1,466 research outputs found
The O VI Absorbers Toward PG0953+415: High Metallicity, Cosmic-Web Gas Far From Luminous Galaxies
The spectrum of the low-redshift QSO PG0953+415 shows two strong, intervening
O VI absorption systems. To study the nature of these absorbers, we have used
the Gemini Multiobject Spectrograph to conduct a deep spectroscopic galaxy
redshift survey in the 5' x 5' field centered on the QSO. This survey is fully
complete for r' < 19.7 and is 73% complete for r' < 21.0. We find three
galaxies at the redshift of the higher-z O VI system (z = 0.14232) including a
galaxy at projected distance rho = 155 kpc. We find no galaxies in the Gemini
field at the redshift of the lower-z O VI absorber (z = 0.06807), which
indicates that the nearest galaxy is more than 195 kpc away or has L < 0.04 L*.
Previous shallower surveys covering a larger field have shown that the z =
0.06807 O VI absorber is affiliated with a group/filament of galaxies, but the
nearest known galaxy has rho = 736 kpc. The z = 0.06807 absorber is notable for
several reasons. The absorption profiles reveal simple kinematics indicative of
quiescent material. The H I line widths and good alignment of the H I and metal
lines favor photoionization and, moreover, the column density ratios imply a
high metallicity: [M/H] = -0.3 +/- 0.12. The z = 0.14232 O VI system is more
complex and less constrained but also indicates a relatively high metallicity.
Using galaxy redshifts from SDSS, we show that both of the PG0953+415 O VI
absorbers are located in large-scale filaments of the cosmic web. Evidently,
some regions of the web filaments are highly metal enriched. We discuss the
origin of the high-metallicity gas and suggest that the enrichment might have
occurred long ago (at high z).Comment: Submitted for publication in the Astrophysical Journal Letters. Figs.
1 and 2 compressed for astro-ph. High-resolution version available at
http://www.astro.umass.edu/~tripp/astro/qualitypreps/pg0953tripp.pd
Ring Formation in Magnetically Subcritical Clouds and Multiple Star Formation
We study numerically the ambipolar diffusion-driven evolution of
non-rotating, magnetically subcritical, disk-like molecular clouds, assuming
axisymmetry. Previous similar studies have concentrated on the formation of
single magnetically supercritical cores at the cloud center, which collapse to
form isolated stars. We show that, for a cloud with many Jeans masses and a
relatively flat mass distribution near the center, a magnetically supercritical
ring is produced instead. The supercritical ring contains a mass well above the
Jeans limit. It is expected to break up, through both gravitational and
possibly magnetic interchange instabilities, into a number of supercritical
dense cores, whose dynamic collapse may give rise to a burst of star formation.
Non-axisymmetric calculations are needed to follow in detail the expected ring
fragmentation into multiple cores and the subsequent core evolution.
Implications of our results on multiple star formation in general and the
northwestern cluster of protostars in the Serpens molecular cloud core in
particular are discussed.Comment: 25 pages, 4 figures, to appear in Ap
Neural NILM: Deep Neural Networks Applied to Energy Disaggregation
Energy disaggregation estimates appliance-by-appliance electricity
consumption from a single meter that measures the whole home's electricity
demand. Recently, deep neural networks have driven remarkable improvements in
classification performance in neighbouring machine learning fields such as
image classification and automatic speech recognition. In this paper, we adapt
three deep neural network architectures to energy disaggregation: 1) a form of
recurrent neural network called `long short-term memory' (LSTM); 2) denoising
autoencoders; and 3) a network which regresses the start time, end time and
average power demand of each appliance activation. We use seven metrics to test
the performance of these algorithms on real aggregate power data from five
appliances. Tests are performed against a house not seen during training and
against houses seen during training. We find that all three neural nets achieve
better F1 scores (averaged over all five appliances) than either combinatorial
optimisation or factorial hidden Markov models and that our neural net
algorithms generalise well to an unseen house.Comment: To appear in ACM BuildSys'15, November 4--5, 2015, Seou
Recurrent Latent Variable Networks for Session-Based Recommendation
In this work, we attempt to ameliorate the impact of data sparsity in the
context of session-based recommendation. Specifically, we seek to devise a
machine learning mechanism capable of extracting subtle and complex underlying
temporal dynamics in the observed session data, so as to inform the
recommendation algorithm. To this end, we improve upon systems that utilize
deep learning techniques with recurrently connected units; we do so by adopting
concepts from the field of Bayesian statistics, namely variational inference.
Our proposed approach consists in treating the network recurrent units as
stochastic latent variables with a prior distribution imposed over them. On
this basis, we proceed to infer corresponding posteriors; these can be used for
prediction and recommendation generation, in a way that accounts for the
uncertainty in the available sparse training data. To allow for our approach to
easily scale to large real-world datasets, we perform inference under an
approximate amortized variational inference (AVI) setup, whereby the learned
posteriors are parameterized via (conventional) neural networks. We perform an
extensive experimental evaluation of our approach using challenging benchmark
datasets, and illustrate its superiority over existing state-of-the-art
techniques
Envelope Structure of Starless Core L694-2 Derived from a Near-Infrared Extinction Map
We present a near-infrared extinction study of the dark globule L694-2, a
starless core that shows strong evidence for inward motions in molecular line
profiles. The J,H, and K band data were taken using the European Southern
Observatory New Technology Telescope. The best fit simple spherical power law
model has index p=2.6 +/- 0.2, over the 0.036--0.1 pc range in radius sampled
in extinction. This power law slope is steeper than the value of p=2 for a
singular isothermal sphere, the initial condition of the inside-out model for
protostellar collapse. Including an additional extinction component along the
line of sight further steepens the inferred profile. Fitting a Bonnor-Ebert
sphere results in a super-critical value of the dimensionless radius xi_max=25
+/- 3. The unstable configuration of material may be related to the observed
inward motions. The Bonnor-Ebert model matches the shape of the observed
profile, but significantly underestimates the amount of extinction (by a factor
of ~4). This discrepancy in normalization has also been found for the nearby
protostellar core B335 (Harvey et al. 2001). A cylindrical density model with
scale height H=0.0164+/- 0.002 pc viewed at a small inclination to the cylinder
axis provides an equally good radial profile as a power law model, and
reproduces the asymmetry of the core remarkably well. In addition, this model
provides a basis for understanding the discrepancy in the normalization of the
Bonnor-Ebert model, namely that L694-2 has prolate structure, with the full
extent (mass) of the core being missed by assuming symmetry between the
profiles in the plane of the sky and along the line-of-sight. If the core is
sufficiently magnetized then fragmentation may be avoided, and later evolution
might produce a protostar similar to B335.Comment: 38 pages, 7 figures, accepted to Astrophysical Journa
Qucs: an introduction to the new simulation and compact device modelling features implemented in release 0.0.19/0.0.19Src2 of the popular GPL circuit simulator.
Following the release of Qucs-0.0.18 in August 2014 the Qucs Development Team have considered in detail a number of possible directions that future versions of the software could take. Spice4qucs is one of these routes. Spice4qucs is a Qucs extension that allows Qucs schematics, including non-linear Equation-Defined Device compact models, to be simulated with external SPICE-compatible simulation engines. Qucs-0.0.19Src2 uses ngspice and Xyce for this purpose. The recent release of Qucs-0.0.19Src2 includes the latest spice4qucs extensions and is made available as an experimental feature. All Qucs legacy simulation and modelling features remain unchanged. The spice4qucs extensions introduce in release Qucs-0.0.19Src2 include the majority of the SPICE components distributed with the ngspice and Xyce circuit simulation packages. Circuits can be simulated with Qucs qucsator, ngspice and Xyce in particular circuit investigations can be centred around SPICE, .FOUR, .NOISE, and .DISTO analysis, a SPICE compatible parametrization system employing the SPICE .PARAM statement, SPICE behavioural sources, ngnutneg scripts, and an innovative form of Qucs simulation called “Custom Simulation”. The“Custom Simulation” facility allows uses to develop ngnutmeg scripts for circuit design, compact device modelling, Monte-Carlo simulation (and other related circuit statistical routines) and to process simulation output data. Combinations of these new features significantly increases Qucs simulation and modelling capabilities, particularly in established areas like power/RF electronics and new emerging technologies. Qucs-0.0.19Src2 also introduces a preliminary version of a new Qucs EDD to Verilog-A compact model synthesis tool. This feature is released as a GPL open source software tool in support of current compact modelling research
Building a second generation Qucs GPL circuit simulator: package structure, simulation features and compact device modelling capabilities
In 2013 a new Qucs development team started work on an extended version of the popular Qucs GPL circuit simulator. The second generation development team undertook the task of eliminating a number of bugs in the Qucs software, improving its performance and extending its capabilities into new circuit and simulation domains. This presentation outlines the most important changes that have taken place with Qucs releases 0.0.17 and 0.0.18. These include, GUI and Qucssator improvements, post-simulation data processing using Qucs, Octave and Python, compact semiconductor modelling with equation-defined devices and Verilog-A code models and the introduction of a non-linear differential equation library and Octave/Matlab interface. Throughout the presentation a number of behavioural models for two and three terminal devices are introduced and their performance evaluated with data obtained from simulation tests undertaken with the Qucs and Xyce GPL circuit simulators
Existence and Stability of Steady Fronts in Bistable CML
We prove the existence and we study the stability of the kink-like fixed
points in a simple Coupled Map Lattice for which the local dynamics has two
stable fixed points. The condition for the existence allows us to define a
critical value of the coupling parameter where a (multi) generalized
saddle-node bifurcation occurs and destroys these solutions. An extension of
the results to other CML's in the same class is also displayed. Finally, we
emphasize the property of spatial chaos for small coupling.Comment: 18 pages, uuencoded PostScript file, J. Stat. Phys. (In press
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