7,121 research outputs found
Estimating Heterogeneous Primal Capacity and Capacity Utilization Measures in a Multi-Species Fishery
We use a stochastic production frontier model to investigate the presence of heterogeneous production and its impact on fleet capacity and capacity utilization in a multi-species fishery. Furthermore, we propose a new fleet capacity estimate that incorporates complete information on the stochastic differences between each vessel-specific technical efficiency distribution. Results indicate that ignoring heterogeneity in production technologies within a multi-species fishery, as well as the complete distribution of a vessel's technical efficiency score, may yield erroneous fleet-wide production profiles and estimates of capacity.Resource /Energy Economics and Policy,
Traffic placement policies for a multi-band network
Recently protocols were introduced that enable the integration of synchronous traffic (voice or video) and asynchronous traffic (data) and extend the size of local area networks without loss in speed or capacity. One of these is DRAMA, a multiband protocol based on broadband technology. It provides dynamic allocation of bandwidth among clusters of nodes in the total network. A number of traffic placement policies for such networks are proposed and evaluated. Metrics used for performance evaluation include average network access delay, degree of fairness of access among the nodes, and network throughput. The feasibility of the DRAMA protocol is established through simulation studies. DRAMA provides effective integration of synchronous and asychronous traffic due to its ability to separate traffic types. Under the suggested traffic placement policies, the DRAMA protocol is shown to handle diverse loads, mixes of traffic types, and numbers of nodes, as well as modifications to the network structure and momentary traffic overloads
CSMA/RN: A universal protocol for gigabit networks
Networks must provide intelligent access for nodes to share the communications resources. In the range of 100 Mbps to 1 Gbps, the demand access class of protocols were studied extensively. Many use some form of slot or reservation system and many the concept of attempt and defer to determine the presence or absence of incoming information. The random access class of protocols like shared channel systems (Ethernet), also use the concept of attempt and defer in the form of carrier sensing to alleviate the damaging effects of collisions. In CSMA/CD, the sensing of interference is on a global basis. All systems discussed above have one aspect in common, they examine activity on the network either locally or globally and react in an attempt and whatever mechanism. Of the attempt + mechanisms discussed, one is obviously missing; that is attempt and truncate. Attempt and truncate was studied in a ring configuration called the Carrier Sensed Multiple Access Ring Network (CSMA/RN). The system features of CSMA/RN are described including a discussion of the node operations for inserting and removing messages and for handling integrated traffic. The performance and operational features based on analytical and simulation studies which indicate that CSMA/RN is a useful and adaptable protocol over a wide range of network conditions are discussed. Finally, the research and development activities necessary to demonstrate and realize the potential of CSMA/RN as a universal, gigabit network protocol is outlined
Symmetron Cosmology
The symmetron is a scalar field associated with the dark sector whose
coupling to matter depends on the ambient matter density. The symmetron is
decoupled and screened in regions of high density, thereby satisfying local
constraints from tests of gravity, but couples with gravitational strength in
regions of low density, such as the cosmos. In this paper we derive the
cosmological expansion history in the presence of a symmetron field, tracking
the evolution through the inflationary, radiation- and matter-dominated epochs,
using a combination of analytical approximations and numerical integration. For
a broad range of initial conditions at the onset of inflation, the scalar field
reaches its symmetry-breaking vacuum by the present epoch, as assumed in the
local analysis of spherically-symmetric solutions and tests of gravity. For the
simplest form of the potential, the energy scale is too small for the symmetron
to act as dark energy, hence we must add a cosmological constant to drive
late-time cosmic acceleration. We briefly discuss a class of generalized,
non-renormalizable potentials that can have a greater impact on the late-time
cosmology, though cosmic acceleration requires a delicate tuning of parameters
in this case.Comment: 42 page
Extremely high data-rate, reliable network systems research
Significant progress was made over the year in the four focus areas of this research group: gigabit protocols, extensions of metropolitan protocols, parallel protocols, and distributed simulations. Two activities, a network management tool and the Carrier Sensed Multiple Access Collision Detection (CSMA/CD) protocol, have developed to the point that a patent is being applied for in the next year; a tool set for distributed simulation using the language SIMSCRIPT also has commercial potential and is to be further refined. The year's results for each of these areas are summarized and next year's activities are described
The Kinematics of Morphologically Selected z~2 Galaxies in the GOODS-N Field
We present near-IR spectra of H-alpha emission from 13 galaxies at z~2 in the
GOODS-N field. The galaxies were selected primarily because they appear to have
elongated morphologies, and slits were aligned with the major axes (as
determined from the rest-frame UV emission) of 11 of the 13. If the galaxies
are elongated because they are highly inclined, alignment of the slit and major
axis should maximize the observed velocity and reveal velocity shear, if
present. In spite of this alignment, we see spatially resolved velocity shear
in only two galaxies. We show that the seeing makes a large difference in the
observed velocity spread of a tilted emission line, and use this information to
place limits on the velocity spread of the ionized gas of the galaxies in the
sample: we find that all 13 have v_{0.5} < 110 km/s, where v_{0.5} is the
velocity shear (half of the velocity range of a tilted emission line) that
would be observed under our best seeing conditions of ~0.5". When combined with
previous work, our data also indicate that aligning the slit along the major
axis does not increase the probability of observing a tilted emission line. We
then focus on the one-dimensional velocity dispersion \sigma, which is much
less affected by the seeing, and see that the elongated subsample exhibits a
significantly lower velocity dispersion than galaxies selected at random from
our total H-alpha sample, not higher as one might have expected. We also see
some evidence that the elongated galaxies are less reddened than those randomly
selected using only UV colors. Both of these results are counter to what would
be expected if the elongated galaxies were highly inclined disks. It is at
least as likely that the galaxies' elongated morphologies are due to merging
subunits.Comment: 9 pages, 5 figures. Accepted for publication in Ap
The Direct Detection of Lyman Continuum Emission from Star-forming Galaxies at z~3
We present the results of rest-frame UV spectroscopic observations of a sample of 14 z ~ 3 star-forming galaxies in the SSA 22a field. These spectra are characterized by unprecedented depth in the Lyman continuum region. For the first time, we have detected escaping ionizing radiation from individual galaxies at high redshift, with 2 of the 14 objects showing significant emission below the Lyman limit. We also measured the ratio of emergent flux density at 1500 Å to that in the Lyman continuum region, for the individual detections (C49 and D3) and the sample average. If a correction for the average IGM opacity is applied to the spectra of the objects C49 and D3, we find f_(1500)/f_(900,corr,C49) = 4.5 and f_(1500)/f_(900,corr,D3) = 2.9. The average emergent flux density ratio in our sample is = 22, implying an escape fraction ~4.5 times lower than inferred from the composite spectrum from Steidel and coworkers. If this new estimate is representative of LBGs, their contribution to the metagalactic ionizing radiation field is J_ν(900) ~ 2.6 × 10^(-22) ergs s^(-1) cm^(-2) Hz^(-1) sr^(-1), comparable to the contribution of optically selected quasars at the same redshift. The sum of the contributions from galaxies and quasars is consistent with recent estimates of the level of the ionizing background at z ~ 3, inferred from the H I Lyα forest optical depth. There is significant variance among the emergent far-UV spectra in our sample, yet the factors controlling the detection or nondetection of Lyman continuum emission from galaxies are not well determined. Because we do not yet understand the source of this variance, significantly larger samples will be required to obtain robust constraints on the galaxy contribution to the ionizing background at z ~ 3 and beyond
Evidence for Solar Metallicities in Massive Star-forming Galaxies at z>~2
We present results of near-IR spectroscopic measurements of 7 star-forming
galaxies at 2.1<z<2.5. Drawn from a large spectroscopic survey of galaxies
photometrically pre-selected by their U_nGR colors to lie at z~2, these
galaxies were chosen for their bright rest-frame optical luminosities
(K_s<=20.0). Most strikingly, the majority of the sample of 7 galaxies exhibit
[NII]/Ha nebular emission line ratios indicative of at least solar HII region
metallicities, at a lookback time of 10.5 Gyr. The broadband colors of the
K_s-bright sample indicate that most have been forming stars for more than a
Gyr at z~2, and have already formed stellar masses in excess of 10^11 Msun. The
descendants of these galaxies in the local universe are most likely metal-rich
and massive spiral and elliptical galaxies, while plausible progenitors for
them can be found among the population of z~3 Lyman Break Galaxies. While the
K_s-bright z~2 galaxies appear to be highly evolved systems, their large Ha
luminosities and uncorrected Ha star-formation rates of 24-60 Msun/yr indicate
that active star formation is still ongoing. The luminous UV-selected objects
presented here comprise more than half of the high-redshift (z>1.5) tails of
current K-band-selected samples such as the K20 and Gemini Deep Deep surveys.Comment: 15 pages including 5 figures. Accepted for publication in Ap
Fixed-Effect Estimation of Highly-Mobile Production Technologies
We consider fixed-effect estimation of a production function where inputs and outputs vary over time, space, and cross-sectional unit. Variability in the spatial dimension allows for time-varying individual effects, without parametric assumptions on the effects. Asymptotics along the spatial dimension provide consistency and normality of the marginal products. A finite-sample example is provided: a production function for bottom-trawler fishing vessels in the flatfish fisheries of the Bering Sea. We find significant spatial variability of output (catch) which we exploit in estimation of a harvesting function
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