5,167 research outputs found
Feasibility and benefits of laminar flow control on supersonic cruise airplanes
An evaluation was made of the applicability and benefits of laminar flow control (LFC) technology to supersonic cruise airplanes. Ancillary objectives were to identify the technical issues critical to supersonic LFC application, and to determine how those issues can be addressed through flight and wind-tunnel testing. Vehicle types studied include a Mach 2.2 supersonic transport configuration, a Mach 4.0 transport, and two Mach 2-class fighter concepts. Laminar flow control methodologies developed for subsonic and transonic wing laminarization were extended and applied. No intractible aerodynamic problems were found in applying LFC to airplanes of the Mach 2 class, even ones of large size. Improvements of 12 to 17 percent in lift-drag ratios were found. Several key technical issues, such as contamination avoidance and excresence criteria were identified. Recommendations are made for their resolution. A need for an inverse supersonic wing design methodology is indicated
Measuring the Cosmic Equation of State with Counts of Galaxies
The classical dN/dz test allows the determination of fundamental cosmological
parameters from the evolution of the cosmic volume element. This test is
applied by measuring the redshift distribution of a tracer whose evolution in
number density is known. In the past, ordinary galaxies have been used as such
a tracer; however, in the absence of a complete theory of galaxy formation,
that method is fraught with difficulties. In this paper, we propose studying
instead the evolution of the apparent abundance of dark matter halos as a
function of their circular velocity, observable via the linewidths or rotation
speeds of visible galaxies. Upcoming redshift surveys will allow the linewidth
distribution of galaxies to be determined at both z~1 and the present day. In
the course of studying this test, we have devised a rapid, improved
semi-analytic method for calculating the circular velocity distribution of dark
halos based upon the analytic mass function of Sheth et al. (1999) and the
formation time distribution of Lacey & Cole (1993). We find that if selection
effects are well-controlled and minimal external constraints are applied, the
planned DEEP Redshift Survey should allow the measurement of the cosmic
equation-of-state parameter w to 10% (as little as 3% if Omega_m has been
well-determined from other observations). This type of test has the potential
also to provide a constraint on any evolution of w such as that predicted by
``tracker'' models.Comment: 4 pages plus 3 embedded figures; version approved by Ap. J. Letters.
A greatly improved error analysis has been added, along with a figure showing
complementarity to other cosmological test
Initial temperature and EoS of quark matter from direct photons
The time evolution of the quark gluon plasma created in gold-gold collisions
of the Relativistic Heavy Ion Collider (RHIC) can be described by
hydrodynamical models. Distribution of hadrons reflects the freeze-out state of
the matter. To investigate the time evolution one needs to analyze penetrating
probes, such as direct photon spectra. Distributions of low energy photons was
published in 2010 by PHENIX. In this paper we analyze a 3+1 dimensional
solution of relativistic hydrodynamics and calculate momentum distribution of
direct photons. Using earlier fits of this model to hadronic spectra, we
compare photon calculations to measurements and find that the initial
temperature of the center of the fireball is at least 519+-12 MeV, while for
the equation of state we get c_s= 0.36+-0.02.Comment: Talk at the VI Workshop on Particle Correlations and Femtoscopy,
Kiev, September 14-18, 2010. 6 pages, 1 figure. This work was supported by
the OTKA grant NK73143 and M. Csanad's Bolyai scholarshi
Forming Clusters of Galaxies as the Origin of Unidentified GeV Gamma-Ray Sources
Over half of GeV gamma-ray sources observed by the EGRET experiment have not
yet been identified as known astronomical objects. There is an isotropic
component of such unidentified sources, whose number is about 60 in the whole
sky. Here we calculate the expected number of dynamically forming clusters of
galaxies emitting gamma-rays by high energy electrons accelerated in the shock
wave when they form, in the framework of the standard theory of structure
formation. We find that a few tens of such forming clusters should be
detectable by EGRET and hence a considerable fraction of the isotropic
unidentified sources can be accounted for, if about 5% of the shock energy is
going into electron acceleration. We argue that these clusters are very
difficult to detect in x-ray or optical surveys compared with the conventional
clusters, because of their extended angular size of about 1 degree. Hence they
define a new population of ``gamma-ray clusters''. If this hypothesis is true,
the next generation gamma-ray telescopes such as GLAST will detect more than a
few thousands of gamma-ray clusters. It would provide a new tracer of
dynamically evolving structures in the universe, in contrast to the x-ray
clusters as a tracer of hydrodynamically stabilized systems. We also derive the
strength of magnetic field required for the extragalactic gamma-ray background
by structure formation to extend up to 100 GeV as observed, that is about
10^{-5} of the shock-heated baryon energy density.Comment: Accepted by ApJ after minor revisions. Received May 9, Accepted
August 3. 8 pages including 2 figure
Conceptual design study of a Harrier V/STOL research aircraft
MCAIR recently completed a conceptual design study to define modification approaches to, and derive planning prices for the conversion of a two place Harrier to a V/STOL control, display and guidance research aircraft. Control concepts such as rate damping, attitude stabilization, velocity command, and cockpit controllers are to be demonstrated. Display formats will also be investigated, and landing, navigation and guidance systems flight tested. The rear cockpit is modified such that it can be quickly adapted to faithfully simulate the controls, displays and handling qualities of a Type A or Type B V/STOL. The safety pilot always has take command capability. The modifications studied fall into two categories: basic modifications and optional modifications. Technical descriptions of the basic modifications and of the optional modifications are presented. The modification plan and schedule as well as the test plan and schedule are presented. The failure mode and effects analysis, aircraft performance, aircraft weight, and aircraft support are discussed
Cosmological Implications of the Fundamental Relations of X-ray Clusters
Based on the two-parameter family nature of X-ray clusters of galaxies
obtained in a separate paper, we discuss the formation history of clusters and
cosmological parameters of the universe. Utilizing the spherical collapse model
of cluster formation, and assuming that the cluster X-ray core radius is
proportional to the virial radius at the time of the cluster collapse, the
observed relations among the density, radius, and temperature of clusters imply
that cluster formation occurs in a wide range of redshift. The observed
relations favor the low-density universe. Moreover, we find that the model of
is preferable.Comment: 7 pages, 4 figures. To be published in ApJ Letter
High-Redshift Galaxies: Their Predicted Size and Surface Brightness Distributions and Their Gravitational Lensing Probability
Direct observations of the first generation of luminous objects will likely
become feasible over the next decade. The advent of the Next Generation Space
Telescope (NGST) will allow imaging of numerous galaxies and mini-quasars at
redshifts z>5. We apply semi-analytic models of structure formation to estimate
the rate of multiple imaging of these sources by intervening gravitational
lenses. Popular CDM models for galaxy formation yield a lensing optical depth
of about 1% for sources at redshift 10. The expected slope of the luminosity
function of the early sources implies an additional magnification bias of about
5, bringing the fraction of lensed sources at z=10 to about 5%. We estimate the
angular size distribution of high-redshift disk galaxies and find that most of
them are more extended than the resolution limit of NGST, roughly 0.06
arcseconds. We also show that there is only a modest redshift evolution in the
mean surface brightness of galaxies at z>2. The expected increase by 1-2 orders
of magnitude in the number of resolved sources on the sky, due to observations
with NGST, will dramatically improve upon the statistical significance of
existing weak lensing measurements. We show that, despite this increase in the
density of sources, confusion noise from z>2 galaxies is expected to be small
for NGST observations.Comment: 27 pages, 8 PostScript figures (of which two are new), revised
version accepted for Ap
Nonlinear stochastic biasing from the formation epoch distribution of dark halos
We propose a physical model for nonlinear stochastic biasing of one-point
statistics resulting from the formation epoch distribution of dark halos. In
contrast to previous works on the basis of extensive numerical simulations, our
model provides for the first time an analytic expression for the joint
probability function. Specifically we derive the joint probability function of
halo and mass density contrasts from the extended Press-Schechter theory. Since
this function is derived in the framework of the standard gravitational
instability theory assuming the random-Gaussianity of the primordial density
field alone, we expect that the basic features of the nonlinear and stochastic
biasing predicted from our model are fairly generic. As representative
examples, we compute the various biasing parameters in cold dark matter models
as a function of a redshift and a smoothing length. Our major findings are (1)
the biasing of the variance evolves strongly as redshift while its
scale-dependence is generally weak and a simple linear biasing model provides a
reasonable approximation roughly at R\simgt 2(1+z)\himpc, and (2) the
stochasticity exhibits moderate scale-dependence especially on R\simlt
20\himpc, but is almost independent of . Comparison with the previous
numerical simulations shows good agreement with the above behavior, indicating
that the nonlinear and stochastic nature of the halo biasing is essentially
understood by taking account of the distribution of the halo mass and the
formation epoch.Comment: 34 pages, 11 figures, ApJ (2000) in pres
Merging history as a function of halo environment
According to the hierarchical scenario, galaxies form via merging and
accretion of small objects. Using N-body simulations, we study the frequency of
merging events in the history of the halos. We find that at z<~2 the merging
rate of the overall halo population can be described by a simple power law
(1+z)^3. The main emphasis of the paper is on the effects of environment of
halos at the present epoch (z=0). We find that the halos located inside
clusters have formed earlier (dz \approx 1) than isolated halos of the same
mass. At low redshifts (z<1), the merger rate of cluster halos is 3 times lower
than that of isolated halos and 2 times lower than merger rate of halos that
end up in groups by z=0. At higher redshifts (z~1-4), progenitors of cluster
and group halos have 3--5 times higher merger rates than isolated halos. We
briefly discuss implications of our results for galaxy evolution in different
environments.Comment: submitted to the Astrophys. Journal; 11 pages, 9 figs., LaTeX (uses
emulateapj.sty
The Clustering of Massive Halos
The clustering properties of dark matter halos are a firm prediction of
modern theories of structure formation. We use two large volume,
high-resolution N-body simulations to study how the correlation function of
massive dark matter halos depends upon their mass and formation history. We
find that halos with the lowest concentrations are presently more clustered
than those of higher concentration, the size of the effect increasing with halo
mass; this agrees with trends found in studies of lower mass halos. The
clustering dependence on other characterizations of the full mass accretion
history appears weaker than the effect with concentration. Using the integrated
correlation function, marked correlation functions, and a power-law fit to the
correlation function, we find evidence that halos which have recently undergone
a major merger or a large mass gain have slightly enhanced clustering relative
to a randomly chosen population with the same mass distribution.Comment: 10 pages, 8 figures; text improved, references and one figure added;
accepted for publication in Ap
- …