266 research outputs found
On The Complexity and Completeness of Static Constraints for Breaking Row and Column Symmetry
We consider a common type of symmetry where we have a matrix of decision
variables with interchangeable rows and columns. A simple and efficient method
to deal with such row and column symmetry is to post symmetry breaking
constraints like DOUBLELEX and SNAKELEX. We provide a number of positive and
negative results on posting such symmetry breaking constraints. On the positive
side, we prove that we can compute in polynomial time a unique representative
of an equivalence class in a matrix model with row and column symmetry if the
number of rows (or of columns) is bounded and in a number of other special
cases. On the negative side, we show that whilst DOUBLELEX and SNAKELEX are
often effective in practice, they can leave a large number of symmetric
solutions in the worst case. In addition, we prove that propagating DOUBLELEX
completely is NP-hard. Finally we consider how to break row, column and value
symmetry, correcting a result in the literature about the safeness of combining
different symmetry breaking constraints. We end with the first experimental
study on how much symmetry is left by DOUBLELEX and SNAKELEX on some benchmark
problems.Comment: To appear in the Proceedings of the 16th International Conference on
Principles and Practice of Constraint Programming (CP 2010
Tractable Combinations of Global Constraints
We study the complexity of constraint satisfaction problems involving global
constraints, i.e., special-purpose constraints provided by a solver and
represented implicitly by a parametrised algorithm. Such constraints are widely
used; indeed, they are one of the key reasons for the success of constraint
programming in solving real-world problems.
Previous work has focused on the development of efficient propagators for
individual constraints. In this paper, we identify a new tractable class of
constraint problems involving global constraints of unbounded arity. To do so,
we combine structural restrictions with the observation that some important
types of global constraint do not distinguish between large classes of
equivalent solutions.Comment: To appear in proceedings of CP'13, LNCS 8124. arXiv admin note: text
overlap with arXiv:1307.179
VLT FORS2 optical imaging and spectroscopy of nine luminous type 2 AGN at 0.3 < z < 0.6-I. Ionized gas nebulae
We present optical imaging and long slit spectroscopic observations of nine luminous type
2 active galactic nuclei (AGN) within the redshift range 0.3 <z< 0.6 based on Very Large
Telescope Focal Reducer and Low Dispersion Spectrograph (VLT FORS2) data. Most objects
(6/9) are high luminosity Seyfert 2, and three are type 2 quasars (QSO2), with our sample
extending to lower luminosity than previous works. Seven out of nine objects (78 per cent)
show morphological evidence for interactions or mergers in the form of disturbed morphologies
and/or peculiar features such as tidal tails, amorphous haloes, or compact emission line knots.
The detection rate of morphological evidence for interaction is consistent with those found
during previous studies of QSO2 at similar z, suggesting that the merger rate is independent
of AGN power at the high end of the AGN luminosity function. We find the emission line flux
spatial profiles are often dominated by the often spatially unresolved central source. In addition,
all but one of our samples is associated with much fainter, extended line emission. We find
these extended emission line structures have a variety of origins and ionization mechanisms:
star-forming companions, tidal features, or extended ionized nebulae. AGN related processes
dominate the excitation of the nuclear gas. Stellar photoionization sometimes plays a role in
extended structures often related to mergers/interactions
A distributed optimization method for the geographically distributed data centres problem
The geographically distributed data centres problem (GDDC) is a naturally distributed resource allocation problem. The problem involves allocating a set of virtual machines (VM) amongst the data centres (DC) in each time period of an operating horizon. The goal is to optimize the allocation of workload across a set of DCs such that the energy cost is minimized, while respecting limitations on data centre capacities, migrations of VMs, etc. In this paper, we propose a distributed optimization method for GDDC using the distributed constraint optimization (DCOP) framework. First, we develop a new model of the GDDC as a DCOP where each DC operator is represented by an agent. Secondly, since traditional DCOP approaches are unsuited to these types of large-scale problem with multiple variables per agent and global constraints, we introduce a novel semi-asynchronous distributed algorithm for solving such DCOPs. Preliminary results illustrate the benefits of the new method
Young stellar populations in type II quasars: timing the onset of star formation and nuclear activity
Despite the emerging morphological evidence that luminous quasar-like active galactic nuclei
(AGN) are triggered in galaxy mergers, the natures of the triggering mergers and the order
of events in the triggering sequence remain uncertain. In this work, we present a detailed
study of the stellar populations of the host galaxies of 21 type II quasars, with the aim of
understanding the sequence of events between the onset of the merger, the triggering of the
associated starburst and the initiation of the quasar activity. To this end, we model highquality,
wide spectral coverage, intermediate-resolution optical spectra of the type II quasars.
We find that of the 21 objects, the higher order Balmer absorption lines, characteristic of
young stellar populations (YSPs), are directly detected in ∼62 per cent of the sample. We also
fit these spectra using a number of combinations of stellar and/or power-law components,
representative of viable formation histories, as well as including the possibility of scattered
AGN light. We find that ∼90 per cent of the type II quasar host galaxies require the inclusion of
a YSP to adequately model their spectra, whilst 71 per cent of the sample require the inclusion
of a YSP with age <100 Myr. Since the ages of the YSP in most type II quasar host galaxies
are comparable with the expected lifetimes of the AGN activity, these results provide strong
evidence that the quasars are triggered close to the peaks of the merger-induced starbursts
Mergers and interactions in SDSS type 2 quasars at z~0.3-0.4. SDSS J143027.66-005614.8: a case study
We present a compilation of HST images of 58 luminous SDSS type 2 AGNs at
0.3<z<0.4. 42 of them are type 2 quasars, which are a good representation of
all optically selected SDSS type 2 quasars in this range. We find that the
majority of the host galaxies are ellipticals (30/42 or 71%). This is
consistent with studies of radio loud and radio quiet type 1 quasars which show
that their host galaxies are in general ellipticals. A significant fraction of
type 2 quasars (>25/42 or >59%) show clear signatures of morphological
disturbance which are in most cases identified with merger/interaction
processes. We discuss this in the context of related works on type 2 quasars
and powerful radio galaxies.
We study in detail the particular case of the radio quiet type 2 quasar SDSS
J143027.66-005614.8 at z=0.32 based on VLT, HST and SDSS imaging and
spectroscopic data. We discuss the global properties of the object in the
context of theoretical and observational studies of galaxy mergers/interactions
and their role in the triggering of the nuclear and star formation activities
in the most luminous active galaxies.Comment: Accepted for publication in MNRAS. 25 pages, 21 figure
Investigating the impact of quasar-driven outflows on galaxies at redshift 0.3-0.4
We present a detailed study of the kinematics of 19 QSO2s in the range
0.3 10^{8.5}_{\odot}\lambda\r{A}_{OF}_{\odot}\pm_{OF}\pm_{\odot}^{-1}_{kin}\pm^{-1}\pm_{\odot}^{-1}\pm^{-1}$). We do not find any significant
correlation between the outflow properties and the previously mentioned galaxy
properties.
4 out of 5 QSO2s without a YS<100 Myr show highly disturbed kinematics,
whereas only 5 out of the 14 QSO2s with YSPs show similarly asymmetric [OIII]
profiles. This might be indicative of negative feedback. The lack of
correlation between the outflow properties and the presence of mergers in
different interaction stages might be due to their different dynamical
timescales. Lastly, the small radio luminosity range covered by our sample may
be impeding the detection of any correlation between radio emission and outflow
properties.Comment: 26 pages, 9 figures. Accepted for publication in A&
Galaxy-wide radio-induced feedback in a radio-quiet quasar
We report the discovery of a radio-quiet type 2 quasar (SDSS J165315.06+234943.0 nicknamed the ‘Beetle’ at z = 0.103) with unambiguous evidence for active galactic nucleus (AGN) radio-induced feedback acting across a total extension of ∼46 kpc and up to ∼26 kpc from the AGN. To the best of our knowledge, this is the first radio-quiet system where radio-induced feedback has been securely identified at ≫several kpc from the AGN. The morphological, ionization and kinematic properties of the extended ionized gas are correlated with the radio structures. We find along the radio axis (a) enhancement of the optical line emission at the location of the radio hotspots (b) turbulent gas kinematics (FWHM ∼ 380–470 km s−1) across the entire spatial range circumscribed by them (c) ionization minima for the turbulent gas at the location of the hot spots, (d) high temperature Te ≳ 1.9 × 104 K at the NE hotspot. Turbulent gas is also found far from the radio axis, ∼25 kpc in the perpendicular direction. We propose a scenario in which the radio structures have perforated the interstellar medium of the galaxy and escaped into the circumgalactic medium. While advancing, they have interacted with in situ gas modifying its properties. Our results show that jets of modest power can be the dominant feedback mechanism acting across huge volumes in radio-quiet systems, including highly accreting luminous AGNs, where radiative mode feedback may be expected
Consistency Checking for the Evolution of Cardinality-based Feature Models
International audienceFeature models (FMs) are a widely used approach to specify the commonalities and variability in variable systems and software product lines. Various works have addressed edits to FMs for FM evolution and tool support to ensure consistency of FMs. An important extension to FMs are feature cardinalities and related constraints, as extensively used e.g., when modeling variability of cloud computing environments. Since cardinality-based FMs pose additional complexity, additional support for evolution and consistency checking with respect to feature cardinalities would be desirable, but has not been addressed yet. In this paper, we discuss common cardinality-based FM edits and resulting inconsistencies based on experiences with FMs in cloud domain. We introduce tool-support for automated inconsistency detection and explanation based on an off-the-shelf solver. We demonstrate the feasibility of the approach by an empirical evaluation showing the performance of the tool
- …