PUB - Publikationen an der Universität Bielefeld

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.