Cracking into RACS - I. Measuring the cosmic dipole in the RACS-low3 and RACS-mid1 radio surveys with simulation-based inference

Abstract

We measure the dipole in the count of radio galaxies over the sky as seen by the Australian SKA Pathfinder (ASKAP) in the Rapid ASKAP Continuum Survey (RACS). Our analysis focuses on the third observational epoch in the 'low' passband (943.5 MHz; RACS-low3) and the first epoch in the 'mid' passband (1367.5 MHz; RACS-mid1). We find strong evidence for a systematic relation between flux density and the temperature at the time of observation, whereby reported flux densities are suppressed to a greater degree when the ASKAP antennas are warmer. We include this temperature response in our forward model as part of our Simulation-based Inference (SBI) pipeline, generating data realisations that are then used by a neural network to learn the likelihood function. Despite this model, RACS-low3 and RACS-mid1 both exhibit a dipole inconsistent with the kinematic dipole of the Cosmic Microwave Background (CMB), adding further evidence for the 'cosmic dipole tension'. For RACS-low3, the dipole agrees in direction but has an amplitude four times in excess, while the dipole for RACS-mid1 is offset from the CMB and has an amplitude roughly five times in excess. Our findings underscore the importance of interrogating the data for systematic effects and showcase the power of SBI in handling them.