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

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 $\sim250μ$as and $\sim2500μ$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 $>100μ$as. Our analysis reveals a Gaussian feature centered at ($Δ$RA $\approx320μ$as, $Δ$Dec $\approx60~μ$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.

💡 Research Summary

This paper exploits the dramatically improved (u,v) coverage of the 2021 Event Horizon Telescope (EHT) array to search for faint, extended emission associated with the jet base of M87* at 230 GHz. The addition of two new stations – the 12‑m Kitt Peak telescope in the United States and the Northern Extended Millimeter Array (NOEMA) in France – creates two intermediate‑length baselines: SMT–KP (~100 km) and PV–NOEMA (~1100 km). These baselines probe angular scales of roughly 250 µas and 2500 µas (≈0.02 pc and 0.2 pc), respectively, filling the gap between the ultra‑long baselines that resolve the ∼40 µas black‑hole shadow and the trivial intra‑site baselines that are sensitive only to arcsecond‑scale structure.

Previous EHT campaigns (2017–2018) lacked such intermediate baselines, leaving a “missing flux” problem of order 1 Jy: the total flux measured on short intra‑site baselines (e.g., ALMA–APEX) exceeded the flux accounted for by compact ring models (0.5–1.0 Jy). The 2021 data therefore provide a unique opportunity to determine whether part of this missing flux originates on scales of a few hundred to a few thousand micro‑arcseconds, i.e., the expected size of the jet launching region.

Because the intermediate baselines are sparse (only two), direct imaging of any extended component is impossible. The authors therefore turn to closure phases, which are immune to station‑based atmospheric and instrumental phase errors. Closure triangles that include the most sensitive station (ALMA) and the short baselines (SMT–KP, PV–NOEMA) are examined. For a perfectly symmetric ring or a point source, the closure phase should be zero; however, the measured closure phases show statistically significant non‑zero residuals when compared with synthetic data generated from ring‑only reconstructions. This discrepancy indicates the presence of additional asymmetric structure on the angular scales sampled by the intermediate baselines.

To quantify the excess emission, the authors augment a compact ring model with a single offset Gaussian component. They explore the parameter space of the Gaussian’s position and flux while keeping the ring parameters fixed to those derived from previous EHT imaging. The best‑fit solution places the Gaussian at ΔRA ≈ +320 µas, ΔDec ≈ +60 µas relative to the ring centre, i.e., roughly 5500 AU projected separation in the direction of the large‑scale jet (south‑west). The inferred flux density of this component is only ∼60 mJy. This modest flux implies that the bulk of the ∼1 Jy missing flux must arise on still larger scales (≫2500 µas) that are not sampled even by the new intermediate baselines.

The authors test the robustness of their result using a suite of image‑reconstruction algorithms (regularized maximum likelihood, DoG‑HIT, CLEAN, COMRADE, THESIS) applied to synthetic data that mimic the 2021 (u,v) coverage. All methods recover the compact ring faithfully but require an additional large‑scale component to match the observed closure phases, confirming that the detection is not an artifact of a particular reconstruction technique. Nevertheless, with only two intermediate baselines the morphology of the jet‑base emission cannot be resolved; the Gaussian should therefore be interpreted as an upper limit on the true flux and a rough localization of where the jet base contributes at 230 GHz.

The paper discusses the astrophysical implications. The offset Gaussian aligns with the known direction of M87’s kiloparsec‑scale jet, supporting the interpretation that the detected component is genuine jet‑base emission rather than a spurious artifact. Its low flux density suggests that the jet becomes radio‑bright only beyond ∼0.2 pc from the black hole, consistent with previous 86 GHz GMVA studies that identified a collimated, edge‑brightened jet base at larger radii. The result also clarifies that the missing flux problem in earlier EHT epochs is not solved by a modest jet‑base component; instead, extended emission on scales of several thousand micro‑arcseconds (perhaps a diffuse sheath or larger‑scale outflow) must dominate.

Finally, the authors outline future prospects. To fully characterize the jet base, a denser network of intermediate baselines is essential. Adding more 30‑m class stations, employing additional short baselines between existing sites, or integrating next‑generation facilities such as the ngEHT or expanded GMVA+ALMA will dramatically improve (u,v) coverage in the 200–2000 µas regime. Such data will enable direct imaging of the jet‑launching region, constrain its brightness profile, and test theoretical models of jet acceleration and collimation in the immediate vicinity of a supermassive black hole. In summary, the 2021 EHT observations provide the first quantitative detection of a faint, offset jet‑base component at 230 GHz, establishing an upper limit of ~60 mJy and a location consistent with the large‑scale jet, while highlighting the need for richer intermediate‑baseline coverage to resolve the detailed structure of M87*’s jet launch zone.