Cycle 8 NRAO ALMA Development Study Proposal – Beyond Black Hole Images: Extending New Imaging Techniques from EHT to ALMA

PI: Lynn Matthews, MIT

ABSTRACT 

The scientific demands of high angular resolution, high-fidelity imaging at millimeter and submillimeter wavelengths are a fundamental driver for future ALMA development. The current ALMA development plan aims to achieve an additional factor of 2 to 3 improvement in angular resolution, demanding either observing at high frequencies with the current longest baselines or at intermediate frequencies with 2 to 3 times longer baselines. However, both options are technically challenging for high fidelity imaging, due to larger calibration errors and/or much less uniform uv-coverage on the longest baselines. A key technical frontier is therefore the development of robust, high-fidelity imaging algorithms which can deal with larger calibration uncertainties and effectively use the planned outrigger stations for enhancing the angular resolution of ALMA images. In the last decade, the technical challenges of imaging with the Event Horizon Telescope (EHT) have accelerated the development of new imaging techniques, collectively known as regularized maximum likelihood (RML) methods. RML methods have strong potential to overcome and improve current and future challenges of ALMA imaging in three ways: (1) allowing highfidelity reconstructions, even at modest super resolution 2-3 times finer than that of traditional CLEAN; (2) the capability to reconstruct images directly from closure quantities, free from antenna-based calibration errors; and (3) the ability to handle intrinsically multi-dimensional emission, such as time-variable emission structures. The aforementioned advantages of RML methods over CLEAN have now been demonstrated not only for very long baseline interferometry, but also for single-band ALMA continuum observations. However, since the current RML packages were designed for performing single-band continuum observations with only a few antennas, the general application of RML methods to ALMA observations will require further developments of both software packages and imaging algorithms. This ALMA Study project aims to improve the effective spatial resolution and image fidelity of the current ALMA array and its planned extended array, by extending of RML methods from the EHT to ALMA. In the Study, we aim to develop the RML imaging algorithms to make them more general, flexible, and powerful, including: (a) the extension of imageable dimensions, enabling multi-band or multi-spectral channel imaging, and (b) developments of gridding-based RML imaging techniques involving minor and major cycles to accelerate the algorithm. Algorithms developed in the Study will be implemented in SMILI, one of the RML packages which is publicly available, open-source and python-interfaced. We will also implement IO functions for CASA Measurement Sets into SMILI, so that it can work as an external library of CASA. We will provide documentation, including an online manual and tutorials with example imaging scripts for ALMA, allowing the broader radio astronomy community to utilize RML imaging techniques for various interferometric data sets.