5,556 research outputs found
A BTP-Based Family of Variable Elimination Rules for Binary CSPs
International audienceThe study of broken-triangles is becoming increasingly ambitious , by both solving constraint satisfaction problems (CSPs) in polynomial time and reducing search space size through value merging or variable elimination. Considerable progress has been made in extending this important concept, such as dual broken-triangle and weakly broken-triangle, in order to maximize the number of captured tractable CSP instances and/or the number of merged values. Specifically, m-wBTP allows to merge more values than BTP. k-BTP, WBTP and m-BTP permit to capture more tractable instances than BTP. Here, we introduce a new weaker form of BTP, which will be called m-fBTP for flexible broken-triangle property. m-fBTP allows on the one hand to eliminate more variables than BTP while preserving satisfiability and on the other to define new bigger tractable class for which arc consistency is a decision procedure. Likewise, m-fBTP permits to merge more values than BTP but less than m-wBTP
New schemes for simplifying binary constraint satisfaction problems
Finding a solution to a Constraint Satisfaction Problem (CSP) is known to be an NP-hard task. This has motivatedthe multitude of works that have been devoted to developing techniques that simplify CSP instances before or duringtheir resolution.The present work proposes rigidly enforced schemes for simplifying binary CSPs that allow the narrowing of valuedomains, either via value merging or via value suppression. The proposed schemes can be viewed as parametrizedgeneralizations of two widely studied CSP simplification techniques, namely, value merging and neighbourhoodsubstitutability. Besides, we show that both schemes may be strengthened in order to allow variable elimination,which may result in more significant simplifications. This work contributes also to the theory of tractable CSPs byidentifying a new tractable class of binary CSP
Examining alternatives to wavelet de-noising for astronomical source finding
The Square Kilometre Array and its pathfinders ASKAP and MeerKAT will produce
prodigious amounts of data that necessitate automated source finding. The
performance of automated source finders can be improved by pre-processing a
dataset. In preparation for the WALLABY and DINGO surveys, we have used a test
HI datacube constructed from actual Westerbork Telescope noise and WHISP HI
galaxies to test the real world improvement of linear smoothing, the {\sc
Duchamp} source finder's wavelet de-noising, iterative median smoothing and
mathematical morphology subtraction, on intensity threshold source finding of
spectral line datasets. To compare these pre-processing methods we have
generated completeness-reliability performance curves for each method and a
range of input parameters. We find that iterative median smoothing produces the
best source finding results for ASKAP HI spectral line observations, but
wavelet de-noising is a safer pre-processing technique.
In this paper we also present our implementations of iterative median
smoothing and mathematical morphology subtraction.Comment: 7 pages, 4 figures, 3 colour figures. Accepted as part of the
Publications of the Astronomical Society of Australia's special issue on
source finding and visualisatio
The Galaxy Population of Abell 1367: The Stellar Mass-Metallicity Relation
Using wide baseline broad-band photometry, we analyse the stellar population
properties of a sample of 72 galaxies, spanning a wide range of stellar masses
and morphological types, in the nearby spiral-rich and dynamically young galaxy
cluster Abell 1367. The sample galaxies are distributed from the cluster centre
out to approximately half the cluster Abell radius. The optical/near-infrared
colours are compared with simple stellar population synthesis models from which
the luminosity-weighted stellar population ages and metallicities are
determined. The locus of the colours of elliptical galaxies traces a sequence
of varying metallicity at a narrow range of luminosity-weighted stellar ages.
Lenticular galaxies in the red sequence, however, exhibit a substantial spread
of luminosity-weighted stellar metallicities and ages. For red sequence
lenticular galaxies and blue cloud galaxies, low mass galaxies tend to be on
average dominated by stellar populations of younger luminosity-weighted ages.
Sample galaxies exhibit a strong correlation between integrated stellar mass
and luminosity-weighted stellar metallicity. Galaxies with signs of
morphological disturbance and ongoing star formation activity, tend to be
underabundant with respect to passive galaxies in the red sequence of
comparable stellar masses. We argue that this could be due to tidally-driven
gas flows toward the star-forming regions, carrying less enriched gas and
diluting the pre-existing gas to produce younger stellar populations with lower
metallicities than would be obtained prior to the interaction. Finally, we find
no statistically significant evidence for changes in the luminosity-weighted
ages and metallicities for either red sequence or blue cloud galaxies, at fixed
stellar mass, with location within the cluster.Comment: 15 pages, 10 figures, MNRAS in pres
Walking near a Conformal Fixed Point: the 2-d O(3) Model at theta near pi as a Test Case
Slowly walking technicolor models provide a mechanism for electroweak
symmetry breaking whose nonperturbative lattice investigation is rather
challenging. Here we demonstrate walking near a conformal fixed point
considering the 2-d lattice O(3) model at vacuum angle .
The essential features of walking technicolor models are shared by this toy
model and can be accurately investigated by numerical simulations. We show
results for the running coupling and the beta-function and we perform a finite
size scaling analysis of the massgap close to the conformal point.Comment: 5 pages, 4 figure
What is a Cool-Core Cluster? A Detailed Analysis of the Cores of the X-ray Flux-Limited HIFLUGCS Cluster Sample
We use the largest complete sample of 64 galaxy clusters (HIghest X-ray FLUx
Galaxy Cluster Sample) with available high-quality X-ray data from Chandra, and
apply 16 cool-core diagnostics to them, some of them new. We also correlate
optical properties of brightest cluster galaxies (BCGs) with X-ray properties.
To segregate cool core and non-cool-core clusters, we find that central cooling
time, t_cool, is the best parameter for low redshift clusters with high quality
data, and that cuspiness is the best parameter for high redshift clusters. 72%
of clusters in our sample have a cool core (t_cool < 7.7 h_{71}^{-1/2} Gyr) and
44% have strong cool cores (t_cool <1.0 h_{71}^{-1/2} Gyr). For the first time
we show quantitatively that the discrepancy in classical and spectroscopic mass
deposition rates can not be explained with a recent formation of the cool
cores, demonstrating the need for a heating mechanism to explain the cooling
flow problem. [Abridged]Comment: 45 pages, 19 figures, 7 tables. Accepted for publication in A&A.
Contact Person: Rupal Mittal ([email protected]
The Complete Local Volume Groups Sample - I. Sample Selection and X-ray Properties of the High-Richness Subsample
We present the Complete Local-Volume Groups Sample (CLoGS), a statistically
complete optically-selected sample of 53 groups within 80 Mpc. Our goal is to
combine X-ray, radio and optical data to investigate the relationship between
member galaxies, their active nuclei, and the hot intra-group medium (IGM). We
describe sample selection, define a 26-group high-richness subsample of groups
containing at least 4 optically bright (log L_B>=10.2 LBsol) galaxies, and
report the results of XMM-Newton and Chandra observations of these systems. We
find that 14 of the 26 groups are X-ray bright, possessing a group-scale IGM
extending at least 65kpc and with luminosity >10^41 erg/s, while a further 3
groups host smaller galaxy-scale gas halos. The X-ray bright groups have masses
in the range M_500=0.5-5x10^13 Msol, based on system temperatures of 0.4-1.4
keV, and X-ray luminosities in the range 2-200x10^41 erg/s. We find that
~53-65% of the X-ray bright groups have cool cores, a somewhat lower fraction
than found by previous archival surveys. Approximately 30% of the X-ray bright
groups show evidence of recent dynamical interactions (mergers or sloshing),
and ~35% of their dominant early-type galaxies host AGN with radio jets. We
find no groups with unusually high central entropies, as predicted by some
simulations, and confirm that CLoGS is in principle capable of detecting such
systems. We identify three previously unrecognized groups, and find that they
are either faint (L_X,R500<10^42 erg/s) with no concentrated cool core, or
highly disturbed. This leads us to suggest that ~20% of X-ray bright groups in
the local universe may still be unidentified.Comment: Accepted for publication by MNRAS, 25 Manuscript pages with 6 tables
and 10 figures, plus 30 pages of appendices. v2 corrects minor typographical
errors identified at proof stag
- …