Beyond Sgr A* and M87*: Sub-Microarcsecond Black Hole Shadow Detection via Lunar-based Extremely Long Baseline Interferometry

TL;DR

This paper evaluates the feasibility of sub-microarcsecond black hole shadow imaging by adding a lunar-based telescope to the 230 GHz EHT, achieving an approximate $0.85\,\mu\mathrm{as}$ resolution and enabling shadows beyond the well-studied M87* and Sgr A*. Using a simple geometric ring model and Moon–Earth baselines, the authors assess shadow detectability for 31 SMBH candidates, identify six high-priority targets (M104, NGC 524, PGC 049940, NGC 5077, NGC 5252, NGC 1052) that would be shadow-detectable with a 100 m lunar antenna, and analyze how baseline geometry and sensitivity govern detectability. They also examine photon-ring detectability under optimistic space-baseline augmentation, finding that, with current assumptions, only Sgr A* remains viable for photon-ring imaging, highlighting the dominant role of sensitivity and the need for broad bandwidth and longer integrations. The work outlines a clear scientific and technical motivation for lunar-based VLBI and provides guidance for future mission concepts (e.g., multiple space-based telescopes and ILRS/LOVEX initiatives) to enable broader tests of GR in strong gravity and to extend shadow studies to a larger SMBH population.

Abstract

The 1.3 mm ground-based very long baseline interferometry (VLBI) array, the Event Horizon Telescope (EHT), is limited by the Earth's diameter and can image the supermassive black hole (SMBH) shadows of only M87* and Sgr A*. Extending the array with an assumed lunar-based telescope could achieve $\sim 0.85\ μ$as angular resolution at 230 GHz, enabling black hole shadow detection for a larger SMBH sample. The concept is motivated by space VLBI missions and lunar exploration, including the ongoing Lunar Orbit VLBI Experiment (LOVEX) aboard QueQiao-2 (Chang'E-7) and the planned International Lunar Research Station (ILRS). We assess shadow detectability for 31 SMBH with predicted large angular sizes, exploring different telescope location and antenna size. Assuming a telescope at the lunar antipode, we simulate the Moon-Earth (u,v) coverage and show that source geometry relative to the Moon's orbit determines whether the primary indicator of shadow, first visibility null, can be sampled. Using a geometric ring model, we identify six high-priority targets: M104, NGC 524, PGC 049940, NGC 5077, NGC 5252, and NGC 1052. Shadows of M104, NGC 5077, and NGC 1052 are detectable with a 5 m lunar-based telescope; PGC 049940 requires 20 m; NGC 524 and NGC 5252 require 100 m. Photon ring detection for Sgr A*, M87*, NGC 1600, and M31 is possible if space telescopes fill the baseline coverage gaps and sensitivity requirements are met. These results provide a clear scientific and technical motivation for lunar-based telescopes in future black hole shadow studies.

Figures (9)

• Figure 1: Ring model images of 29 SMBH candidates (Table \ref{['tab:BHimages_candidates']} excludes Sgr A* and M87*), ordered by their ring sizes. All images are shown with a common angular scale and color map. The green circle in the lower right indicates a 0.85 $\mu$as beam estimated from Eq. \ref{['eq:theta_beam']}.
• Figure 2: Map of telescope sites on the Moon.
• Figure 3: Observable duration for different sites over one lunar sidereal month (27.32 days)
• Figure 4: Simulated ($u,v$) coverage for a Moon-Earth VLBI array, formed by a lunar-based telescope at S1 and the EHT 2025 array. Data points for the Moon-Earth baselines are shown in red, and for the ground-based EHT baselines in blue. The grey circle indicates a baseline of 30 $D_\oplus$.
• Figure 5: Distribution of the minimal projected Moon-Earth baseline on the celestial sphere, with marked positions of SMBH candidates. The sinusoidal pattern reflects the geometric relationship between the source position and the Moon's orbital path (with a 5.15$^\circ$ mean inclination relative to the equator); sources located near this orbital path exhibit smaller minimal projected baselines, corresponding to the linear ($u,v$) coverage cases presented in Fig. \ref{['fig:uv_coverage']}.