 12 pages, 8 figures. Submitted to MNRA
Nose controls on delta wings at supersonic speeds
Expressions are derived for lξ and a2 of nose ailerons and nose elevators on a delta wing, as depicted in Fig. 1, in supersonic flight. Nose and trailing edge controls on delta wings in supersonic flight are compared
A New Ant Species of the Genus Tetramorium Mayr, 1855 (Hymenoptera: Formicidae) from Saudi Arabia, with a Revised Key to the Arabian Species
Tetramorium amalae sp. n. is described and illustrated from Saudi Arabia based on two worker caste specimens collected in Al Bahah region. The new species belongs to the T. shilohense group and appears to be closely related to T. dysderke Bolton from Nigeria. T. amalae is distinguished by having well-developed frontal carinae, smaller eyes, greater head length and width, greater pronotal width, and the petiole node is longer than broad. Tetramorium latinode Collingwood & Agosti is recorded for the first time from Saudi Arabia and for only the second time since the original description. The worker caste of T. latinode is redescribed and illustrated using scanning electron micrographs to facilitate recognition and the gyne is described for the first time with observations given on species relationships, biology and habitat. A revised key to the nineteen Tetramorium species recorded from Arabian Peninsula based on worker castes is provided. Tetramorium bicarinatum (Nylander) is recorded for the first time from Saudi Arabia. It is suggested that T. amalae and T. latinode are endemic to the Arabian Peninsula
How neutral is the intergalactic medium surrounding the redshift z=7.085 quasar ULAS J1120+0641?
The quasar ULAS J1120+0641 at redshift z=7.085 has a highly ionised near zone
which is smaller than those around quasars of similar luminosity at z~6. The
spectrum also exhibits evidence for a damping wing extending redward of the
systemic Lya redshift. We use radiative transfer simulations in a cosmological
context to investigate the implications for the ionisation state of the
inhomogeneous IGM surrounding this quasar. Our simulations show that the
transmission profile is consistent with an IGM in the vicinity of the quasar
with a volume averaged HI fraction of f_HI>0.1 and that ULAS J1120+0641 has
been bright for 10^6--10^7 yr. The observed spectrum is also consistent with
smaller IGM neutral fractions, f_HI ~ 10^-3--10-4, if a damped Lya system in an
otherwise highly ionised IGM lies within 5 proper Mpc of the quasar. This is,
however, predicted to occur in only ~5 per cent of our simulated sight-lines
for a bright phase of 10^6--10^7 yr. Unless ULAS J1120+0641 grows during a
previous optically obscured phase, the low age inferred for the quasar adds to
the theoretical challenge of forming a 2x10^9 M_sol black hole at this high
redshift.Comment: 5 pages, 4 figures, accepted to MNRAS letter
Ultraviolet observations of the X-ray photoionized wind of Cygnus X-1 during X-ray soft/high state
(Shortened) Ultraviolet observations of the black hole X-ray binary Cygnus
X-1 were obtained using the STIS on HSTubble. We detect P Cygni line features
show strong, broad absorption components when the X-ray source is behind the
companion star and noticeably weaker absorption when the X-ray source is
between us and the companion star. We fit the P Cygni profiles using the SEI
method applied to a spherically symmetric stellar wind subject to X-ray
photoionization from the black hole. The Si IV doublet provides the most
reliable estimates of the parameters of the wind and X-ray illumination. The
velocity increases with radius according to
, with and
km s.The microturbulent velocity was
km s. Our fit implies a ratio of X-ray luminosity to wind mass-loss rate
of L, measured at = 4.8. Our
models determine parameters that may be used to estimate the accretion rate
onto the black hole and independently predict the X-ray luminosity. Our
predicted L matches that determined by contemporaneous RXTE ASM remarkably
well, but is a factor of 3 lower than the rate according to
Bondi-Hoyle-Littleton spherical wind accretion. We suggest that some of the
energy of accretion may go into powering a jet.Comment: 34 pages, 21 figures, 4 tables, accepted for publication in Ap
Fast, large volume, GPU enabled simulations for the Ly-alpha forest: power spectrum forecasts for baryon acoustic oscillation experiments
High redshift measurements of the baryonic acoustic oscillation scale (BAO)
from large Ly-alpha forest surveys represent the next frontier of dark energy
studies. As part of this effort, efficient simulations of the BAO signature
from the Ly-alpha forest will be required. We construct a model for producing
fast, large volume simulations of the Ly-alpha forest for this purpose.
Utilising a calibrated semi-analytic approach, we are able to run very large
simulations in 1 Gpc^3 volumes which fully resolve the Jeans scale in less than
a day on a desktop PC using a GPU enabled version of our code. The Ly-alpha
forest spectra extracted from our semi-analytical simulations are in excellent
agreement with those obtained from a fully hydrodynamical reference simulation.
Furthermore, we find our simulated data are in broad agreement with
observational measurements of the flux probability distribution and 1D flux
power spectrum. We are able to correctly recover the input BAO scale from the
3D Ly-alpha flux power spectrum measured from our simulated data, and estimate
that a BOSS-like 10^4 deg^2 survey with ~15 background sources per square
degree and a signal-to-noise of ~5 per pixel should achieve a measurement of
the BAO scale to within ~1.4 per cent. We also use our simulations to provide
simple power-law expressions for estimating the fractional error on the BAO
scale on varying the signal-to-noise and the number density of background
sources. The speed and flexibility of our approach is well suited for exploring
parameter space and the impact of observational and astrophysical systematics
on the recovery of the BAO signature from forthcoming large scale spectroscopic
surveys.Comment: 16 pages, 11 figures, accepted to MNRA
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