Into Supermassive Black Holes. Cutting-Edge Research by the Event Horizon Telescope

In 2019, the international collaboration of the Event Horizon Telescope (EHT) published the first “photographs” of the supermassive black hole at the center of the M87 galaxy (see p. XX in this issue and TEHPC, 2019a). These images confirmed the model predictions based on Albert Einstein’s theory of general relativity by showing the presence of a central dark shadow surrounded by a bright ring associated with a so-called photon sphere (TEHPC, 2019b; figure 1, left). A few years later, the EHT collaboration published images of the same black hole in polarized light, which revealed the presence of prominent magnetic fields near the event horizon and allowed to constrain the physical conditions of the surrounding plasma (TEHPC 2021a, 2021b, 2023). A second image of M87 was obtained a year after the original was announced (TEHPC, 2024). Although the overall structure is similar, the brightest region of the ring shifted position (figure 1, right), as expected theoretically in the dynamical environment of supermassive black holes. The magnitude of this change could favor the hypothesis of an opposite orientation between the rotation directions of the black hole and the surrounding material (TEHPC, 2025).

In a series of papers published in parallel with the M87 ones, the EHT collaboration also revealed images of the black hole Sagittarius A* (Sgr A*), located at the center of our galaxy, the Milky Way. These images show the same general structure as in the case of M87 (with a shadow and a bright ring) but test the theory of general relativity more directly,  since the mass of Sgr A* is known to greater precision. This is due to the work of Andrea Ghez’s and Reinhard Genzel’s groups, which measured the mass by monitoring the orbits of stars around the galactic center. However, the analysis of the EHT results on Sgr A* is more complex than for M87 due to its faster variability and the presence of ionized material in the line of sight, which causes twinkling.

Despite the great progress made thanks to the EHT images of M87 and Sgr A*, several fundamental questions remain unanswered. One of them concerns the variability of the observed emission. The black hole characteristic variability time is proportional to its mass, in the sense that more massive black holes exhibit slower variations than less massive ones. In the case of the two objects mentioned above, the characteristic variability times are minutes for Sgr A* and days for M87. The rapid variability of Sgr A* implies challenges for the EHT data analysis: the current sensitivity the instrument has makes us average hours of data to obtain the images, but over such a long time, the emission structure can change drastically, resulting in images with a lower sharpness. Besides, the changes observed in M87 over the years allow us to constrain some of its properties, such as the relative orientation of the black hole rotation and the surrounding material. Ideally, a more sensitive and versatile instrument than the EHT, capable of monitoring both M87 and Sgr A* with cadences that allow the tracking of their variations would be preferred.

The Next Generation Event Horizon Telescope (ngEHT) is precisly such an instrument, which will be adding a dozen telescopes to the existing EHT array (figure 2). In addition, the ngEHT will observe simultaneously in three frequency bands (whereas the current EHT only observes in one), which will mitigate the effect of the Earth’s atmosphere on observations and the twinkling caused by ionized gas in the direction of Sgr A*. Thanks to the ngEHT, it will be possible to obtain films of M87 and Sgr A* that will allow tracing their temporal variations. A second very important unanswered question that ngEHT will make possible to address is the origin of the powerful relativistic jets that often emanate from some supermassive black holes, including M87. Several existing theories attempt to explain the formation of these jets, but observations on appropriate spatial scales are needed to constrain these theories. The EHT has too high an angular resolution and it filters the emission to the jet scales, revealing only structures comparable to the event horizon. Other instruments have insufficient  resolution to resolve the internal structure of the jet in the black hole vicinity. This observational gap will be closed with the ngEHT by incorporating strategically placed telescopes that optimize spatial coverage and sensitivity at intermediate scales. In other words, the ngEHT will allow us, for the first time, to “film” the formation of a relativistic jet.

The new radio telescopes to be incorporated into the ngEHT are all identical, with a thirteen-meter diameter. The exact location of each new telescope was determined by detailed studies that considered meteorological and topographical conditions, as well as the optimization of the final arrangement for the scientific purposes of the instrument as a whole. One of the selected locations to receive an ngEHT antenna is the National Astronomical Observatory in San Pedro Mártir, Baja California. This site combines exceptional meteorological conditions with an ideal logistical infrastructure for a frontier instrument such as the ngEHT. In addition, the proximity to existing telescopes in Arizona and Sierra Negra in Puebla allows to access the intermediate spatial scales needed for the study of relativistic jet formation.

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