13 research outputs found
The cosmic radio dipole
No full textRubart M. The cosmic radio dipole. Bielefeld: Universität Bielefeld; 2015.The velocity of the Milky Way with respect to the (at large scales) homogeneous universe seemed to be well known from observations of the cosmic microwave background. Such a peculiar motion also affects radio observations and should lead to a dipole anisotropy in number counts of radio sources. This anisotropy has been estimated in this thesis using the NRAO VLA Sky Survey and the Westerbork Northern Sky Survey. On the one hand the obtained directions agree with the expectation within the measurement uncertainties. The radio dipole amplitudes on the other hand show a significant excess, which would correspond to an increased velocity of the Milky Way with respect to those radio sources, if no other explanation can be found.
In this thesis, a general introduction to the topic is given and the relations to other scientific questions (e.g. large bulk flows) are discussed. Previous results concerning the cosmic radio dipole are presented, compared and analysed. The radio dipole is estimated by means of linear estimators, including a newly developed two-dimensional version, and the obtained amplitudes are analysed in detail. Certain issues (i.e. shot noise contributions) of the linear estimator are revealed and the effects of masking sources and calibration errors on the dipole estimation are discussed.
A quadratic estimator was developed and the radio dipole was re-obtained, refining the results from linear estimators. It is shown that the quadratic estimator has many advantages compared to the linear ones. The quadratic estimator is more stable w.r.t. masking effects and reveals new insights concerning the dipole in radio surveys. For example the radio sky cannot be described well by a mono- and dipole term alone.
In a final step the effect of local structures (z<<1) on the dipole estimation is investigated. For example, the effect of a local void contribution on the dipole amplitude is analysed. Such structures may reduce the discrepancy between the dipole in radio and microwave observations. At the end, all obtained results are compared and discussed
Impact of local structure on the cosmic radio dipole
No full textRubart M, Bacon D, Schwarz D. Impact of local structure on the cosmic radio dipole. Astronomy & Astrophysics. 2014;565: A111.We investigate the contribution that a local over- or under-density can haveon linear cosmic dipole estimations. We focus here on radio surveys, such asthe NRAO VLA Sky Survey (NVSS), and forthcoming surveys such as those with theLOw Frequency ARray (LOFAR), the Australian Square Kilometre Array Pathfinder(ASKAP) and the Square Kilometre Array (SKA). The NVSS has already been used toestimate the cosmic radio dipole; it was shown recently that this radio dipoleamplitude is larger than expected from a purely kinematic effect, assuming thevelocity inferred from the dipole of the cosmic microwave background. We showhere that a significant contribution to this excess could come from a localvoid or similar structure. In contrast to the kinetic contribution to the radiodipole, the structure dipole depends on the flux threshold of the survey andthe wave band, which opens the chance to distinguish the two contributions
Cosmic radio dipole from NVSS and WENSS
No full textRubart M, Schwarz D. Cosmic radio dipole from NVSS and WENSS. Astronomy & Astrophysics. 2013;555: A117.We use linear estimators to determine the magnitude and direction of thecosmic radio dipole from the NRAO VLA Sky Survey (NVSS) and the WesterborkNorthern Sky Survey (WENSS). We show that special attention has to be given tothe issues of bias due to shot noise, incomplete sky coverage and masking ofthe Milky Way. We compare several different estimators and show thatconflicting claims in the literature can be attributed to the use of differentestimators. We find that the NVSS and WENSS estimates of the cosmic radiodipole are consistent with each other and with the direction of the cosmicmicrowave background (CMB) dipole. We find from the NVSS a dipole amplitude of in direction . This amplitude exceeds theone expected from the CMB by a factor of about 3 and is inconsistent with theassumption of a pure kinetic origin of the radio dipole at 99.5% CL
Cosmic radio dipole: Estimators and frequency dependence
No full textSiewert T, Schmidt-Rubart M, Schwarz D. Cosmic radio dipole: Estimators and frequency dependence. Astronomy and Astrophysics. 2021;653: A9.The cosmic radio dipole is of fundamental interest to studies of cosmology. Recent works have put forth open questions about the nature of the observed cosmic radio dipole. In the current work, we use simulated source count maps to test a linear and a quadratic estimator for possible biases in the estimated dipole amplitude with respect to the masking procedure. We find a superiority on the part of the quadratic estimator, which we used to analyse the TGSS-ADR1, WENSS, SUMSS, and NVSS radio source catalogues, spread over a decade of frequencies. We applied the same masking strategy to all four surveys to produce comparable results. In order to address the differences in the observed dipole amplitudes, we cross-matched the two surveys located at both ends of the analysed frequency range. For the linear estimator, we identified a general bias in the estimated dipole directions. The positional offsets of the quadratic estimator to the cosmic microwave background (CMB) dipole for skies with 10(7) simulated sources is found to be below one degree and the absolute accuracy of the estimated dipole amplitudes is better than 10(-3). For the four radio source catalogues, we find an increasing dipole amplitude with decreasing frequency, which is consistent with results from the literature and the results of the cross-matched catalogue. We conclude that for all analysed surveys, the observed cosmic radio dipole amplitudes exceed the expectations derived from the CMB dipole, which cannot strictly be explained by a kinematic dipole alone
Requirements and Design of a Training System for Domestic Workers
No full textEmployees in household-related services have so far been neglected in research and practice. The overall goal of our project is to identify work-related stress of this special target group, develop recommendations, and disseminate them using low-threshold, attractive edutainment offers. In this context, this contribution presents a learning platform design for the special target group of domestic workers, such as gardeners or cleaners. The design is based on a requirements analysis with respect to this special target group, which we as well outline in this contribution
Authoring Educational Hypercomics assisted by Large Language Models
No full textInteractive stories can be an effective approach for teaching purposes. One shortcoming is the effort necessary to author and create these stories, especially complex storylines with choices for the readers. Based on recent advances in Natural Language Processing (NLP), new opportunities arise for assistance systems in the context of interactive stories. In our work, we present an authoring approach and prototypical tool for the creation of visual comic-strip like interactive stories, a type of hypercomics, that integrate an Artificial Intelligence (AI) assistance. Such comics are already used in our Gekonnt hanDeln web platform. The AI assistance provides suggestions for the overall story outline as well as how to design and write individual story frames. We provide a detailed description about the approach and its prototypical implementation. Furthermore, we present a study evaluating the prototype with student groups and how the prototype evolved in an iterative style based on the students’ feedback
Testing foundations of modern cosmology with SKA all-sky surveys
No full textSchwarz D, Bacon D, Chen S, et al. Testing foundations of modern cosmology with SKA all-sky surveys. 2015.Continuum and HI surveys with the Square Kilometre Array (SKA) will allow usto probe some of the most fundamental assumptions of modern cosmology,including the Cosmological Principle. SKA all-sky surveys will map an enormousslice of space-time and reveal cosmology at superhorizon scales and redshiftsof order unity. We illustrate the potential of these surveys and discuss theprospects to measure the cosmic radio dipole at high fidelity. We outlineseveral potentially transformational tests of cosmology to be carried out bymeans of SKA all-sky surveys
Cosmic radio dipole: Estimators and frequency dependence
No full textThe cosmic radio dipole is of fundamental interest to studies of cosmology. Recent works have put forth open questions about the nature of the observed cosmic radio dipole. In the current work, we use simulated source count maps to test a linear and a quadratic estimator for possible biases in the estimated dipole amplitude with respect to the masking procedure. We find a superiority on the part of the quadratic estimator, which we used to analyse the TGSS-ADR1, WENSS, SUMSS, and NVSS radio source catalogues, spread over a decade of frequencies. We applied the same masking strategy to all four surveys to produce comparable results. In order to address the differences in the observed dipole amplitudes, we cross-matched the two surveys located at both ends of the analysed frequency range. For the linear estimator, we identified a general bias in the estimated dipole directions. The positional offsets of the quadratic estimator to the cosmic microwave background (CMB) dipole for skies with 107 simulated sources is found to be below one degree and the absolute accuracy of the estimated dipole amplitudes is better than 10−3. For the four radio source catalogues, we find an increasing dipole amplitude with decreasing frequency, which is consistent with results from the literature and the results of the cross-matched catalogue. We conclude that for all analysed surveys, the observed cosmic radio dipole amplitudes exceed the expectations derived from the CMB dipole, which cannot strictly be explained by a kinematic dipole alone
