4,987 research outputs found
Using 21-cm absorption surveys to measure the average HI spin temperature in distant galaxies
We present a statistical method for measuring the average HI spin temperature
in distant galaxies using the expected detection yields from future wide-field
21cm absorption surveys. As a demonstrative case study we consider a simulated
all-southern-sky survey of 2-h per pointing with the Australian Square
Kilometre Array Pathfinder for intervening HI absorbers at intermediate
cosmological redshifts between and . For example, if such a survey
yielded absorbers we would infer a harmonic-mean spin temperature of
K for the population of damped Lyman
(DLAs) absorbers at these redshifts, indicating that more than
per cent of the neutral gas in these systems is in a cold neutral medium (CNM).
Conversely, a lower yield of only 100 detections would imply
K and a CNM fraction less than per
cent. We propose that this method can be used to provide independent
verification of the spin temperature evolution reported in recent 21cm surveys
of known DLAs at high redshift and for measuring the spin temperature at
intermediate redshifts below , where the Lyman- line is
inaccessible using ground-based observatories. Increasingly more sensitive and
larger surveys with the Square Kilometre Array should provide stronger
statistical constraints on the average spin temperature. However, these will
ultimately be limited by the accuracy to which we can determine the HI column
density frequency distribution, the covering factor and the redshift
distribution of the background radio source population.Comment: 11 pages, 9 figures, 1 table. Proof corrected versio
Ultra-violet photo-ionisation in far-infrared selected sources
It has been reported that there is a deficit of stellar heated dust, as
evident from the lack of far-infrared (FIR) emission, in sources within the
Herschel-SPIRE sample with X-ray luminosities exceeding a ``critical value'' of
L~10^37 W. Such a scenario would be consistent with the suppression of star
formation by the AGN, required by current theoretical models. Since absorption
of the 21-cm transition of neutral hydrogen (HI), which traces the star-forming
reservoir, also exhibits a critical value in the ultra-violet band (above
ionising photon rates of Q ~ 3 x 10^56 s^-1), we test the SPIRE sample for the
incidence of the detection of 250 micron emission with Q. The highest value at
which FIR emission is detected above the SPIRE confusion limit is Q = 8.9 x
10^57 s^-1, which is ~30 times that for the HI, with no critical value
apparent. Since complete ionisation of the neutral atomic gas is expected at Q
> 3 x 10^56 s-1., this may suggest that much of the FIR must arise from heating
of the dust by the AGN. However, integrating the ionising photon rate of each
star over the initial mass function, we cannot rule out that the high observed
ionising photon rates are due to a population of hot, massive stars.Comment: Accepted by A&
System configuration and executive requirements specifications for reusable shuttle and space station/base
System configuration and executive requirements specifications for reusable shuttle and space station/bas
Medium power hydrogen arcjet performance
An experimental investigation was performed to evaluate hydrogen arcjet operating characteristics in the range of 1 to 4 kW. A series of nozzles were operated in modular laboratory thrusters to examine the effects of geometric parameters such as constrictor diameter and nozzle divergence angle. Each nozzle was tested over a range of current and mass flow rates to explore stability and performance. In the range of mass flow rates and power levels tested, specific impulse values between 650 and 1250 sec were obtained at efficiencies between 30 and 40 percent. The performance of the two larger half angle (20, 15 deg) nozzles was similar for each of the two constrictor diameters tested. The nozzles with the smallest half angle (10 deg) were difiicult to operate. A restrike mode of operation was identified and described. Damage in the form of melting was observed in the constrictor region of all the nozzle inserts tested. Arcjet ignition was also difficult in many tests and a glow discharge mode that prevents starting was identified
Postrelease survival, vertical and horizontal movements, and thermal habitats of five species of pelagic sharks in the central Pacific Ocean
From 2001 to 2006, 71 pop-up satellite archival tags (PSATs)
were deployed on five species of pelagic shark (blue shark [Prionace glauca]; shortfin mako [Isurus oxyrinchus]; silky shark [Carcharhinus falciformis]; oceanic whitetip shark
[C. longimanus]; and bigeye thresher [Alopias superciliosus]) in the central Pacific Ocean to determine species-specific movement patterns and survival rates after release from longline fishing gear. Only a single postrelease mortality could be unequivocally documented:
a male blue shark which succumbed seven days after release.
Meta-analysis of published reports and the current study (n=78 reporting PSATs) indicated that the summary
effect of postrelease mortality for blue sharks was 15% (95% CI, 8.5–25.1%) and suggested that catch-and-release
in longline fisheries can be a viable management tool to protect parental biomass in shark populations. Pelagic sharks displayed species-specific depth and temperature ranges, although with significant individual temporal and spatial variability in vertical movement patterns, which
were also punctuated by stochastic events (e.g., El Niño-Southern Oscillation). Pelagic species can be separated
into three broad groups based on daytime temperature preferences by using the unweighted pair-group method with arithmetic averaging clustering on a Kolmogorov-Smirnov
Dmax distance matrix: 1) epipelagic species (silky and oceanic whitetip sharks), which spent >95% of their
time at temperatures within 2°C of sea surface temperature; 2) mesopelagic-I species (blue sharks and shortfin makos, which spent 95% of their time at temperatures from 9.7°
to 26.9°C and from 9.4° to 25.0°C, respectively; and 3) mesopelagic-II species (bigeye threshers), which spent 95% of their time at temperatures from 6.7° to 21.2°C. Distinct
thermal niche partitioning based on body size and latitude was also evident within epipelagic species
Tracemaking Activities of Crabs and Their Environmental Significance: The Ichnogenus \u3ci\u3ePsilonichnus\u3c/i\u3e
Modem crabs are common inhabitants of shallow subtidal, intertidal, and supratidal environments, and many crabs are capable of producing traces that can be preserved in the rock record. The first crabs, Early Jurassic in age, probably were not fossorial. By Cretaceous time, however, diverse endobenthic lineages were established. Many representatives of these lineages undoubtedly produced domiciles that are preserved in shallow marine to quasimarine sediments and that should be useful in characterizing the depositional environment of the sediments. Nonetheless, most such dwelling structures have been studied little and remain essentially unnamed.
The ichnogenus Psilonichnus Fiirsich is amenable to the taxonomic concept of several forms of crab burrows; presently recognized ichnospecies include P. tubiformis Fiirsich and P. upsilon (n. ichnosp.). Future work may reveal the need for further ichnospecific differentiation. The occurrence of Psilonichnus upsilon and related burrow forms should prove to be a useful criterion for the identification of marine-margin facies in the rock record.
Certain crabs also produce domiciles referable to Thalassinoides, Gyrolithes, and Skolithos, and possibly Macanopsis and Spongeliomorpha. Except for Skolithos, such structures traditionally have been attributed to shrimp, lobsters, or stomatopods. Ethologic and taxonomic re-evaluation of these burrow forms is needed
Further Observational Evidence for a Critical Ionising Luminosity in Active Galaxies
We report the results of a survey for HI 21-cm absorption at redshifts of z >
2.6 in a new sample of radio sources with the Green Bank and Giant Metrewave
Radio Telescopes. From a total of 25 targets, we report zero detections in the
16 for which optical depth limits could be obtained. Based upon the detection
rate for z > 0.1 associated absorption, we would expect approximately four
detections. Of the 11 which have previously not been searched, there is
sufficient source-frame optical/ultra-violet photometry to determine the
ionising photon rate for four. Adding these to the literature, the hypothesis
that there is a critical rate of logQ = 56 ionising photons per second is now
significant at ~7 sigma. This reaffirms our assertion that searching z > 3
active galaxies for which optical redshifts are available selects sources in
which the ultra-violet luminosity is sufficient to ionise all of the neutral
gas in the host galaxy.Comment: Accepted by MNRA
On Detection of Black Hole Quasi-Normal Ringdowns: Detection Efficiency and Waveform Parameter Determination in Matched Filtering
Gravitational radiation from a slightly distorted black hole with ringdown
waveform is well understood in general relativity. It provides a probe for
direct observation of black holes and determination of their physical
parameters, masses and angular momenta (Kerr parameters). For ringdown searches
using data of gravitational wave detectors, matched filtering technique is
useful. In this paper, we describe studies on problems in matched filtering
analysis in realistic gravitational wave searches using observational data.
Above all, we focus on template constructions, matches or signal-to-noise
ratios (SNRs), detection probabilities for Galactic events, and accuracies in
evaluation of waveform parameters or black hole hairs. We have performed
matched filtering analysis for artificial ringdown signals which are generated
with Monte-Carlo technique and injected into the TAMA300 observational data. It
is shown that with TAMA300 sensitivity, the detection probability for Galactic
ringdown events is about 50% for black holes of masses greater than with SNR . The accuracies in waveform parameter estimations
are found to be consistent with the template spacings, and resolutions for
black hole masses and the Kerr parameters are evaluated as a few % and , respectively. They can be improved up to and for events
of by using fine-meshed template bank in the hierarchical
search strategy.Comment: 10 pages, 10 figure
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