820 research outputs found
Investigation of Dynamic Store Separation out of a Weapons Bay Cavity Utilizing a Low Speed Wind Tunnel
Characterizing mission store trajectories as they separate from a weapons bay cavity is highly relevant to the Air Force mission. The flow around a weapons bay is unsteady. The unsteady flow can cause a mission store separation trajectory to be unpredictable, and such is the case for what some have termed a pitch bifurcation. Traditional wind tunnel testing is incapable of detecting a bifurcation because traditional wind tunnel testing records time-averaged data. In this study, an experimental testing system was developed and refined in order to support the time-accurate characterization of dynamic mission store separation events. A Motion Test Apparatus integrated with a low-speed wind tunnel maneuvers a model within the wind tunnel test section along a prescribed trajectory. A dedicated data acquisition system, along with sensors, record time-accurate force-and-moment measurements as well as model attitude. Two mission store geometries fabricated of two different materials were studied as they performed a one-off store separation trajectory from a weapons bay cavity. The mission store models separated, alternatively, from forward and aft positions from the weapons bay. Data confirmed that variability in pitch moment experienced by the models was higher for store separation from the aft position. Force-and-moment data also suggests a bifurcation was present for certain test cases
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
Red Galaxy Growth and the Halo Occupation Distribution
We have traced the past 7 Gyr of red galaxy stellar mass growth within dark
matter halos. We have determined the halo occupation distribution, which
describes how galaxies reside within dark matter halos, using the observed
luminosity function and clustering of 40,696 0.2<z<1.0 red galaxies in Bootes.
Half of 10^{11.9} Msun/h halos host a red central galaxy, and this fraction
increases with increasing halo mass. We do not observe any evolution of the
relationship between red galaxy stellar mass and host halo mass, although we
expect both galaxy stellar masses and halo masses to evolve over cosmic time.
We find that the stellar mass contained within the red population has doubled
since z=1, with the stellar mass within red satellite galaxies tripling over
this redshift range. In cluster mass halos most of the stellar mass resides
within satellite galaxies and the intra-cluster light, with a minority of the
stellar mass residing within central galaxies. The stellar masses of the most
luminous red central galaxies are proportional to halo mass to the power of a
third. We thus conclude that halo mergers do not always lead to rapid growth of
central galaxies. While very massive halos often double in mass over the past 7
Gyr, the stellar masses of their central galaxies typically grow by only 30%.Comment: Accepted for publication in the ApJ. 34 pages, 22 Figures, 5 Table
A major radio outburst in III Zw 2 with an extremely inverted, millimeter-peaked spectrum
III Zw 2 is a spiral galaxy with an optical spectrum and faint extended radio
structure typical of a Seyfert galaxy, but also with an extremely variable,
blazar-like radio core. We have now discovered a new radio flare where the
source has brightened more than twenty-fold within less than two years. A
broad-band radio spectrum between 1.4 and 666 GHz shows a textbook-like
synchrotron spectrum peaking at 43 GHz, with a self-absorbed synchrotron
spectral index +2.5 at frequencies below 43 GHz and an optically thin spectral
index -0.75 at frequencies above 43 GHz. The outburst spectrum can be well
fitted by two homogenous, spherical components with equipartition sizes of 0.1
and 0.2 pc at 43 and 15 GHz, and with magnetic fields of 0.4 and 1 Gauss. VLBA
observations at 43 GHz confirm this double structure and these sizes. Time
scale arguments suggest that the emitting regions are shocks which are
continuously accelerating particles. This could be explained by a frustrated
jet scenario with very compact hotspots. Similar millimeter-peaked spectrum
(MPS) sources could have escaped our attention because of their low flux
density at typical survey frequencies and their strong variability.Comment: ApJ Letters, in press, (AAS)LaTeX, 3 figures, available at
http://www2.mpifr-bonn.mpg.de/staff/hfalcke/publications.html#iiizw2 or in a
few weeks at
http://www.mpifr-bonn.mpg.de/staff/falcke/publications.html#iiizw
The DEEP Groth Strip Galaxy Redshift Survey. III. Redshift Catalog and Properties of Galaxies
The Deep Extragalactic Evolutionary Probe (DEEP) is a series of spectroscopic
surveys of faint galaxies, targeted at the properties and clustering of
galaxies at redshifts z ~ 1. We present the redshift catalog of the DEEP 1 GSS
pilot phase of this project, a Keck/LRIS survey in the HST/WFPC2 Groth Survey
Strip. The redshift catalog and data, including reduced spectra, are publicly
available through a Web-accessible database. The catalog contains 658 secure
galaxy redshifts with a median z=0.65, and shows large-scale structure walls to
z = 1. We find a bimodal distribution in the galaxy color-magnitude diagram
which persists to z = 1. A similar color division has been seen locally by the
SDSS and to z ~ 1 by COMBO-17. For red galaxies, we find a reddening of only
0.11 mag from z ~ 0.8 to now, about half the color evolution measured by
COMBO-17. We measure structural properties of the galaxies from the HST
imaging, and find that the color division corresponds generally to a structural
division. Most red galaxies, ~ 75%, are centrally concentrated, with a red
bulge or spheroid, while blue galaxies usually have exponential profiles.
However, there are two subclasses of red galaxies that are not bulge-dominated:
edge-on disks and a second category which we term diffuse red galaxies
(DIFRGs). The distant edge-on disks are similar in appearance and frequency to
those at low redshift, but analogs of DIFRGs are rare among local red galaxies.
DIFRGs have significant emission lines, indicating that they are reddened
mainly by dust rather than age. The DIFRGs in our sample are all at z>0.64,
suggesting that DIFRGs are more prevalent at high redshifts; they may be
related to the dusty or irregular extremely red objects (EROs) beyond z>1.2
that have been found in deep K-selected surveys. (abridged)Comment: ApJ in press. 24 pages, 17 figures (12 color). The DEEP public
database is available at http://saci.ucolick.org
A unified multiwavelength model of galaxy formation
We present a new version of the GALFORM semi-analytical model of galaxy formation. This brings together several previous developments of GALFORM into a single unified model, including a different initial mass function (IMF) in quiescent star formation and in starbursts, feedback from active galactic nuclei supressing gas cooling in massive halos, and a new empirical star formation law in galaxy disks based on their molecular gas content. In addition, we have updated the cosmology, introduced a more accurate treatment of dynamical friction acting on satellite galaxies, and updated the stellar population model. The new model is able to simultaneously explain both the observed evolution of the K-band luminosity function and stellar mass function, and the number counts and redshift distribution of sub-mm galaxies selected at 850ÎŒm. This was not previously achieved by a single physical model within the ÎCDM framework, but requires having an IMF in starbursts that is somewhat top-heavy. The new model is tested against a wide variety of observational data covering wavelengths from the far-UV to sub-mm, and redshifts from z = 0 to z = 6, and is found to be generally successful. These observations include the optical and near-IR luminosity functions, HI mass function, fraction of early type galaxies, Tully-Fisher, metallicity-luminosity and size-luminosity relations at z = 0, as well as far-IR number counts, and far-UV luminosity functions at z ⌠3 â 6. Discrepancies are however found in galaxy sizes and metallicities at low luminosities, and in the abundance of low mass galaxies at high-z, suggesting the need for a more sophisticated model of supernova feedback
The dearth of halo dwarf galaxies: is there power on short scales?
N-body simulations of structure formation with scale-invariant primordial
perturbations show significantly more virialized objects of dwarf-galaxy mass
in a typical galactic halo than are observed around the Milky Way. We show that
the dearth of observed dwarf galaxies could be explained by a dramatic downturn
in the power spectrum at small distance scales. This suppression of small-scale
power might also help mitigate the disagreement between cuspy simulated halos
and smooth observed halos, while remaining consistent with Lyman-alpha-forest
constraints on small-scale power. Such a spectrum could arise in inflationary
models with broken scale invariance.Comment: 5 pages LaTeX, 3 figure
Cosmic-Ray Positrons: Are There Primary Sources?
Cosmic rays at the Earth include a secondary component originating in
collisions of primary particles with the diffuse interstellar gas. The
secondary cosmic rays are relatively rare but carry important information on
the Galactic propagation of the primary particles. The secondary component
includes a small fraction of antimatter particles, positrons and antiprotons.
In addition, positrons and antiprotons may also come from unusual sources and
possibly provide insight into new physics. For instance, the annihilation of
heavy supersymmetric dark matter particles within the Galactic halo could lead
to positrons or antiprotons with distinctive energy signatures. With the
High-Energy Antimatter Telescope (HEAT) balloon-borne instrument, we have
measured the abundances of positrons and electrons at energies between 1 and 50
GeV. The data suggest that indeed a small additional antimatter component may
be present that cannot be explained by a purely secondary production mechanism.
Here we describe the signature of the effect and discuss its possible origin.Comment: 15 pages, Latex, epsfig and aasms4 macros required, to appear in
Astroparticle Physics (1999
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