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

TL;DR

This paper leverages the enhanced intermediate-baseline coverage of the 2021 Event Horizon Telescope observations to search for resolved jet-base emission near M87*. By analyzing closure phases and fitting a simple ring+Gaussian model to the 230 GHz data, the authors identify a faint, SW-offset Gaussian component with a flux of order $F_0\sim$ few×$10^{1}$ mJy and width $\mathrm{fwhm}\sim180~\mu\\mathrm{as}$, located at a projected offset of a few hundred microarcseconds from the ring. The Gaussian component modestly improves the fit to the ALMA-PV-NOEMA and ALMA-KP-SMT triangles, suggesting the presence of extended jet-base emission at intermediate scales while remaining conservative about morphology due to limited baseline sampling. Crucially, the results imply that the bulk of the previous “missing flux” likely resides on even larger scales, and that past 2017–2018 ring-only reconstructions remain robust. The study underscores the need for additional intermediate baselines and higher sensitivity to firmly map the jet base at horizon scales and connect it to the larger-scale jet structure.

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 $\sim250~μ$as and $\sim2500~μ$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 $>100~μ$as. Our analysis reveals a Gaussian feature centered at ($Δ$RA $\approx320~μ$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.

Figures (18)

• Figure 1: $(u,v)$ coverage of the EHT and representative ring image obtained by DoG-HITM87_2021 in 2017, 2018, and 2021. Upper panels: $(u,v)$ with the SMT-KP (red) and PV-NOEMA baselines (green) highlighted. These are used to constrain the jet base emission. Middle panels: a zoomed-in view of the $(u,v)$ coverage, highlighting the short and intermediate baselines. We also highlight the $(u,v)$ coordinates related to spatial scales of $1000\,\mu\mathrm{as}$ and $200\,\mu\mathrm{as}$, corresponding to the expected range for extended emission. Bottom panels: DoG-HIT image of the central ring obtained from these data M87_2021.
• Figure 2: ALMA-APEX-PV (top) closure phases of M87* as observed in 2017, 2018 and 2021. The ALMA-PV-NOEMA (middle) and ALMA-SMT-KP (bottom) closure phases on M87* observed on April 18, 2021 (figure is adapted from M87_2021) are shown together with ring-image models obtained with different imaging algorithms (see text for details). The top panel shows an example of a trivial closure phase triangle, where two stations are co-located, see Georgiev2025 for details. The image models are discrepant with the observed data by a few degrees, which may be attributable to intermediate-scale emission observed on the PV-NOEMA and SMT-KP baselines.
• Figure 3: Symmetric Gaussian component with varying parameters over the reconstructed DoG-HIT image as represented in Figure \ref{['fig: uv-coverage']} represented in log scale. Different rows correspond to changes in the values of different parameters (see text for details). The solid lines represents the corresponding complete model (ring + Gaussian) closure phases on the respective triangles.
• Figure 4: $\chi^2_{\nu}$ achieved with the Gaussian model for a fixed width and brightness ratio, but varying position. The left and middle panels show the $\chi^2_\nu$ for the ALMA-PV-NOEMA and the ALMA-SMT-KP triangles, respectively, and the right panel shows the combined $\chi^2_\nu$. The contours indicate the position of the best-fit model.
• Figure 5: Ring-only DoG-HIT image (panel a: linear scale, panel b: logarithmic scale) compared to the DoG-HIT ring image with an additional Gaussian (panel c and d). In the bottom row, we show the fit to the bump and offset in closure phases and the fit to the amplitudes for DoG-HIT (panels e,f,g) and for DoG-HIT with the symmetric Gaussian component added (panels h,i,j).