15,264 research outputs found
Extending and Exploring the 2 cm Survey Sample
We present new results from the VLBA 2 cm Survey, an imaging survey of active
galactic nuclei (AGN) at sub-milliarcsecond resolution. We extend the
structural variability monitoring program of more than 130 extragalactic
parsec-scale radio jets from over 170 AGN to a total of eight years. The sample
is explored further in time for all sources, and intensively for individual
objects. We report new detailed results on the compact sources III Zw 2, AO
0235+16, and NRAO 512.Comment: Proceedings of the 7th European VLBI Network Symposium (October 12-15
2004, Toledo, Spain), eds. Bachiller, R., Colomer, F., Desmurs, J. F., & de
Vicente, P., 2 tables, 3 figures, needs evn2004.cl
Fast Conical Hull Algorithms for Near-separable Non-negative Matrix Factorization
The separability assumption (Donoho & Stodden, 2003; Arora et al., 2012)
turns non-negative matrix factorization (NMF) into a tractable problem.
Recently, a new class of provably-correct NMF algorithms have emerged under
this assumption. In this paper, we reformulate the separable NMF problem as
that of finding the extreme rays of the conical hull of a finite set of
vectors. From this geometric perspective, we derive new separable NMF
algorithms that are highly scalable and empirically noise robust, and have
several other favorable properties in relation to existing methods. A parallel
implementation of our algorithm demonstrates high scalability on shared- and
distributed-memory machines.Comment: 15 pages, 6 figure
On the nature of the near-UV extended light in Seyfert galaxies
We study the nature of the extended near-UV emission in the inner kiloparsec
of a sample of 15 Seyfert galaxies which have both near-UV (F330W) and narrow
band [OIII] high resolution Hubble images. For the majority of the objects we
find a very similar morphology in both bands. From the [OIII] images we
construct synthetic images of the nebular continuum plus the emission line
contribution expected through the F330W filter, which can be subtracted from
the F330W images. We find that the emission of the ionised gas dominates the
near-UV extended emission in half of the objects. A further broad band
photometric study, in the bands F330W (U), F547M (V) and F160W (H), shows that
the remaining emission is dominated by the underlying galactic bulge
contribution. We also find a blue component whose nature is not clear in 4 out
of 15 objects. This component may be attributed to scattered light from the
AGN, to a young stellar population in unresolved star clusters, or to
early-disrupted clusters. Star forming regions and/or bright off-nuclear star
clusters are observed in 4/15 galaxies of the sample.Comment: 23 pages, 6 figures, 3 tables; accepted for publication in MNRA
GALFIT-CORSAIR: implementing the core-Sersic model into GALFIT
We introduce GALFIT-CORSAIR: a publicly available, fully retro-compatible
modification of the 2D fitting software GALFIT (v.3) which adds an
implementation of the core-Sersic model.
We demonstrate the software by fitting the images of NGC 5557 and NGC 5813,
which have been previously identified as core-Sersic galaxies by their 1D
radial light profiles. These two examples are representative of different dust
obscuration conditions, and of bulge/disk decomposition. To perform the
analysis, we obtained deep Hubble Legacy Archive (HLA) mosaics in the F555W
filter (~V-band). We successfully reproduce the results of the previous 1D
analysis, modulo the intrinsic differences between the 1D and the 2D fitting
procedures.
The code and the analysis procedure described here have been developed for
the first coherent 2D analysis of a sample of core-Sersic galaxies, which will
be presented in a forth-coming paper. As the 2D analysis provides better
constraining on multi-component fitting, and is fully seeing-corrected, it will
yield complementary constraints on the missing mass in depleted galaxy cores.Comment: Accepted for publication in PASP; A binary version of GALFIT-CORSAIR
is publicly available at
http://astronomy.swin.edu.au/~pbonfini/galfit-corsair
Does the extension of primary care practice opening hours reduce the use of emergency services?
Over-crowding in Emergency Departments (EDs) generates potential inefficiencies. Using regional administrative data, we investigate the impact of an increase in the accessibility of primary care on ED visits in Italy. We test whether extending practice opening hours up to 12 hours/day reduces inappropriate ED visits. We estimate count data models, considering different measures for ED visits recorded at the list level. Since the extension programme is voluntary, we also account for the potential endogeneity of participation, using a two-stage residual inclusion and a GMM approach. Our results show that improving primary care accessibility favours a more appropriate use of EDs
ForestHash: Semantic Hashing With Shallow Random Forests and Tiny Convolutional Networks
Hash codes are efficient data representations for coping with the ever
growing amounts of data. In this paper, we introduce a random forest semantic
hashing scheme that embeds tiny convolutional neural networks (CNN) into
shallow random forests, with near-optimal information-theoretic code
aggregation among trees. We start with a simple hashing scheme, where random
trees in a forest act as hashing functions by setting `1' for the visited tree
leaf, and `0' for the rest. We show that traditional random forests fail to
generate hashes that preserve the underlying similarity between the trees,
rendering the random forests approach to hashing challenging. To address this,
we propose to first randomly group arriving classes at each tree split node
into two groups, obtaining a significantly simplified two-class classification
problem, which can be handled using a light-weight CNN weak learner. Such
random class grouping scheme enables code uniqueness by enforcing each class to
share its code with different classes in different trees. A non-conventional
low-rank loss is further adopted for the CNN weak learners to encourage code
consistency by minimizing intra-class variations and maximizing inter-class
distance for the two random class groups. Finally, we introduce an
information-theoretic approach for aggregating codes of individual trees into a
single hash code, producing a near-optimal unique hash for each class. The
proposed approach significantly outperforms state-of-the-art hashing methods
for image retrieval tasks on large-scale public datasets, while performing at
the level of other state-of-the-art image classification techniques while
utilizing a more compact and efficient scalable representation. This work
proposes a principled and robust procedure to train and deploy in parallel an
ensemble of light-weight CNNs, instead of simply going deeper.Comment: Accepted to ECCV 201
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