Radio weak lensing shear measurement in the visibility domain – II. Source extraction

Abstract

This paper extends the method introduced in Rivi et al. (2016b) to measure galaxy ellipticities in the visibility domain for radio weak lensing surveys. In that paper we focused on the development and testing of the method for the simple case of individual galaxies located at the phase centre, and proposed to extend it to the realistic case of many sources in the field of view by isolating visibilities of each source with a faceting technique. In this second paper we present a detailed algorithm for source extraction in the visibility domain and show its effectiveness as a function of the source number density by running simulations of SKA1-MID observations in the band 950-1150 MHz and comparing original and measured values of galaxies' ellipticities. Shear measurements from a realistic population of 10 4 galaxies randomly located in a field of view of 1 deg 2 (i.e. the source density expected for the current radio weak lensing survey proposal with SKA1) are also performed. At SNR 10, the multiplicative bias is only a factor 1.5 worse than what found when analysing individual sources, and is still comparable to the bias values reported for similar measurement methods at optical wavelengths. The additive bias is unchanged from the case of individual sources, but it is significantly larger than typically found in optical surveys. This bias depends on the shape of the uv coverage and we suggest that a uv-plane weighting scheme to produce a more isotropic shape could reduce and control additive bias.

SuperCLASS – III. Weak lensing from radio and optical observations in Data Release 1

“…the compilation of Mandelbaum et al 2015), but for radio interferometer data, the situation is less well developed. A number of approaches have been proposed (Rivi & Miller 2018;Rivi et al 2019) but have only been tested on simulations, and not real data. Conversely the method used in Chang et al (2004) was applied directly to data, but not simulations (still less simulations of the more recent data sets which are far greater in volume).…”

Radio galaxy detection in the visibility domain

“…We adopt a uniform prior on source position over the interferometer's field of view (FOV). For the other galaxy parameters we use distributions presented in Rivi & Miller (2018) and estimated from measurements of faint sources observed with the Very Large Array (VLA) radio telescope. In particular the flux and scale-length priors are obtained by the analysis of the deep radio VLA-SWIRE field catalog:…”

“…We use the antennae configuration provided in Heystek (2015) to simulate 8 hour observations of a 1 deg 2 FOV at declination of −30 • (i.e. at the zenith), in a single smeared-out frequency channel between 1280-1520 MHz and with visibilities sampled once every τ acc = 60 s. We generate realistic star-forming galaxy populations as in Rivi & Miller (2018), where sources are randomly distributed within the FOV and scale-lengths are flux dependent according to the following linear relation between the log median values: ln [α med /arcsec] = −0.93 + 0.33 ln [S/µJy]. Observed visibilities are computed adopting the exponential profile, as for the model visibilities, and we add uncorrelated Gaussian noise to these with variance given in Wrobel & Walker (1999) assuming all the antennas are SKA1-MID dish antennae.…”

“…The first one decomposes galaxy shapes through an orthonormal basis of functions, shapelets (Refregier & Bacon 2003;Refregier & Alexandre 2003), and coefficients are jointly fitted solving normal equations (Chang et al 2004). The second one, RadioLensfit (Rivi et al 2016;Rivi & Miller 2018), is an adaptation to the radio data of the optical method lensfit (Miller et al 2013), where a source extraction technique has been implemented in order to fit visibilities of a single galaxy at a time. The single source ellipticity fitting is performed adopting a realistic star-forming galaxy brightness profile and marginalising the likelihood over flux, position and size.…”

SuperCLASS – I. The super cluster assisted shear survey: Project overview and data release 1

“…Massey et al 2013, and references therein), but high resolution radio observations necessarily require an interferometric array. This can be a positive in terms of reducing the impact of the atmosphere, but it also means that the ideas used in optical shape measurement need to be reassessed at the very least and, indeed, the fact that the observations are done in the Fourier domain allows for new possibilities (Rivi et al 2016;Rivi & Miller 2018;Rivi et al 2019).…”