First M87 Event Horizon Telescope Results and the Role of ALMA (1910.10193v1)

Abstract: In April 2019, the Event Horizon Telescope (EHT) collaboration revealed the first image of the candidate super-massive black hole (SMBH) at the centre of the giant elliptical galaxy Messier 87 (M87). This event-horizon-scale image shows a ring of glowing plasma with a dark patch at the centre, which is interpreted as the shadow of the black hole. This breakthrough result, which represents a powerful confirmation of Einstein's theory of gravity, or general relativity, was made possible by assembling a global network of radio telescopes operating at millimetre wavelengths that for the first time included the Atacama Large Millimeter/ submillimeter Array (ALMA). The addition of ALMA as an anchor station has enabled a giant leap forward by increasing the sensitivity limits of the EHT by an order of magnitude, effectively turning it into an imaging array. The published image demonstrates that it is now possible to directly study the event horizon shadows of SMBHs via electromagnetic radiation, thereby transforming this elusive frontier from a mathematical concept into an astrophysical reality. The expansion of the array over the next few years will include new stations on different continents - and eventually satellites in space. This will provide progressively sharper and higher-fidelity images of SMBH candidates, and potentially even movies of the hot plasma orbiting around SMBHs. These improvements will shed light on the processes of black hole accretion and jet formation on event-horizon scales, thereby enabling more precise tests of general relativity in the truly strong field regime.

• The paper presents the first resolved image of M87*’s black hole shadow, measuring a 42 ± 3 microarcsecond ring diameter that validates general relativity.
• It highlights ALMA's phased-array contribution in enhancing EHT sensitivity and uv-plane coverage to capture unprecedented event horizon details.
• Future directions include expanding the global VLBI network and incorporating additional telescopes to enable finer resolution tests of gravitational physics.

Overview of "First M87 Event Horizon Telescope Results and the Role of ALMA"

This paper focuses on the achievements and methodologies pertaining to the first image of a supermassive black hole (SMBH), specifically the black hole M87*, obtained by the Event Horizon Telescope (EHT) collaboration. The paper highlights the pivotal contributions of the Atacama Large Millimeter/submillimeter Array (ALMA) as an integral part of the EHT network in increasing sensitivity and resolving power to achieve this groundbreaking result.

Background and Methodology

The EHT is a global very long baseline interferometry (VLBI) array combining radio facilities around the world to achieve an angular resolution high enough to discern event horizon-scale structures. Observing at a wavelength of 1.3 mm, the EHT effectively simulates a telescope the size of Earth. In 2017, it included stations from continents across the globe, such as ALMA in Chile, the IRAM 30-meter telescope in Spain, and the South Pole Telescope, among others.

ALMA's role as a phased-array station formed a cornerstone for the EHT. By enhancing the array's sensitivity by an order of magnitude, ALMA made it feasible to image the event horizon itself by improving signal-to-noise ratios across VLBI baselines and providing critical uv-plane coverage. These contributions were crucial in capturing the first resolved image of a black hole shadow, a significant validation of general relativity (GR) predictions at such scales.

Results

The image obtained by the EHT revealed a bright ring—interpreted as the shadow cast by the event horizon of M87*—that confirmed GR's predictions about the light behavior near a black hole. Analysis showed that the diameter of this ring is in close agreement with the Schwarzschild radius of a black hole of approximately 6.5 billion solar masses. The dark region in the center is consistent with the event horizon's shadow.

One of the most noteworthy results of the paper is the 42 ± 3 microarcseconds measurement of the ring diameter, confirming the estimation of M87*’s mass as 6.5 billion times that of the Sun, providing a significant affirmation of GR and the existence of SMBHs.

Implications

The results have significant implications for astrophysics and our understanding of black holes. Achieving such an image marks a monumental advance in testing GR in the strong-field regime, providing new insights into accretion processes and jet formation in proximity to event horizons. The ability to directly image a black hole shadow opens new avenues for precision tests of GR, potentially assessing alternatives to the classical GR framework.

Future Directions

The paper outlines future enhancements for the EHT, involving the expansion of the array with additional telescopes from diverse continents, and potentially incorporating VLBI from space. Such expansions will improve the baseline coverage and the fidelity of future observations, enabling finer-resolu... tion images and potential time-variable studies or "movies" of black hole environments. Furthermore, ongoing and future observations of Sgr A* (the black hole at the center of our galaxy) and other AGNs using improved arrays could reveal more about black hole parameters such as spin, thereby enhancing our understanding of black hole physics and gravity.

In conclusion, the paper establishes the technical and scientific framework that led to the first capture of a black hole’s shadow, with ALMA playing an instrumental role in the EHT array's success. This pioneering observation sets the stage for future explorations of gravitational physics and SMBHs, poised to redefine numerous paradigms within astrophysics.