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The cosmic radio dipole: Bayesian estimators on new and old radio surveys

Astronomy & Astrophysics, Volume 675, id.A72, 14 pp.

Authors:

J. D. Wagenveld, H-R. Klöckner, D. J. Schwarz

Abstract:

he cosmic radio dipole is an anisotropy in the number counts of radio sources and is analogous to the dipole seen in the cosmic microwave background (CMB). Measurements of source counts of large radio surveys have shown that, although the radio dipole is generally consistent in direction with the CMB dipole, the amplitudes are in tension. These observations present an intriguing puzzle, namely the cause of this discrepancy, with a true anisotropy breaking with the assumptions of the cosmological principle, invalidating the most common cosmological models that are built on these assumptions. We present a novel set of Bayesian estimators to determine the cosmic radio dipole and compare the results with those of commonly used methods applied to the Rapid ASKAP Continuum Survey (RACS) and the NRAO VLA Sky Survey (NVSS) radio surveys. In addition, we adapt the Bayesian estimators to take into account systematic effects known to influence large radio surveys of this kind, folding information such as the local noise floor or array configuration directly into the parameter estimation. The enhancement of these estimators allows us to greatly increase the number of sources used in the parameter estimation, yielding tighter constraints on the cosmic radio dipole estimation than previously achieved with NVSS and RACS. We extend the estimators further to work on multiple catalogues simultaneously, leading to a combined parameter estimation using both NVSS and RACS. The result is a dipole estimate that perfectly aligns with the CMB dipole in terms of direction but with an amplitude that is three times as large, and a significance of 4.8σ. This new dipole measurement is made to an unprecedented level of precision for radio sources, which is only matched by recent results using infrared quasars.
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The environment and host haloes of the brightest z~6 Lyman-break galaxies

MNRAS

Authors:

PW Hatfield, RAA Bowler, MJ Jarvis, CL Hale

Abstract:

By studying the large-scale structure of the bright high-redshift Lyman-break galaxy (LBG) population it is possible to gain an insight into the role of environment in galaxy formation physics in the early Universe. We measure the clustering of a sample of bright ($-22.7
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The kinematic contribution to the cosmic number count dipole

Astronomy & Astrophysics, Volume 697, id.A112, 11 pp.

Authors:

J. D. Wagenveld, S. von Hausegger, H.-R. Klöckner, D. J. Schwarz

Abstract:

Measurements of the number-count dipole with large surveys have shown amplitudes in tension with kinematic predictions based on the observed Doppler dipole of the cosmic microwave background (CMB). These observations seem to be in direct conflict with a homogeneous and isotropic universe as asserted by the cosmological principle, demanding further investigation into the origin of the tension. Here, we investigated whether the observed number-count dipoles are consistent with being fully kinematic, regardless of boost, or if there is any residual anisotropy contributing to the total observed dipole, independent of the kinematic part. To disentangle these contributions, we aim to leverage the fact that the kinematic matter dipole expected in a given galaxy catalogue scales with observed properties of the sample, and different catalogues used in the literature therefore have different kinematic dipole expectations. We performed joint dipole fits using the NRAO VLA Sky Survey (NVSS), the Rapid ASKAP Continuum Survey (RACS), and the active galactic nuclei (AGN) catalogue derived from the Wide-field Infrared Survey Explorer (CatWISE). The direction of the common dipole between these catalogues is offset from the CMB dipole direction by 23 ± 5 degrees. Assuming a common kinematic and non-kinematic dipole component between all catalogues, we find that a large residual, non-kinematic dipole anisotropy is detected, though a common direction between the two components is disfavoured by model selection. Freeing up both amplitude and direction for this residual dipole while fixing the kinematic dipole to the CMB dipole expectation, we recover a significant residual dipole with 𝒟_resid = (0.81 ± 0.14)×10−2, which is offset from the CMB dipole direction by 39 ± 8 degrees. While these results cannot explain the origin of the unexpectedly large number-count dipoles, they offer a rephrasing of the anomaly in terms of kinematic and non-kinematic contributions, providing evidence for the existence of the latter within the models explored here. The present work provides a valuable first test of this concept, although its scrutinising power is limited by the currently employed catalogues. Larger catalogues, especially in radio, will be needed to further lift the degeneracy between the kinematic and residual dipole components.
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Understanding radio pulsars using modern broad-band instruments

Abstract:

The canonical model of a pulsar is insufficient to describe the variety and variability of its radio emission. Pulsars are neutron stars, with intense gravitational and magnetic fields, which emit bright beams of radio waves that co-rotate with the spinning star. The regularity of pulsar rotation, and hence the arrival times of pulses of radio emission, means that pulsars are used as clocks in space to test fundamental theories of physics and to search for gravitational waves. However, their accuracy as clocks is limited by the fact that we do not fully understand how pulsars produce their radio beams, and so cannot predict their emission behaviour completely.

Throughout the history of pulsar science, new telescopes and updated technology have expanded the complexity observable in the shapes and properties of observed pulse profiles from the radio pulsar population. This growing dataset has raised as many new questions as it has answered about the nature of pulsar radio emission, the properties of the pulsar population and our ability to characterize the behaviour of pulsars and their environments with physical laws. However, modern instruments offer a broad-band view of radio pulsars. This allows us to probe, for the first time, the continuous evolution of pulsar radio emission over a wide frequency range with a single instrument.

This thesis uses the expanded observational capabilities of new broad-band instruments to make progress in answering fundamental questions about pulsar radio emission. I apply a statistical approach to the frequency evolution of single pulses of PSR J1136+1551 observed by the GMRT, in order to constrain the cross-section and frequency-dependent emission heights of its radio beam structure. This work shows that the beam structure of J1136+1551 is best described by a fan beam model and that it is important to include the effects of orthogonal polarization mode interaction to explain the frequency-dependent behaviour. In order to maximize the understanding available with broad-band observations, it is important to address the interaction between pulsar radio emission and the interstellar medium through which it propagates. I create an algorithm for correcting the effects of the interstellar medium in broad-band polarimetric data from the Parkes Ultra-Wideband receiver to reveal the intrinsic polarization behaviour of PSRs~J1056--6258 and J1359--6038. Finally, I apply understanding of intrinsic pulsar behaviour to constrain models of pulsar scattering by the interstellar medium, performing a survey of the scattering properties of 84 single-component pulsars observed with the MeerKAT telescope.

This work reveals the capacity of new broad-band observations to expand our understanding of pulsar radio emission. It constrains understanding of both pulsar beam structure, including frequency-dependent emission heights, and the structures that make up the interstellar medium. The results highlight the importance of accounting for the behaviour of both the pulsar and the interstellar medium simultaneously when analysing broad-band observations, and future work will focus on applying these modelling approaches to pulsars with complex profile shapes and polarization properties.

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