A Quantitative Evaluation of the Galaxy Component of COSMOS and APM Catalogs

We have carried out an independent quantitative evaluation of the galaxy component of the "COSMOS/UKST Southern Sky Object Catalogue" (SSC) and the "APM/UKST J Catalogue" (APM). Using CCD observations our results corroborate the accuracy of the photometry of both catalogs, which have an overall dispersion of about 0.2 mag in the range 17 <= b_J <= 21.5. The SSC presents externally calibrated galaxy magnitudes that follow a linear relation, while the APM instrumental magnitudes of galaxies, only internally calibrated by the use of stellar profiles, require second-order corrections. The completeness of both catalogs in a general field falls rapidly fainter than b_J = 20.0, being slightly better for APM. The 90% completeness level of the SSC is reached between b_J = 19.5 and 20.0, while for APM this happens between b_J = 20.5 and 21.0. Both SSC and APM are found to be less complete in a galaxy cluster field. Galaxies misclassified as stars in the SSC receive an incorrect magnitude because the stellar ones take saturation into account besides using a different calibration curve. In both cases, the misclassified galaxies show a large diversity of colors that range from typical colors of early-types to those of blue star-forming galaxies. A possible explanation for this effect is that it results from the combination of low sampling resolutions with properties of the image classifier for objects with characteristic sizes close to the instrumental resolution. We find that the overall contamination by stars misclassified as galaxies is < 5% to b_J = 20.5, as originally estimated for both catalogs. Although our results come from small areas of the sky, they are extracted from two different plates and are based on the comparison with two independent datasets.Comment: 14 pages of text and tables, 8 figures; to be published in the Astronomical Journal; for a single postscript version file see ftp://danw.on.br/outgoing/caretta/caretta.p

Partitioning DAG Computations: a Cautionary Note

The representation of a parallel computation as a directed acyclic graph can help the programmer to analyse the properties of the computation in order to optimise partitioning and scheduling. We point out that the results obtained from such &quot;optimizations&quot; are only as good as the underlying cost model. 1 Introduction Some parallel computations can be represented as a directed acyclic graph (DAG) in which the nodes represent operations and there is an arc from a node v to a node w if the output from the operation performed at v is needed as one of the inputs to the operation at w. Examples of DAG computation are the fast Fourier transform graph, the binary tree for arithmetic expression evaluation and the diamond for computing the longest common subsequence. Various researchers have used this representation of a computation in order to investigate the input/output complexity of a computation [Kung and Hong 1981], the effects of introducing communications as a resource in the PRAM ..

Energy-Efficient Query Processing in a Combined Database and Web Service Environment

The energy-efficiency of server hardware, web server software, and databases has been widely studied. However, studies that combine these aspects are rare. In this chapter, the authors present an energy-efficiency evaluation of a web/database application in a Windows/IIS/MSSQL environment running on an industrial grade Intel server. Moreover, they provide a wide overview of related research and technologies. Researchers have noticed that despite energy-saving technologies, energy consumption of data centers is still growing. To resolve this dilemma, the authors explore the background and propose concrete solutions. They concentrate on the following aspects: server BIOS/operating system energy optimization (limited impact) and “bursting” (i.e., queuing requests and then executing them in bursts). The authors have used the bursting method with both database and web/database applications. Their results indicate about 10% energy savings using this method. The authors analyse the model using statistical tools and present an equation to express the quality of service vs. burst wait time relationship