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

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 Murchison Widefield Array 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.

A map of diffuse radio emission at 182 MHz to enhance epoch of reionization observations in the Southern hemisphere

“…To enable accurate visibility modelling, the map must be combined with a source catalogue such as the modified GLEAM catalogue discussed in Section 2.2 or the Long Baseline EoR Surv e y (Lynch et al 2021 ). The success of precision calibration for 21 cm cosmology depends on highly accurate and complete modelling of point-like and compact extended sources (Carroll et al 2016 ;Line et al 2020 ;Zhang et al 2020 ).…”

WODEN: A CUDA-enabled package to simulate low-frequency radio interferometric data

“…Another excellent CUDA accelerated simulation package, OSKAR (Mort et al, 2010), addresses these two points. However, the RTS also generates parts of the sky model via shapelets (see Line et al, 2020 for an overview), which OSKAR cannot. Furthermore, in real data, the precession/nutation of the Earth's rotational axis causes sources to move from the sky coordinates as specified in the RA, DEC J2000 coordinate system.…”

“…In Line et al (2020), we compared two methods to model Fornax A: a combination of point and elliptical Gaussians, compared to shapelets (see Figure 2). We were able to quickly compare the computational efficiency of the methods using a desktop, and comment on their respective strengths and weaknesses in regard to foreground removal for EoR purposes.…”

Epoch of Reionization Power Spectrum Limits from Murchison Widefield Array Data Targeted at EoR1 Field

“…The source catalogue is created by cross-matching with PUMA (Line et al 2017). Full details of PUMA and this crossmatched catalogue can be found in Line et al (2020), but we include a brief description in section 3.3. The single compound calibrator provides a model for the visibilities.…”