541 research outputs found
Program for refan JT8D engine design, fabrication and test, phase 2
The objective of the JT8D refan program was to design, fabricate, and test certifiable modifications of the JT8D engine which would reduce noise generated by JT8D powered aircraft. This was to be accomplished without affecting reliability and maintainability, at minimum retrofit cost, and with no performance penalty. The mechanical design, engine performance and stability characteristics at sea-level and altitude, and the engine noise characteristics of the test engines are documented. Results confirmed the structural integrity of the JT8D-109. Engine operation was stable throughout the airplane flight envelope. Fuel consumption of the test engines was higher than that required to meet the goal of no airplane performance penalty, but the causes were identified and corrected during a normal pre-certification engine development program. Compared to the baseline JT8D-109 engine, the acoustically treated JT8D-109 engine showed noise reductions of 6 PNdB at takeoff and 11 PNdB at a typical approach power setting
Does the galaxy correlation length increase with the sample depth?
We have analyzed the behavior of the correlation length, , as a function
of the sample depth by extracting from the CfA2 redshift survey volume--limited
samples out to increasing distances. For a fractal distribution, the value of
would increase with the volume occupied by the sample. We find no linear
increase for the CfA2 samples of the sort that would be expected if the
Universe preserved its small scale fractal character out to the distances
considered (60--100\hmpc). The results instead show a roughly constant value
for as a function of the size of the sample, with small fluctuations due
to local inhomogeneities and luminosity segregation. Thus the fractal picture
can safely be discarded.Comment: Accepted for publication in ApJ
The dwarf low surface brightness population in different environments of the Local Universe
The nature of the dwarf galaxy population as a function of location in the
cluster and within different environments is investigated. We have previously
described the results of a search for low surface brightness objects in data
drawn from an East-West strip of the Virgo cluster (Sabatini et al., 2003) and
have compared this to a large area strip outside of the cluster (Roberts et
al., 2004). In this talk I compare the East-West data (sampling sub-cluster A
and outward) to new data along a North-South cluster strip that samples a
different region (part of sub-cluster A, and the N,M clouds) and with data
obtained for the Ursa Major cluster and fields around the spiral galaxy M101.
The sample of dwarf galaxies in different environments is obtained from uniform
datasets that reach central surface brightness values of ~26 B mag/arcsec^2 and
an apparent B magnitude of 21 (M_B=-10 for a Virgo Cluster distance of 16 Mpc).
We discuss and interpret our results on the properties and distribution of
dwarf low surface brightness galaxies in the context of variuos physical
processes that are thought to act on galaxies as they form and evolve.Comment: 10 pages, 3 figures, to appear in "Dark Galaxies and Lost Baryons",
IAU244 conference proceeding
Summary and analysis of performance and stability characteristics of the refan JT8D-109 engine
The refan JT8D-109 performance and stability characteristics are reported as determined from sea level testing, altitude testing, and DC-9 flight testing. The test results are summarized as follows: (1) TSFC at SLS achieved design goal of 12.66 percent reduction. (2) TSFC at altitude average cruise power 0.5 percent higher than design goal, (3) TSFC at altitude maximum cruise power 1.7-3.7 percent higher than design goal, (4) ground starting consistent with JT8D-9 base engine, (5) successful flight starts without starter assist, (6) transient surge margin equivalent to JT8D-9, (7) stable engine operation with inlet distortion, and (8) stable engine operation during snap acceleration and deceleration. A flight idle setting is required for acceptable aborted-landing go-around acceleration time due to increase in low-rotor moment of inertia, and a performance improvement program should be conducted as part of any future certification program
Calibration of colour gradient bias in shear measurement using HST/CANDELS data
Accurate shape measurements are essential to infer cosmological parameters from large area weak gravitational lensing studies. The compact diffraction-limited point-spread function (PSF) in space-based observations is greatly beneficial, but its chromaticity for a broad band observation can lead to new subtle effects that could hitherto be ignored: the PSF of a galaxy is no longer uniquely defined and spatial variations in the colours of galaxies result in biases in the inferred lensing signal. Taking Euclid as a reference, we show that this colourgradient bias (CG bias) can be quantified with high accuracy using available multi-colour Hubble Space Telescope (HST) data. In particular we study how noise in the HST observations might impact such measurements and find this to be negligible. We determine the CG bias using HST observations in the F606W and F814W filters and observe a correlation with the colour, in line with expectations, whereas the dependence with redshift is weak. The biases for individual galaxies are generally well below 1%, which may be reduced further using morphological information from the Euclid data. Our results demonstrate that CG bias should not be ignored, but it is possible to determine its amplitude with sufficient precision, so that it will not significantly bias the weak lensing measurements using Euclid data
Large-scale retrospective relative spectro-photometric self-calibration in space
We consider the application of relative self-calibration using overlap
regions to spectroscopic galaxy surveys that use slit-less spectroscopy. This
method is based on that developed for the SDSS by Padmanabhan at al. (2008) in
that we consider jointly fitting and marginalising over calibrator brightness,
rather than treating these as free parameters. However, we separate the
calibration of the detector-to-detector from the full-focal-plane
exposure-to-exposure calibration. To demonstrate how the calibration procedure
will work, we simulate the procedure for a potential implementation of the
spectroscopic component of the wide Euclid survey. We study the change of
coverage and the determination of relative multiplicative errors in flux
measurements for different dithering configurations. We use the new method to
study the case where the flat-field across each exposure or detector is
measured precisely and only exposure-to-exposure or detector-to-detector
variation in the flux error remains. We consider several base dither patterns
and find that they strongly influence the ability to calibrate, using this
methodology. To enable self-calibration, it is important that the survey
strategy connects different observations with at least a minimum amount of
overlap, and we propose an "S"-pattern for dithering that fulfills this
requirement. The final survey strategy adopted by Euclid will have to optimise
for a number of different science goals and requirements. The large-scale
calibration of the spectroscopic galaxy survey is clearly cosmologically
crucial, but is not the only one.Comment: 23 pages, 19 figures, Accepted for publication in MNRAS, 201
Cosmic homogeneity demonstrated with luminous red galaxies
We test the homogeneity of the Universe at with the Luminous Red
Galaxy (LRG) spectroscopic sample of the Sloan Digital Sky Survey. First, the
mean number of LRGs within completely surveyed LRG-centered spheres of
comoving radius is shown to be proportional to at radii greater than
. The test has the virtue that it does not rely
on the assumption that the LRG sample has a finite mean density; its results
show, however, that there \emph{is} such a mean density. Secondly, the survey
sky area is divided into 10 disjoint solid angular regions and the fractional
rms density variations of the LRG sample in the redshift range
among these () regions is found to be 7
percent of the mean density. This variance is consistent with typical biased
\lcdm models and puts very strong constraints on the quality of SDSS
photometric calibration.Comment: submitted to Ap
Constraining Modified Gravity with Euclid
Future proposed satellite missions as Euclid can offer the opportunity to
test general relativity on cosmic scales through mapping of the galaxy weak
lensing signal. In this paper we forecast the ability of these experiments to
constrain modified gravity scenarios as those predicted by scalar-tensor and
theories. We found that Euclid will improve constraints expected from
the PLANCK satellite on these modified gravity models by two orders of
magnitude. We discuss parameter degeneracies and the possible biases introduced
by modified gravity
Analytical modeling of large-angle CMBR anisotropies from textures
We propose an analytic method for predicting the large angle CMBR temperature
fluctuations induced by model textures. The model makes use of only a small
number of phenomenological parameters which ought to be measured from simple
simulations. We derive semi-analytically the -spectrum for together with its associated non-Gaussian cosmic variance error bars. A
slightly tilted spectrum with an extra suppression at low is found, and we
investigate the dependence of the tilt on the parameters of the model. We also
produce a prediction for the two point correlation function. We find a high
level of cosmic confusion between texture scenarios and standard inflationary
theories in any of these quantities. However, we discover that a distinctive
non-Gaussian signal ought to be expected at low , reflecting the prominent
effect of the last texture in these multipoles
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