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Search Results: 1 - 10 of 1135 matches for " Rene Andrae "
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Error estimation in astronomy: A guide
Rene Andrae
Physics , 2010,
Abstract: Estimating errors is a crucial part of any scientific analysis. Whenever a parameter is estimated (model-based or not), an error estimate is necessary. Any parameter estimate that is given without an error estimate is meaningless. Nevertheless, many (undergraduate or graduate) students have to teach such methods for error estimation to themselves when working scientifically for the first time. This manuscript presents an easy-to-understand overview of different methods for error estimation that are applicable to both model-based and model-independent parameter estimates. These methods are not discussed in detail, but their basics are briefly outlined and their assumptions carefully noted. In particular, the methods for error estimation discussed are grid search, varying $\chi^2$, the Fisher matrix, Monte-Carlo methods, error propagation, data resampling, and bootstrapping. Finally, a method is outlined how to propagate measurement errors through complex data-reduction pipelines.
Only marginal alignment of disc galaxies
Rene Andrae,Knud Jahnke
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.19620.x
Abstract: Testing theories of angular-momentum acquisition of rotationally supported disc galaxies is the key to understand the formation of this type of galaxies. The tidal-torque theory tries to explain this acquisition process in a cosmological framework and predicts positive autocorrelations of angular-momentum orientation and spiral-arm handedness on distances of 1Mpc/h. This disc alignment can also cause systematic effects in weak-lensing measurements. Previous observations claimed discovering such correlations but did not account for errors in redshift, ellipticity and morphological classifications. We explain how to rigorously propagate all important errors. Analysing disc galaxies in the SDSS database, we find that positive autocorrelations of spiral-arm handedness and angular-momentum orientations on distances of 1Mpc/h are plausible but not statistically significant. This result agrees with a simple hypothesis test in the Local Group, where we find no evidence for disc alignment. Moreover, we demonstrate that ellipticity estimates based on second moments are strongly biased by galactic bulges, thereby corrupting correlation estimates and overestimating the impact of disc alignment on weak-lensing studies. Finally, we discuss the potential of future sky surveys. We argue that photometric redshifts have too large errors, i.e., PanSTARRS and LSST cannot be used. We also discuss potentials and problems of front-edge classifications of galaxy discs in order to improve estimates of angular-momentum orientation.
Soft clustering analysis of galaxy morphologies: A worked example with SDSS
Rene Andrae,Peter Melchior,Matthias Bartelmann
Physics , 2010, DOI: 10.1051/0004-6361/201014169
Abstract: Context: The huge and still rapidly growing amount of galaxies in modern sky surveys raises the need of an automated and objective classification method. Unsupervised learning algorithms are of particular interest, since they discover classes automatically. Aims: We briefly discuss the pitfalls of oversimplified classification methods and outline an alternative approach called "clustering analysis". Methods: We categorise different classification methods according to their capabilities. Based on this categorisation, we present a probabilistic classification algorithm that automatically detects the optimal classes preferred by the data. We explore the reliability of this algorithm in systematic tests. Using a small sample of bright galaxies from the SDSS, we demonstrate the performance of this algorithm in practice. We are able to disentangle the problems of classification and parametrisation of galaxy morphologies in this case. Results: We give physical arguments that a probabilistic classification scheme is necessary. The algorithm we present produces reasonable morphological classes and object-to-class assignments without any prior assumptions. Conclusions: There are sophisticated automated classification algorithms that meet all necessary requirements, but a lot of work is still needed on the interpretation of the results.
Quantifying galaxy shapes: Sersiclets and beyond
Rene Andrae,Peter Melchior,Knud Jahnke
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.19348.x
Abstract: Parametrising galaxy morphologies is a challenging task, e.g., in shear measurements of weak lensing or investigations of galaxy evolution. The huge variety of morphologies requires an approach that is highly flexible, e.g., accounting for azimuthal structure. We revisit the method of sersiclets, where galaxy morphologies are decomposed into basis functions based on the Sersic profile. This approach is justified by the fact that the Sersic profile is the first-order Taylor expansion of any real light profile. We show that sersiclets overcome the modelling failures of shapelets. However, sersiclets implicate an unphysical relation between the steepness of the light profile and the spatial scale of azimuthal structures, which is not obeyed by real galaxy morphologies and can therefore give rise to modelling failures. Moreover, we demonstrate that sersiclets are prone to undersampling, which restricts sersiclet modelling to highly resolved galaxy images. Analysing data from the Great08 challenge, we demonstrate that sersiclets should not be used in weak-lensing studies. We conclude that although the sersiclet approach appears very promising at first glance, it suffers from conceptual and practical problems that severly limit its usefulness. The Sersic profile can be enhanced by higher-order terms in the Taylor expansion, which can drastically improve model reconstructions of galaxy images. If orthonormalised, these higher-order profiles can overcome the problems of sersiclets while preserving their mathematical justification.
Parametrising arbitrary galaxy morphologies: potentials and pitfalls
Rene Andrae,Knud Jahnke,Peter Melchior
Physics , 2010, DOI: 10.1111/j.1365-2966.2010.17690.x
Abstract: We demonstrate that morphological observables (e.g. steepness of the radial light profile, ellipticity, asymmetry) are intertwined and cannot be measured independently of each other. We present strong arguments in favour of model-based parametrisation schemes, namely reliability assessment, disentanglement of morphological observables, and PSF modelling. Furthermore, we demonstrate that estimates of the concentration and Sersic index obtained from the Zurich Structure & Morphology catalogue are in excellent agreement with theoretical predictions. We also demonstrate that the incautious use of the concentration index for classification purposes can cause a severe loss of the discriminative information contained in a given data sample. Moreover, we show that, for poorly resolved galaxies, concentration index and M_20 suffer from strong discontinuities, i.e. similar morphologies are not necessarily mapped to neighbouring points in the parameter space. This limits the reliability of these parameters for classification purposes. Two-dimensional Sersic profiles accounting for centroid and ellipticity are identified as the currently most reliable parametrisation scheme in the regime of intermediate signal-to-noise ratios and resolutions, where asymmetries and substructures do not play an important role. We argue that basis functions provide good parametrisation schemes in the regimes of high signal-to-noise ratios and resolutions. Concerning Sersic profiles, we show that scale radii cannot be compared directly for profiles of different Sersic indices. Furthermore, we show that parameter spaces are typically highly nonlinear. This implies that significant caution is required when distance-based classificaton methods are used.
Dos and don'ts of reduced chi-squared
Rene Andrae,Tim Schulze-Hartung,Peter Melchior
Physics , 2010,
Abstract: Reduced chi-squared is a very popular method for model assessment, model comparison, convergence diagnostic, and error estimation in astronomy. In this manuscript, we discuss the pitfalls involved in using reduced chi-squared. There are two independent problems: (a) The number of degrees of freedom can only be estimated for linear models. Concerning nonlinear models, the number of degrees of freedom is unknown, i.e., it is not possible to compute the value of reduced chi-squared. (b) Due to random noise in the data, also the value of reduced chi-squared itself is subject to noise, i.e., the value is uncertain. This uncertainty impairs the usefulness of reduced chi-squared for differentiating between models or assessing convergence of a minimisation procedure. The impact of noise on the value of reduced chi-squared is surprisingly large, in particular for small data sets, which are very common in astrophysical problems. We conclude that reduced chi-squared can only be used with due caution for linear models, whereas it must not be used for nonlinear models at all. Finally, we recommend more sophisticated and reliable methods, which are also applicable to nonlinear models.
Assessment of stochastic and deterministic models of 6304 quasar lightcurves from SDSS Stripe 82
Rene Andrae,Dae-Won Kim,Coryn A. L. Bailer-Jones
Physics , 2013, DOI: 10.1051/0004-6361/201321335
Abstract: The optical light curves of many quasars show variations of tenths of a magnitude or more on time scales of months to years. This variation often cannot be described well by a simple deterministic model. We perform a Bayesian comparison of over 20 deterministic and stochastic models on 6304 QSO light curves in SDSS Stripe 82. We include the damped random walk (or Ornstein-Uhlenbeck [OU] process), a particular type of stochastic model which recent studies have focused on. Further models we consider are single and double sinusoids, multiple OU processes, higher order continuous autoregressive processes, and composite models. We find that only 29 out of 6304 QSO lightcurves are described significantly better by a deterministic model than a stochastic one. The OU process is an adequate description of the vast majority of cases (6023). Indeed, the OU process is the best single model for 3462 light curves, with the composite OU process/sinusoid model being the best in 1706 cases. The latter model is the dominant one for brighter/bluer QSOs. Furthermore, a non-negligible fraction of QSO lightcurves show evidence that not only the mean is stochastic but the variance is stochastic, too. Our results confirm earlier work that QSO light curves can be described with a stochastic model, but place this on a firmer footing, and further show that the OU process is preferred over several other stochastic and deterministic models. Of course, there may well exist yet better (deterministic or stochastic) models which have not been considered here.
Radiological Mimics of Popliteal Cysts: An Algorithmic Approach Using US and MRI to Identify the Potentially Malignant Lesions: Case Series  [PDF]
Andrae Blanchard, Asad A. Naqvi, Zain Badar, Hema Choudur
Open Journal of Clinical Diagnostics (OJCD) , 2016, DOI: 10.4236/ojcd.2016.62002
Abstract: Background: Popliteal cysts are common and present as asymptomatic lumps in the medial popliteal fossa. Some have complex internal characteristics such as septa and loose-bodies. However, not all are popliteal cysts and can be aggressive. These lesions need to be differentiated by the absence of the communicating neck with the joint on ultrasound. Presence of Doppler flow of non-communicating cysts requires further evaluation on MRI, prior to performing a biopsy. Using a case series, we propose an algorithmic approach that is simple and will help identify the malignant lesions and institute appropriate management. Case-Presentation: Popliteal Cyst: On ultrasound: characteristic neck communicating with knee joint. Synovial Sarcoma: Gadolinium enhancement, with areas of low-, iso- and hyper-intense signal to fat on T2. Synovial-Osteochondromatosis: Non-mineralized: T1-low/intermediate intensity; T2-high intensity. Mineralized type: low intensity on T1 & T2. Thrombosed Popliteal Aneurysm: Lamellated appearance-high/low signal intensity on T2. Myxoid-Liposarcomas: Inhomogeneous appearance; homogenous with gadolinium. Usually require a biopsy for diagnosis. Conclusion: The cystic lesions in the medial aspect of the popliteal fossa can be misdiagnosed. Our article reiterates the importance of the communicating neck that separates popliteal cysts from other mimics. We have proposed an algorithm to identify these mimics.
The bulk of the black hole growth since z~1 occurs in a secular universe: No major merger-AGN connection
Mauricio Cisternas,Knud Jahnke,Katherine J. Inskip,Jeyhan Kartaltepe,Anton M. Koekemoer,Thorsten Lisker,Aday R. Robaina,Marco Scodeggio,Kartik Sheth,Jonathan R. Trump,Rene Andrae,Takamitsu Miyaji,Elisabeta Lusso,Marcella Brusa,Peter Capak,Nico Cappelluti,Francesca Civano,Olivier Ilbert,Chris D. Impey,Alexie Leauthaud,Simon J. Lilly,Mara Salvato,Nick Z. Scoville,Yoshi Taniguchi
Physics , 2010, DOI: 10.1088/0004-637X/726/2/57
Abstract: What is the relevance of major mergers and interactions as triggering mechanisms for active galactic nuclei (AGN) activity? To answer this longstanding question, we analyze 140 XMM-selected AGN host galaxies and a matched control sample of 1264 inactive galaxies over z~0.3-1.0 and log(M_*/M_sun)<11.7 with high-resolution HST/ACS imaging from the COSMOS field. The visual analysis of their morphologies by 10 independent human classifiers yields a measure of the fraction of distorted morphologies in the AGN and control samples, i.e. quantifying the signature of recent mergers which might potentially be responsible for fueling/triggering the AGN. We find that (1) the vast majority (>85%) of the AGN host galaxies do not show strong distortions, and (2) there is no significant difference in the distortion fractions between active and inactive galaxies. Our findings provide the best direct evidence that, since z~1, the bulk of black hole accretion has not been triggered by major galaxy mergers, therefore arguing that the alternative mechanisms, i.e., secular processes and minor interactions, are the leading triggers for the episodes of major black hole growth. We also exclude an alternative interpretation of our results: a significant time lag between merging and the observability of the AGN phase could wash out the most significant merging signatures, explaining the lack of enhancement of strong distortions on the AGN hosts. We show that this alternative scenario is unlikely due to: (1) recent major mergers being ruled out for the majority of sources due to the high fraction of disk-hosted AGN, (2) the lack of a significant X-ray signal in merging inactive galaxies as a signature of a potential buried AGN, and (3) the low levels of soft X-ray obscuration for AGN hosted by interacting galaxies, in contrast to model predictions.
Antigen Detection in Canine Blastomycosis: Comparison of Different Antibody-Antigen Combinations in Two Competitive ELISAs  [PDF]
Debra Andrae, Katheryn Birch, Trevor Bybee, Thomas Ritcher, Jason Werth, Gene M. Scalarone
Open Journal of Medical Microbiology (OJMM) , 2012, DOI: 10.4236/ojmm.2012.23016
Abstract: This present study was designed to evaluate four different Blastomyces dermatitidis antibody-antigen combinations (B5896 and T-58 antibodies and B5896 and WI-R antigens) for the detection of antigen in 36 urine specimens from dogs with blastomycosis using a standard indirect ELISA (STD) and a biotin-streptavidin ELISA (B-SA). The antigen detection sensitivity values ranged from 81% (B-SA: T-58 Ab + WI-R Ag) to 100% (STD and B-SA: B5896 Ab + WI-R Ag; B5896 Ab + B5896 Ag) with the antibody-antigen combinations in the two assays. Optimal detection was evidenced when the B5896 Ab was allowed to react with the urine specimens for 30 min at 37?C and then placed in the B-SA ELISA plates containing the B5896 Ag. The greatest absorbance value obtained with this antibody-antigen com-bination was 0.903 (range of 0.596 - 0.903) as compared to the control value of 1.246. The difference between the control absorbance and the test absorbance values was 0.343 which was considerably greater than the control-test values with the other combinations. This study thus showed that the results obtained in antigen detection assays are dependent upon the antibody used to react with the urine specimens as well as the antigen used in the enzyme immunoassay.
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