Eyeing a Supermassive Black Hole
Combining radio telescopes in Arizona, Chile and Hawaii, an international team of astronomers has observed the area around a supermassive black hole with unprecedented sharpness - 2 million times finer than human vision.
Connecting the Submillimeter Telescope at the University of Arizona's Arizona Radio Observatory, or ARO, to the Atacama Pathfinder Experiment, or APEX, telescope in Chile and the Submillimeter Array, or SMA, in Hawaii, astronomers have created a huge, single virtual telescope.
They were able to make the sharpest direct observation ever of the center of a distant galaxy, the bright quasar 3C 279, which contains a supermassive black hole with a mass about 1 billion times that of the sun and is so far from Earth that its light has taken more than 5 billion years to reach us.
The telescopes were linked using a technique known as Very Long Baseline Interferometry, or VLBI. Larger telescopes can make sharper observations, and interferometry allows multiple telescopes to act like a single telescope as large as the separation – or “baseline” – between them. Using VLBI, the sharpest observations can be achieved by making the separation between telescopes as large as possible.
According to Lucy Ziurys, director of the ARO and a professor in the UA departments of astronomy and chemistry and biochemistry, such an undertaking is fraught with extreme difficulties because each telescope has to detect, process and record the signals coming from an object at the exact same time.
“By placing our telescopes further and further apart, we increase our angular resolution, but at the same time, this becomes more challenging in terms of syncing the signal,” Ziurys explained. “The minute you put telescopes more distant from each other, you increase the chance for them being out of sync.”
For their quasar observations, the team used the three telescopes to create an interferometer with transcontinental baseline lengths of 5,870 miles from Chile to Hawaii, 4,458 miles from Chile to Arizona and 2,875 miles from Arizona to Hawaii. Connecting APEX in Chile to the network was crucial, as it contributed the longest baselines.
“Before these observations were conducted, it was unclear that a detection of any quasar or black hole could be made between telescopes as far apart as Chile and the continental United States, let alone between Chile and Hawaii,” Ziurys said. “These observations have shown that it can be done.”
The observations were made in radio waves with a wavelength of 1.3 millimeters, about 1,000 times shorter than FM radio waves. This is the first time observations at a wavelength as short as this have been made using such long baselines.
The observations achieved a sharpness, or angular resolution, of just 28 microarcseconds – about 8 billionths of a degree. This represents the ability to distinguish details an amazing 2 million times sharper than human vision. Observations this sharp can probe scales of less than one light-year across the quasar – a remarkable achievement for a target that is billions of light-years away.
“We chose quasar 3C279 because it is one of the strongest compact sources known at that particular wavelength and therefore ideal for detecting signals between the APEX antenna, the ARO SMT and the telescopes in Hawaii,” Ziurys said.
According to the scientists, the observations are a new milestone toward imaging supermassive black holes and the regions around them. The future holds plans to connect even more telescopes in this way to create the so-called Event Horizon Telescope. The Event Horizon Telescope will be able to image the shadow of the supermassive black hole in the center of our Milky Way galaxy, as well as others in nearby galaxies.
The shadow – a dark region seen against a brighter background – is caused by the bending of light by the black hole and would be the first direct observational evidence for the existence of a black hole’s event horizon, the boundary from within which not even light can escape.
In the absence of visible light, astronomers rely on detecting the high-energy radiation emitted by objects as they fall into the black hole. Most black holes are thought to be surrounded by a swirling disk of matter, joined by jets of energy shooting out of the black hole in opposite directions. Ziurys said that even though the black hole in our Milky Way is tiny by comparison at only 4 million solar masses, the Event Horizon Telescope should be able to resolve the area around it because it is much closer than quasar 3C279.
“There are many other sources such as 3C279 that will be studied by the Event Horizon Telescope in order to understand their structure, in particular their jets, even if the resolution is not sufficient to resolve the black holes they contain.”
Ziurys said that being in Arizona, the UA’s Submillimeter Telescope is in an excellent location for the geographical coverage of the observations, offering the necessary performance to detect the very weak radio signals inherent in the VLBI technique.
“The SMT has played and will continue to play a critical role in the development of millimeter-wave VLBI observations and in the EHT project in particular,” she said. “It has been part of every successful high-frequency VLBI measurements to date and will be crucial in achieving higher spatial resolution by higher-frequency observations.”
The experiment marks the first time that APEX has taken part in VLBI observations and is the culmination of three years hard work at APEX’s high altitude site on the 5,000-meter plateau of Chajnantor in the Chilean Andes, where the atmospheric pressure is only about half that at sea level. To make APEX ready for VLBI, scientists from Germany and Sweden installed new digital data acquisition systems, a very precise atomic clock, and pressurized data recorders capable of recording 4 gigabits per second for many hours under challenging environmental conditions. The data – 4 terabytes from each telescope – were shipped to Germany on hard drives and processed at the Max Planck Institute for Radio Astronomy in Bonn.
The successful addition of APEX is also important for another reason. It shares its location and many aspects of its technology with the new Atacama Large Millimeter/submillimeter Array, or ALMA, telescope. ALMA is nearing completion and will finally consist of 54 dishes with the same 12-meter diameter as APEX, plus 12 smaller dishes with a diameter of 7 meters.
The possibility of connecting ALMA to the network is currently being studied. With the vastly increased collecting area of ALMA’s dishes, the observations could achieve 10 times better sensitivity than these initial tests. This would put the shadow of the Milky Way's supermassive black hole within reach for future observations.
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