Speakers
Description
21 cm Intensity Mapping probes the universe using the spin-flip transition of the neutral hydrogen atom. This cosmological probe has enormous potential to unravel the nature of our cosmos. Multiple next-generation radio instruments such as the Low-Frequency Array (LOFAR)[1], the Murchison Widefield Array (MWA)[2] and the proposed Square Kilometer Array (SKA)[3] are either currently attempting to or hope to detect this signal. However, to detect this signal we must first remove foregrounds which are 4-5 orders of magnitude greater than the signal itself. These foregrounds are principally of two types: galactic synchrotron and extra-galactic foregrounds.
The latter in particular can be extremely bright. Such sources are already problematic within radio astronomy but can be catastrophic for the 21cm signal. Subtraction of the calibrated source in visibility space, also known as peeling, is typically done to combat this problem. However, the inherent chromaticity of various instrumental effects typically causes these bright sources to leak into and contaminate the 21 cm signal despite the peeling.
We propose the bluebild algorithm [4] as an alternate solution. Bluebild is an interferometric imager developed to reconstruct the sky intensity matrix directly on the celestial sphere. It uses functional principal component analysis based on the intensity of sources to separate input visibilities into eigen-visibilities. These eigen-visibilities are then reconstructed into eigenimages, which can be optimally combined into energy levels for source separation. Since the PCA is done on the visibilities directly, all energy levels contain artefacts only from the sources present within themselves. This would in principle allow us to separate bright sources from the 21 cm signal
We use this principle to image a model field using bluebild. The field consists of 21 cm brightness fluctuations, synchrotron and extra-galactic foregrounds. These are in turn modulated by directed dependent and direction-independent effects. Finally, additive noise is also incorporated into this model. We use the model as an input to the Oxford SKA Radio Telescope Simulator to produce simulated visibilities and subsequently image this output using the bluebild algorithm. We then evaluate the cosmological signal extracted from the bluebild output.
[1]:Mertens, F., et. al. (2020). Improved upper limits on the 21 cm signal power spectrum of neutral hydrogen at z ≈ 9.1 from LOFAR. Monthly Notices of the Royal Astronomical Society.
[2]:Trott, C.M., et. al. (2020). Deep multiredshift limits on Epoch of Reionization 21 cm power spectra from four seasons of Murchison Widefield Array observations. Monthly Notices of the Royal Astronomical Society, 493, 4711-4727.
[3]:Santos, M.G., et. al. (2015). Cosmology with a SKA HI intensity mapping survey. arXiv: Cosmology and Nongalactic Astrophysics.
[4]:(Submitted) Tolley, E., et. al. (2023). BIPP: An efficient HPC implementation of the Bluebild algorithm for radio astronomy
