Probing jet base emission of M87* with the 2021 Event Horizon Telescope observations

TL;DR

This study uses enhanced 2021 EHT observations to detect and characterize the jet base emission in M87*, revealing a faint Gaussian feature consistent with the jet origin, and highlights the need for expanded baseline coverage for detailed imaging.

Contribution

First detection of jet base emission in M87* at 230 GHz using new intermediate baselines, providing constraints on large-scale emission structures and informing future observations.

Findings

Detected a Gaussian component offset from the core at ~5500 AU.

Most missing flux in previous studies originates from larger scales.

Current data do not favor complex models, only a simple Gaussian.

Abstract

We investigate the presence and spatial characteristics of the jet base emission in M87* at 230 GHz, enabled by the enhanced uv coverage in the 2021 Event Horizon Telescope (EHT) observations. The addition of the 12-m Kitt Peak Telescope and NOEMA provides two key intermediate-length baselines to SMT and the IRAM 30-m, giving sensitivity to emission structures at scales of $\sim250~\mu$as and $\sim2500~\mu$as (0.02 pc and 0.2 pc). Without these baselines, earlier EHT observations lacked the capability to constrain emission on large scales, where a "missing flux" of order $\sim1$ Jy is expected. To probe these scales, we analyzed closure phases, robust against station-based gain errors, and modeled the jet base emission using a simple Gaussian offset from the compact ring emission at separations $>100~\mu$as. Our analysis reveals a Gaussian feature centered at ($\Delta$RA $\approx320~\mu$as, $\Delta$Dec $\approx60~\mu$as), a projected separation of $\approx5500$ AU, with a flux density of only $\sim60$ mJy, implying that most of the missing flux in previous studies must arise from larger scales. Brighter emission at these scales is ruled out, and the data do not favor more complex models. This component aligns with the inferred direction of the large-scale jet and is consistent with emission from the jet base. While our findings indicate detectable jet base emission at 230 GHz, coverage from only two intermediate baselines limits reconstruction of its morphology. We therefore treat the recovered Gaussian as an upper limit on the jet base flux density. Future EHT observations with expanded intermediate-baseline coverage will be essential to constrain the structure and nature of this component.