1,904 research outputs found
High redshift ellipticals: prospects for the VLT
The results and the present limits of the observations of high-z ellipticals
are discussed in the framework of VLT imminent and future instruments.Comment: 6 pages, 3 figures, to appear in the proceedings of the ESO workshop
'Scientific Drivers for ESO Future VLT/VLTI Instrumentation', Garching, June
200
Conformant Planning via Symbolic Model Checking
We tackle the problem of planning in nondeterministic domains, by presenting
a new approach to conformant planning. Conformant planning is the problem of
finding a sequence of actions that is guaranteed to achieve the goal despite
the nondeterminism of the domain. Our approach is based on the representation
of the planning domain as a finite state automaton. We use Symbolic Model
Checking techniques, in particular Binary Decision Diagrams, to compactly
represent and efficiently search the automaton. In this paper we make the
following contributions. First, we present a general planning algorithm for
conformant planning, which applies to fully nondeterministic domains, with
uncertainty in the initial condition and in action effects. The algorithm is
based on a breadth-first, backward search, and returns conformant plans of
minimal length, if a solution to the planning problem exists, otherwise it
terminates concluding that the problem admits no conformant solution. Second,
we provide a symbolic representation of the search space based on Binary
Decision Diagrams (BDDs), which is the basis for search techniques derived from
symbolic model checking. The symbolic representation makes it possible to
analyze potentially large sets of states and transitions in a single
computation step, thus providing for an efficient implementation. Third, we
present CMBP (Conformant Model Based Planner), an efficient implementation of
the data structures and algorithm described above, directly based on BDD
manipulations, which allows for a compact representation of the search layers
and an efficient implementation of the search steps. Finally, we present an
experimental comparison of our approach with the state-of-the-art conformant
planners CGP, QBFPLAN and GPT. Our analysis includes all the planning problems
from the distribution packages of these systems, plus other problems defined to
stress a number of specific factors. Our approach appears to be the most
effective: CMBP is strictly more expressive than QBFPLAN and CGP and, in all
the problems where a comparison is possible, CMBP outperforms its competitors,
sometimes by orders of magnitude
Formal Design of Asynchronous Fault Detection and Identification Components using Temporal Epistemic Logic
Autonomous critical systems, such as satellites and space rovers, must be
able to detect the occurrence of faults in order to ensure correct operation.
This task is carried out by Fault Detection and Identification (FDI)
components, that are embedded in those systems and are in charge of detecting
faults in an automated and timely manner by reading data from sensors and
triggering predefined alarms. The design of effective FDI components is an
extremely hard problem, also due to the lack of a complete theoretical
foundation, and of precise specification and validation techniques. In this
paper, we present the first formal approach to the design of FDI components for
discrete event systems, both in a synchronous and asynchronous setting. We
propose a logical language for the specification of FDI requirements that
accounts for a wide class of practical cases, and includes novel aspects such
as maximality and trace-diagnosability. The language is equipped with a clear
semantics based on temporal epistemic logic, and is proved to enjoy suitable
properties. We discuss how to validate the requirements and how to verify that
a given FDI component satisfies them. We propose an algorithm for the synthesis
of correct-by-construction FDI components, and report on the applicability of
the design approach on an industrial case-study coming from aerospace.Comment: 33 pages, 20 figure
On the robustness of the H Lick index as a cosmic clock in passive early-type galaxies
We examine the H Lick index in a sample of massive () and passive early-type galaxies extracted from SDSS at
z<0.3, in order to assess the reliability of this index to constrain the epoch
of formation and age evolution of these systems. We further investigate the
possibility of exploiting this index as "cosmic chronometer", i.e. to derive
the Hubble parameter from its differential evolution with redshift, hence
constraining cosmological models independently of other probes. We find that
the H strength increases with redshift as expected in passive evolution
models, and shows at each redshift weaker values in more massive galaxies.
However, a detailed comparison of the observed index with the predictions of
stellar population synthesis models highlights a significant tension, with the
observed index being systematically lower than expected. By analyzing the
stacked spectra, we find a weak [NII] emission line (not
detectable in the single spectra) which anti-correlates with the mass, that can
be interpreted as a hint of the presence of ionized gas. We estimated the
correction of the H index by the residual emission component exploiting
different approaches, but find it very uncertain and model-dependent. We
conclude that, while the qualitative trends of the observed H-z
relations are consistent with the expected passive and downsizing scenario, the
possible presence of ionized gas even in the most massive and passive galaxies
prevents to use this index for a quantitative estimate of the age evolution and
for cosmological applications.Comment: 20 pages, 11 figures, 1 table. Accepted for publication in MNRAS Main
Journa
Satisfiability Modulo Transcendental Functions via Incremental Linearization
In this paper we present an abstraction-refinement approach to Satisfiability
Modulo the theory of transcendental functions, such as exponentiation and
trigonometric functions. The transcendental functions are represented as
uninterpreted in the abstract space, which is described in terms of the
combined theory of linear arithmetic on the rationals with uninterpreted
functions, and are incrementally axiomatized by means of upper- and
lower-bounding piecewise-linear functions. Suitable numerical techniques are
used to ensure that the abstractions of the transcendental functions are sound
even in presence of irrationals. Our experimental evaluation on benchmarks from
verification and mathematics demonstrates the potential of our approach,
showing that it compares favorably with delta-satisfiability /interval
propagation and methods based on theorem proving
An improved measurement of baryon acoustic oscillations from the correlation function of galaxy clusters at
We detect the peak of baryon acoustic oscillations (BAO) in the two-point
correlation function of a spectroscopic sample of clusters selected
from the Sloan Digital Sky Survey. Galaxy clusters, as tracers of massive dark
matter haloes, are highly biased structures. The linear bias of the sample
considered in this work, that we estimate from the projected correlation
function, is . Thanks to the high signal in the
cluster correlation function and to the accurate spectroscopic redshift
measurements, we can clearly detect the BAO peak and determine its position,
, with high accuracy, despite the relative paucity of the sample. Our
measurement, , is in good agreement
with previous estimates from large galaxy surveys, and has a similar
uncertainty. The BAO measurement presented in this work thus provides a new
strong confirmation of the concordance cosmological model and demonstrates the
power and promise of galaxy clusters as key probes for cosmological
applications based on large scale structures.Comment: 10 pages, 7 figure, accepted for publication in MNRA
Exploring the formation and evolution of massive ellipticals with Extremely Red Objects
Extremely Red Objects (EROs) provide the important possibility to shed light
on the formation and evolution of the present-day massive ellipticals. On one
hand, they allow to select z>1 old passively evolving spheroidals and to
compare their abundance with the predictions of galaxy formation scenarios. On
the other hand, they provide the possibility to find dust obscured starbursts,
a fraction of which may trace the formation of proto-ellipticals at z>2. In
this paper, the most recent results on EROs are reviewed and the main
implications discussed.Comment: 5 pages, 1 figure, invited contribution, to appear in the proceedings
of the Deep Fields Workshop, Garching, October 200
- …