The work of two professors at NYU Abu Dhabi, Associate Professor of Physics Ingyin Zaw and Assistant Professor of Physics David Russell, is on black holes and the role these massive yet obscure objects play in the formation of galaxies.
Black holes have such dense masses that not even light can escape their gravitational pull, which means that they are essentially invisible. Zaw’s focus is on supermassive black holes, which are hundreds of thousands to billions of times more massive than the Sun and can be found at the center of “every normal galaxy,” she said.
Research in the field has shown there is a correlation between the mass of a black hole and the mass of the surrounding galaxy, which indicates that black holes and galaxies may co-evolve, Zaw explained. But there are cases where the masses of the two bodies don’t correlate, and these are interesting sites for analysis.
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For example, there are a few dwarf galaxies — which, as their name suggests, are much smaller than normal galaxies — that host large black holes that are much more likely to be found in bigger galaxies. Zaw is directing some of her current research to this subset of objects.
To analyze the black holes, Zaw looks at maser emissions, which are like lasers, but instead of being in the visible light spectrum, they exist in the radio spectrum. “Masers are emitted by clouds of water vapor that orbit black holes, and by analyzing them, we can measure the mass of a black hole very well,” she said. Indeed, she explained, analysis of maser emissions is the best method for studying distant black holes that lie outside of our galaxy, as it provides researchers with a high level of accuracy about the mass of the black hole and the surrounding galaxy.
“By looking at black holes, we can learn something about the evolution of galaxies and especially about the evolution of dwarf galaxies,” and perhaps even about the beginning of the universe, Zaw explained. “These galaxies should act more like the galaxies that were present earlier in the universe closer to the Big Bang.”
Russell is interested to learn how many black holes exist in our galaxy, the Milky Way, and how these black holes influence their surroundings.
“We can’t see a black hole itself,” he explained, “but we can spot it when there is a star close by, so I typically analyze these binary systems where there is a star orbiting a black hole.”
Though it was once thought that matter that begins its descent to a black hole was doomed to fall into the void, but “it turns out that some of the matter that gets sucked towards a black hole doesn’t fall in, but ends up getting spewed out in two jets that travel close to the speed of light,” Russell said.
He and his colleagues are trying to understand how much energy and matter these jets contain and how the jets affect surrounding material. “These jets can blow holes through a galaxy and prevent star formation,” he noted, “and in the early universe, we think that this may have played a role in regulating the rate at which early galaxies were formed.”
Russell has collaborated recently with the Al Sadeem observatory in Al Wathba, which is run by Thabet Al Qaissieh, to observe a nearby black hole, named MAXI J1820+070, that experiences outbursts of light. Students and postdoctoral researchers in Russell's group at NYUAD are analyzing those observations trying to determine how these outbursts affect the gas that surrounds the black hole.
“We take a series of observations over a period of 30 minutes and see how much the light flickers in that time,” he explained. “We are working to understand what causes the flickering and we’re trying to correlate the flickering in the visible light spectrum with different wavelengths and understand the inflow and outflow” of matter.
One motivation for studying black holes, Russell added, is to understand how matter behaves in extremely strong gravitational fields. “In space, there are high energies that we can’t probe on Earth by recreating them in an instrument,” he said. “We know from Einstein’s general theory of relativity that when you get close to a compact object like a black hole, space-time itself is being dragged around the black hole as it spins. So far, our observations fit Einstein’s theory, but we need to understand this in a better way.”