Please tell us about yourself
University of New Mexico doctoral student Karishma Bansal presented her recent discoveries on a pair of orbiting supermassive black holes in a galaxy about 750 million light years from Earth, the most compact orbiting system ever detected. She explained how black holes form, how scientists detect them and what the most recent advances in detection have told astronomers about the objects. Speaking to a packed house during the free event, Bansal often waxed poetic as she talked about the black holes she has studied, comparing their motion around one another to a “dance.”
Black holes are celestial objects so dense and massive that nothing, not even light, can escape their gravitational force. Most are formed when massive stars, several times the mass of the sun, die and collapse under their own weight. But as Bansal explained, supermassive black holes are much too big to have formed from collapsing stars, and scientists still aren’t sure how they are created. They do believe that most large galaxies, including the Milky Way, have a supermassive black hole at the center. When galaxies collide, their black holes often begin orbiting one another, and eventually merge to form an even larger black hole.
“They are hungry monsters,” Bansal said. “They just eat and eat all the material around them.”
But despite their size, it’s not easy to detect faraway objects that don’t emit light. Scientists confirm the existence of black holes through a variety of methods, Bansal said, such as tracking the way light from other objects bends around them, and by looking for quasars, huge jets of plasma spewed out by the material around most supermassive black holes. In 2016, a group of scientists working on a giant telescope called the Laser Interferometer Gravitational-Wave Observatory found a new way to learn about the objects, by detecting gravitational waves from the collision of a pair of supermassive black holes.
Bansal said she used about 12 years of data from the Very Long Baseline Array, a collection of radio telescopes located all over North America, to confirm that two supermassive black holes in a galaxy called 0402+379 are moving around one another. She said that discovery, finalized earlier this year, was important because most orbiting black holes scientists have been able to detect in the past are thousands of light years away from each other, and no one has ever actually measured their motion. The two black holes she studied are separated by just 24 light years.
She said her discovery is an example of what scientists can detect with larger telescopes and better resolution. But there’s still much to be learned. Right now she and other scientists have mapped out several possible orbit patterns for the black holes, but they need more data to know which pattern the pair is following. They also want to find more compact systems like this, she said, which will require more high-resolution telescopes and technology that can detect gravitational waves.
While supermassive black holes in distant galaxies might not seem to affect everyday life, Bansal said she believes it’s worthwhile to try and unravel their mysteries, especially since many astronomers believe they are a picture of what the Milky Way may look like in the far distant future.
“You are sitting on Earth, and you get to look up at the dark skies all night,” Bansal said. “Why not enjoy the show?”
What did you study?
Karishma Bansal, an IISER Pune alumnus, is part of a team of researchers that detected for the first time, two supermassive black holes orbiting each other, in a galaxy about 750 million light years away from Earth.
Please tell us about your work
Bansal worked with UNM’s Dr. Gregory Taylor and scientist from other universities on the discovery. For over a decade, Taylor and his colleagues utilized the Very Long Baseline Array – a series of 10 radio telescopes located across much of North America from Hawaii to the Virgin Islands with one telescope in western New Mexico—to conduct very fine measurement from enormous distances away from Earth. Bansal then used the collected data to prove orbital movement between the two black holes from measurements made since 2006 to present.
These orbiting black holes resulted from two galaxies merging together. Scientists have theorized this scenario was possible; as Bansal explains, “Any decent sized galaxy has a super massive black hole at the center, so given all those statistics we would expect these systems to be more common.” However, Bansal and Taylor’s work is the first to actually observe this phenomenon, a system which Bansal says is “the most complex . . . ever detected.” And, since our own Milky Way galaxy is due to collide with the Andromeda galaxy in about a billion years, Bansal and Taylor’s research gives us an advance view of what that collision and its effects could look like. The findings may also shed additional light on how gravitational waves work.
The project has attracted the attention of the campus, local, national, and international news media, even in her home country of India. Despite the press coverage, however, Bansal notes that she only has partial funding for the next school year. “You guys make me famous now, and nobody gives me money,” she says with a laugh, noting that as an international student she is ineligible for the bulk of funding available in her field.
How does your work benefit the community?
Supermassive black holes (SMBHs) are the largest types of black holes, whose formation is still a mystery. The two SMBHs observed in the galaxy 0402+379 are separated by a distance of about 24 light years. On the cosmic scale this separation is very small, and thus hard to detect which marks the technical achievement of this discovery.
This work is the first instance where a supermassive binary black hole system has been imaged as two distinct objects which are orbiting each other. This was done with the VLBA (Very Long Baseline Array), a system of 10 radio telescopes that has been observing signals emitted by this system for many years now.
“In the initial stages of the project, I started learning how to reduce radio data. Later I began to analyse data that my advisor, Dr. Greg Taylor, and team had been gathering for over 10 years”, said Bansal, now a graduate student at the University of New Mexico. “It was very exciting to be able to see black holes in an image for the first time.”
Regarding how this finding would further our knowledge of the universe, she said, “It will enable us to understand formation and evolution of galaxies in general and how supermassive black holes affect them. As we know that (the galaxies) Andromeda and Milky Way are on a collision course, this discovery enables us to see our future from billions of years from now, today.”
Bansal came from India to UNM three years ago to work with Dr. Taylor. When she’s not busy making revolutionary astronomical discoveries, she goes hiking, biking, and participates in mixed martial arts to relax and, as she says, “kick some butt.” She is passionate about astronomy, has several projects in process, and hopes to pursue research in the field past graduation. Where would she go to research if she could choose?
“The South Pole,” she says with a smile. “They have pretty cool telescopes there.”