A New Milestone in Sharpness

Event Horizon Telescope advances to submillimeter observations

In recent years, the Event Horizon Telescope, a global network of radio telescopes, has imaged the shadows around the supermassive black holes in M 87 and the Galactic Center at a wavelength of 1,3 millimeters. Now, in a pilot experiment, astronomers have achieved a record by taking data at an even shorter wavelength of 0,87 millimeters. The shorter the observation wavelength, the sharper the image. Once the data have been fully analyzed, the sharpest images of what are probably the most extreme regions in the universe will be created.

The Event Horizon Telescope (EHT) collaboration, with the participation of the Atacama Large Millimeter/sub-millimeter Array (ALMA), has for the first time received radio waves at 0.87 millimeters. “The only way to further improve the angular resolution, and hence the sharpness of images from ground-based radio telescopes has been to observe at radio wavelengths shorter than 1 millimeter. This has been a huge challenge", says Anton Zensus, founding chair of the EHT collaboration and director at the Max Planck Institute for Radio Astronomy.

Sharper than ever

With these new observations, the Event Horizon Telescope achieved the highest angular resolution among ground-based telescopes to date. It combines radio telescopes around the world, forming a radio interferometer about the size of the globe, to image supermassive black holes and the radiation from their immediate surroundings. These black holes are located in the centers of galaxies, regions of particularly extreme physical conditions in the universe. The shorter the observation wavelength, the higher the angular resolution of the telescope network. It shows, for example, the shadow of the black hole at the center of our Milky Way with unrivaled accuracy.

The precursor measurements were made by detecting radio emission from the centers of distant galaxies at a frequency of about 345 gigahertz, corresponding to a wavelength of 0.87 millimeters. Creating images from these radio interferometric raw data is not straight forward and will take more time. The collaboration expects to obtain images of black holes and their immediate surroundings that are 50 percent more detailed than before. In addition to M87* and Sgr A*, the scientists will also be able to create images of other black holes. “Looking at changes in the surrounding gas at different wavelengths will help us solve the mystery of how black holes attract and accrete matter, and how they can launch powerful jets that stream over galactic distances”, says Sheperd Doeleman, founding director of the Event Horizon Telescope and a staff member at the Harvard Center for Astrophysics.

In search of black holes

The data were already taken between October 18 and 21, 2018, but it was only today that a clear signal of five blazars was detected in the interferometric data stream. Blazars are particularly active cores of galaxies that contain one or more supermassive black holes at their center. During the measurements, the radio telescope network extended over a maximum of 9500 kilometers. At the observation wavelength of 0.87 millimeters, the Event Horizon Telescope will be able to image details at an unprecedented resolution of up to 13 micro-arcseconds. This corresponds to the size of a cap from a water bottle on the moon, as seen from Earth. “At 0.87 millimeters, our images will be sharper and more detailed, which in turn will likely reveal new properties, both those that were previously predicted and maybe some that weren’t”, says Alexander Raymond of NASA's Jet Propulsion Laboratory.

Technically, it has already been possible to observe the night sky at 0.87 millimeters with radio telescopes for quite some time, but recording data of sufficient quality was still a challenge. However, these problems were resolved over time through technological advances. For example, water vapor in the atmosphere absorbs radio waves at wavelengths especially below one millimeter. This makes it more difficult to receive signals with a sufficient signal-to-noise ratio. The key to success was to further improve the sensitivity of the Event Horizon Telescope by increasing the observation bandwidth of the detectors at the individual radio telescopes and defining flexible time slots so that the measurements could be started under optimal weather conditions.

 

Additional Information

The EHT collaboration involves more than 400 researchers from Africa, Asia, Europe, North and South America, with around 270 participating in this paper. The international collaboration aims to capture the most detailed images of black holes using a virtual Earth-sized telescope. Supported by considerable international efforts, the EHT links existing telescopes using novel techniques to create a fundamentally new instrument with the highest angular resolving power that has yet been achieved.

The EHT consortium consists of 13 stakeholder institutes; the Academia Sinica Institute of Astronomy and Astrophysics, the University of Arizona, the Center for Astrophysics | Harvard & Smithsonian, the University of Chicago, the East Asian Observatory, the Goethe University Frankfurt, the Institut de Radioastronomie Millimétrique, the Large Millimeter Telescope, the Max Planck Institute for Radio Astronomy, the MIT Haystack Observatory, the National Astronomical Observatory of Japan, the Perimeter Institute for Theoretical Physics, and the Radboud University.

The telescopes participating in this endeavor include ALMA, APEX, the IRAM 30-meter Telescope, the IRAM NOEMA Observatory, the James Clerk Maxwell Telescope (JCMT), the Large Millimeter Telescope (LMT), the Submillimeter Array (SMA), the Submillimeter Telescope (SMT), the South Pole Telescope (SPT), the Kitt Peak Telescope (KP), and the Greenland Telescope (GLT). Data have been post-processed at correlator facilities at the MPIfR in Bonn, Germany and MIT/Haystack Observatory in Westford, MA, USA.  Further analysis was performed in the framework of the global EHT collaboration.

IRAM is an international research organization for millimeter and submillimeter astronomy supported by the CNRS (France), the Max-Planck Gesellschaft (Germany), and the IGN (Spain). The organization operates two world-class research facilities, the IRAM 30-meter telescope in Spain, and NOEMA (Northern Extended Millimeter Array), the largest millimeter interferometer of the northern hemisphere, in the French Alps.

 

NJ/BEU