The road to Oz may be paved with yellow bricks, but it took time to lay each one of those bricks.
Science is also a journey that requires building the path on which it travels. When astronomers from the Event Horizon Telescope Collaboration published the First image of a black hole silhouette in 2019It was an achievement based on more than a decade of work. The hardware, the software, the observing strategies, even the relationships between continents and disciplines all had to be conceptualized, built and tested before scientists could achieve their goal.
Since that first image of the shadow of the supermassive black hole at the center of the elliptical galaxy M87, the collaboration has continued to obtain images The black hole at the center of our own galaxy, Sagittarius A* (pronounced “A-star”). The team has also produced images that include information about Magnetic fields frolicking in hot gas near the event horizons of both black holes.
The last milestone on this path, published on August 27 in the Astronomical Magazineextends the EHT's work to a shorter wavelength. This addition will allow for sharper images than before.
So far, EHT imaging has been based on measurements at 1.3 mm (230 GHz), but the collaboration has long been planning to extend that to 870 microns (0.87 mm or 345 GHz). The shorter wavelength has several advantages: it sharpens images by 50 percent compared to 1.3 mm, and it’s better at penetrating the plasma clutter and magnetic fields between us and black holes, giving us a clearer view of what’s happening near the event horizon.
However, it is difficult to observe a wavelength of 870 microns. Water vapour in the Earth's atmosphere absorbs signals more easily at this wavelength than at the longer 1.3 mm wavelength. In addition, the equipment is less sensitive.
The collaboration has overcome these obstacles. Thanks to test observations carried out in 2018, the team has successfully combined data streams from several radio telescopes to confirm that they can detect structures at 0.87 mm. This is the first step necessary to be able to reconstruct images with data at this wavelength.
Credit: ESO / M. Kornmesser
Building a planet-sized telescope
The Event Horizon Telescope works by Combining observations from pairs of telescopes in its global network To create an image, a technique called very long baseline interferometryEach pair is sensitive to structure on a certain scale of the observed object, depending on the distance between the two telescopes.
The new analysis uses data obtained in October 2018 with four telescopes: the Atacama Large Millimeter/submillimeter Array (ALMA), linked to one of three observatories in Hawaii, Chile or Spain (Weather did not collaborate with other EHT sites in Greenland and France).
The team achieved a resolution of just 19 microarcseconds, allowing them to see details down to the size of a Brussels sprout as seen from the distance of the Moon. No ground-based observation had ever achieved this before.
Reaching 870 microns will allow EHT scientists to better study he photon ringthe closest path that light can take around the black hole without falling into it. This ring is a unique prediction of Einstein's general theory of relativity, which describes gravity as the warping of space-time by mass. Everything, including light, has to travel along the curved paths these warps create.
“Gravity bends all wavelengths of light in the same way, so the radiation projected by the black hole should produce central ‘rings’ of similar size,” explains Shep Doeleman (Black Hole Initiative, Harvard), founding director of the EHT.
But the supermassive black holes the EHT studies are also surrounded by hot gas entangled with magnetic fields. Shorter wavelengths pass through this hot plasma more easily than longer ones, so the accretion flow and jet will look different at 870 microns than at 1.3 mm, he says. By comparing the observed emission at the two wavelengths, the team can use what we know about gas physics and the physics of light bending to distinguish which light is coming from where and refine the images of both M87* and Sgr A*.
These early test observations can’t produce an image — they’re just a glimpse of where we’re headed on the path to discovery. The team is now working on 870-micron data taken in 2021, which could give us the first look at M87’s black hole at this shorter wavelength. And the next-generation EHT, which will add several new stations around the world, will also include 870 micron where weather permits.
Reference: Alexander W. Raymond et al. “First very long baseline interferometry detections at 870 μm.” Astronomical MagazinePublished on August 27, 2024.