The MeerKAT Absorption Line Survey: Homogeneous continuum catalogues towards a measurement of the cosmic radio dipole^★

¹ Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: wagenveld@mpifr-bonn.mpg.de
² Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune 411 007, India
³ National Radio Astronomy Observatory, Socorro, NM 87801, USA
⁴ The Inter-University Institute for Data Intensive Astronomy, Department of Astronomy, and University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
⁵ Ioffe Institute, 26 Politeknicheskaya st., St. Petersburg 194021, Russia
⁶ HSE University, Saint Petersburg, Russia
⁷ Department of Astronomy, University of Maryland, College park, MD 20742, USA
⁸ X-ray Astrophysics Laboratory, NASA/GSFC, Greenbelt, MD 20771, USA
⁹ Center for Research and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD 20771, USA
¹⁰ Observatoire de Paris, Collège de France, PSL University, Sorbonne University, CNRS, LERMA, Paris, France
¹¹ Jansky Fellow of the National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
¹² Astrophysics Research Centre and School of Mathematics, Statistics and Computer Science, University of KwaZulu-Natal, Durban 4041, South Africa
¹³ Wits Centre for Astrophysics, School of Physics, University of the Witwatersrand, 2050 Johannesburg, South Africa
¹⁴ Department of Physics and Electronics, Rhodes University, PO Box 94 Makhanda 6140, South Africa
¹⁵ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute, 44780 Bochum, Germany
¹⁶ Department of Space, Earth and Environment, Chalmers University of Technology, 43992 Onsala, Sweden
¹⁷ Institut d'astrophysique de Paris, UMR 7095, CNRS-SU, 98bis bd Arago, 75014 Paris, France
¹⁸ Green Bank Observatory, Green Bank, WV 24944, USA

Received: 16 November 2022
Accepted: 16 February 2023

Abstract

The number counts of homogeneous samples of radio sources are a tried and true method of probing the large-scale structure of the Universe, as most radio sources outside the Galactic plane are at cosmological distances. As such, they are expected to trace the cosmic radio dipole, an anisotropy analogous to the dipole seen in the cosmic microwave background (CMB). Results have shown that although the cosmic radio dipole matches the direction of the CMB dipole, it has a significantly larger amplitude. This unexplained result challenges our assumption of the Universe being isotropic, which can have large repercussions for the current cosmological paradigm. Though significant measurements have been made, sensitivity to the radio dipole is generally hampered by systematic effects that can cause large biases in the measurement. Here we assess these systematics with data from the MeerKAT Absorption Line Survey (MALS), a blind search for absorption lines with pointings centred on bright radio sources. With the sensitivity and field of view of MeerKAT, thousands of sources are observed in each pointing, allowing for the possibility of measuring the cosmic radio dipole given enough pointings. We present the analysis of ten MALS pointings, focusing on systematic effects that could lead to an inhomogeneous catalogue. We describe the calibration and creation of full band continuum images and catalogues, producing a combined catalogue containing 16 307 sources and covering 37.5 square degrees of sky down to a sensitivity of 10 μJy beam⁻¹. We measure the completeness, purity, and flux recovery statistics for these catalogues using simulated data. We investigate different source populations in the catalogues by looking at flux densities and spectral indices and how they might influence source counts. Using the noise characteristics of the pointings, we find global measures that can be used to correct for the incompleteness of the catalogue, producing corrected number counts down to 100–200 μJy. We show that we can homogenise the catalogues and properly account for systematic effects. We determine that we can measure the dipole to 3σ significance with 100 MALS pointings.