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MIT astronomers have spotted the elusive starlight around some of the earliest quasars in the universe. The distant signals, which date back more than 13 billion years to the infancy of the universe, reveal clues about how the very first black holes and galaxies developed. Quasars are the vibrant centers of active galaxies, harboring an insatiable supermassive black hole at their core.
Most galaxies have a central black hole that occasionally feeds on gas and stellar debris, producing a brief burst of light in the form of a glowing ring as material swirls toward the black hole. Quasars, on the other hand, can consume enormous amounts of matter over a much longer period of time, creating an extremely bright and long-lasting ring – so bright, in fact, that quasars are among the brightest objects in the universe. Because they are so bright, quasars dwarf the rest of the galaxy in which they reside. But the MIT team was able to observe for the first time the much fainter light from stars in the host galaxies of three ancient quasars. Based on this elusive stellar light, the researchers estimated the mass of each host galaxy relative to the mass of the central supermassive black hole. They found that the central black holes of these quasars were much more massive than those of their host galaxy, compared to their modern counterparts.
The findings, published today in The Astrophysical Journal, may shed light on how the earliest supermassive black holes became so massive despite the relatively short cosmic time in which they could grow. In particular, these earliest monster black holes may have formed from more massive “seeds” than modern black holes. “After the universe was formed, there were black holes that consumed material and grew in a very short time,” says study author Minghao Yue, a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research. “One of the big questions is to understand how those monster black holes got so big so quickly.”
“These black holes are billions of times more massive than the Sun, at a time when the universe is still in its infancy,” said study author Anna-Christina Eilers, an assistant professor of physics at MIT. “Our results imply that supermassive black holes in the early Universe may have gained mass before their host galaxy did, and that the initial seeds of black holes may have been more massive than they are today.” Eilers and Yue’s co-authors include MIT Kavli director Robert Simcoe, MIT Hubble Fellow and postdoc Rohan Naidu, and collaborators in Switzerland, Austria, Japan and at North Carolina State University.
Dazzling cores
The extreme brightness of a quasar has been apparent since astronomers first discovered the objects in the 1960s. They then assumed that the quasar’s light came from a single, star-like “point source.” Scientists called the objects “quasars,” short for “quasi-stellar” object. Since those first observations, scientists have realized that quasars are not in fact of stellar origin, but come from the accumulation of intensely powerful and persistent supermassive black holes at the centers of galaxies that also host stars, which are much fainter compared to their dazzling cores.
It is a huge challenge to separate the light from the central black hole of a quasar from the light from the stars of the host galaxy. It’s a bit like distinguishing a field of fireflies around a central, massive searchlight. But in recent years, astronomers have had many more opportunities to do this thanks to the launch of NASA’s James Webb Space Telescope (JWST), which can see further back in time, and with much higher sensitivity and resolution, than any existing observatory then. In their new study, Yue and Eilers used dedicated time at JWST to observe six known, ancient quasars, intermittently from the fall of 2022 until the following spring. In total, the team collected more than 120 hours of observations of the six distant objects.
“The quasar outshines its host galaxy by orders of magnitude. And previous images were not sharp enough to distinguish what the host galaxy and all its stars look like,” Yue says. “Now, for the first time, we are able to reveal the light from these stars by very carefully modeling the much sharper images of these quasars from JWST.”
A light balance
The team inventoried the imaging data that JWST had collected from each of the six distant quasars, which they estimate to be about 13 billion years old. That data includes measurements of the light from each quasar at different wavelengths. The researchers fed that data into a model to determine how much of that light is likely to come from a compact “point source,” such as the accretion disk of a central black hole, versus a more diffuse source, such as light from the surrounding, scattered stars of the host system.
Through this modeling, the team divided the light from each quasar into two components: light from the luminous disk of the central black hole and light from the more diffuse stars of the host galaxy. The amount of light from both sources is a reflection of their total mass. The researchers estimate that for these quasars the ratio of the mass of the central black hole to the mass of the host galaxy was about 1:10. This, they realized, was in stark contrast to the current mass balance of 1:1,000, in which more recently formed black holes are much less massive compared to their host galaxy.
“This tells us something about what grows first: Is it the black hole that grows first, and the galaxy catches up next? Or does the galaxy and its stars grow first, dominating and regulating the black hole’s growth?” Eilers explains. “We see that in the early universe, black holes appear to grow faster than their host galaxy. This is preliminary evidence that the initial seeds of black holes may have been more massive then.” “There must have been some mechanism by which a black hole gained mass before their host galaxy in those first billions of years,” Yue adds. “It’s kind of the first evidence we’re seeing for this, which is exciting.”
Bron: Phys.org
