Locating the missing large-scale emission in the jet of M87* with short EHT baselines

TL;DR

This paper introduces a technique using short baseline closure phases in VLBI to detect and measure large-scale emission structures in M87*, revealing extended emission aligned with the jet.

Contribution

It presents a new method to measure image moments from trivial closure phases, validated on synthetic data and applied to EHT observations of M87* to detect large-scale emission.

Findings

Detected extended emission aligned with the jet in M87*

Measured the source centroid offset in 2021 data

Validated the technique with synthetic EHT data

Abstract

In Very-Long Baseline Interferometric arrays, nearly co-located stations probe the largest scales and typically cannot resolve the observed source. In the absence of large-scale structure, closure phases constructed with these stations are zero and, since they are independent of station-based errors, they can be used to probe data issues. Here, we show with an expansion about co-located stations, how these trivial closure phases become non-zero with brightness distribution on smaller scales than their short baseline would suggest. When applied to sources that are made up of a bright compact and large-scale diffuse component, the trivial closure phases directly measure the centroid relative to the compact source and higher-order image moments. We present a technique to measure these image moments with minimal model assumptions and validate it on synthetic Event Horizon Telescope (EHT) data. We then apply this technique to 2017 and 2018 EHT observations of M87* and find a weak preference for extended emission in the direction of the large-scale jet. We also apply it to 2021 EHT data and measure the source centroid about 1 mas northwest of the compact ring, consistent with the jet observed at lower frequencies.