Modelling and peeling extended sources with shapelets: a Fornax A case study

TL;DR

This paper compares shapelet-based and multi-scale CLEAN methods for modeling extended radio sources like Fornax A, demonstrating shapelets' superior performance in simulations and discussing their potential for improving Epoch of Reionisation 21cm signal detection.

Contribution

The paper introduces a new shapelet modeling package SHAMFI and a CUDA simulation code WODEN, and evaluates their effectiveness against existing methods for subtracting extended sources in radio astronomy.

Findings

Shapelet method effectively subtracts large-scale emission in simulations.

MS CLEAN performance deteriorates at large scales with increased resolution.

Real data results are limited by other systematics, but simulations show promise.

Abstract

To make a power spectrum (PS) detection of the 21 cm signal from the Epoch of Reionisation (EoR), one must avoid/subtract bright foreground sources. Sources such as Fornax A present a modelling challenge due to spatial structures spanning from arc seconds up to a degree. We compare modelling with multi-scale (MS) CLEAN components to 'shapelets', an alternative set of basis functions. We introduce a new image-based shapelet modelling package, SHAMFI. We also introduce a new CUDA simulation code (WODEN) to generate point source, Gaussian, and shapelet components into visibilities. We test performance by modelling a simulation of Fornax A, peeling the model from simulated visibilities, and producing a residual PS. We find the shapelet method consistently subtracts large-angular-scale emission well, even when the angular-resolution of the data is changed. We find that when increasing the angular-resolution of the data, the MS CLEAN model worsens at large angular-scales. When testing on real MWA data, the expected improvement is not seen in real data because of the other dominating systematics still present. Through further simulation we find the expected differences to be lower than obtainable through current processing pipelines. We conclude shapelets are worthwhile for subtracting extended galaxies, and may prove essential for an EoR detection in the future, once other systematics have been addressed.