2,093 research outputs found
The First Release COSMOS Optical and Near-IR Data and Catalog
We present imaging data and photometry for the COSMOS survey in 15
photometric bands between 0.3um and 2.4um. These include data taken on the
Subaru 8.3m telescope, the KPNO and CTIO 4m telescopes, and the CFHT 3.6m
telescope. Special techniques are used to ensure that the relative photometric
calibration is better than 1% across the field of view. The absolute
photometric accuracy from standard star measurements is found to be 6%. The
absolute calibration is corrected using galaxy spectra, providing colors
accurate to 2% or better. Stellar and galaxy colors and counts agree well with
the expected values. Finally, as the first step in the scientific analysis of
these data we construct panchromatic number counts which confirm that both the
geometry of the universe and the galaxy population are evolving.Comment: 19 pages, 13 figures, 14 tables, Accepted to ApJS for COSMOS speciall
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A wide angle tail radio galaxy in the COSMOS field: evidence for cluster formation
We have identified a complex galaxy cluster system in the COSMOS field via a
wide angle tail (WAT) radio galaxy consistent with the idea that WAT galaxies
can be used as tracers of clusters. The WAT galaxy, CWAT-01, is coincident with
an elliptical galaxy resolved in the HST-ACS image. Using the COSMOS
multiwavelength data set, we derive the radio properties of CWAT-01 and use the
optical and X-ray data to investigate its host environment. The cluster hosting
CWAT-01 is part of a larger assembly consisting of a minimum of four X-ray
luminous clusters within ~2 Mpc distance. We apply hydrodynamical models that
combine ram pressure and buoyancy forces on CWAT-01. These models explain the
shape of the radio jets only if the galaxy's velocity relative to the
intra-cluster medium (ICM) is in the range of about 300-550 km/s which is
higher than expected for brightest cluster galaxies (BCGs) in relaxed systems.
This indicates that the CWAT-01 host cluster is not relaxed, but is possibly
dynamically young. We argue that such a velocity could have been induced
through subcluster merger within the CWAT-01 parent cluster and/or
cluster-cluster interactions. Our results strongly indicate that we are
witnessing the formation of a large cluster from an assembly of multiple
clusters, consistent with the hierarchical scenario of structure formation. We
estimate the total mass of the final cluster to be approximately 20% of the
mass of the Coma cluster.Comment: 18 pages, 13 figures; accepted for publication in ApJS, COSMOS
special issue; added color figure (Fig. 13) which was previously unavailabl
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Combinatorial optimization and metaheuristics
Today, combinatorial optimization is one of the youngest and most active areas of discrete mathematics. It is a branch of optimization in applied mathematics and computer science, related to operational research, algorithm theory and computational complexity theory. It sits at the intersection of several fields, including artificial intelligence, mathematics and software engineering. Its increasing interest arises for the fact that a large number of scientific and industrial problems can be formulated as abstract combinatorial optimization problems, through graphs and/or (integer) linear programs. Some of these problems have polynomial-time (âefficientâ) algorithms, while most of them are NP-hard, i.e. it is not proved that they can be solved in polynomial-time. Mainly, it means that it is not possible to guarantee that an exact solution to the problem can be found and one has to settle for an approximate solution with known performance guarantees. Indeed, the goal of approximate methods is to find âquicklyâ (reasonable run-times), with âhighâ probability, provable âgoodâ solutions (low error from the real optimal solution). In the last 20 years, a new kind of algorithm commonly called metaheuristics have emerged in this class, which basically try to combine heuristics in high level frameworks aimed at efficiently and effectively exploring the search space. This report briefly outlines the components, concepts, advantages and disadvantages of different metaheuristic approaches from a conceptual point of view, in order to analyze their similarities and differences. The two very significant forces of intensification and diversification, that mainly determine the behavior of a metaheuristic, will be pointed out. The report concludes by exploring the importance of hybridization and integration methods
Design, Implementation and Operation of a Sparse Aperture Imaging Satellite Testbed
In order to better understand the technological difficulties involved in designing and building a sparse aperture array, the challenge of building a white light Golay-3 telescope was undertaken. The MIT Adaptive Reconnaissance Golay-3 Optical Satellite (ARGOS) project exploits wide-angle Fizeau interferometer technology with an emphasis on modularity in the optics and spacecraft subsystems. Unique design procedures encompassing the nature of coherent wavefront sensing, control and combining as well as various system engineering aspects to achieve cost effectiveness, are developed. To demonstrate a complete spacecraft in a 1-g environment, the ARGOS system is mounted on a frictionless air-bearing, and has the ability to track fast orbiting satellites like the ISS or the planets. Wavefront sensing techniques are explored to mitigate initial misalignment and to feed back real-time aberrations into the optical control loop. This paper presents the results and the lessons learned from the conceive, design, implement and operate phases of ARGOS. A preliminary assessment shows that the beam combining problem is the most challenging aspect of sparse optical arrays. The need for optical control is paramount due to tight beam combining tolerances. The wavefront sensing/control requirements appear to be a major technology and cost driver
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