Monster black holes grow after galactic mergers
and World Science staff
The monster black holes that sit at the centers of galaxies weren’t born big, but grew over time through repeated mergers, astronomers say.
The findings come from an analysis of the Hubble Space Telescope’s deepest view of the universe, a project called the Hubble Ultra Deep Field.
Supermassive black holes are objects believed to weigh as much as millions or billions of Suns.
They are thought to sit at the centers of most galaxies, and are the giant versions of black holes—objects so dense that their gravitational pull sucks in anything nearby, including light.
Astronomers have found that some supermassive black holes existed as early as one billion years into the universe’s history of 14 billion years, and astronomers have trouble explaining how they got so huge so soon. One theory is that they did so
because the galaxies they inhabited merged, and this provided new food
for the black holes.
The new findings back this idea, the researchers in the new study said.
By looking at distant galaxies, the Ultra Deep Field sees them as they
were early in cosmic history, because light from those galaxies takes
billions of years to reach Earth.
The study provides “the first statistical evidence that supermassive black-hole growth is linked” to galaxy assembly, said Arizona State University’s Rogier Windhorst, a member of the two teams that conducted the analysis. “Black holes grow by drawing in stars, gas and dust. These morsels come more plentifully within their reach when galaxies merge.”
The studies also confirm the predictions of recent computer simulations that newly merging galaxies are enshrouded in so much dust that astronomers cannot see the black-hole feeding frenzy, Windhorst and colleagues said.
The simulations suggest that it takes hundreds of millions of years before dust clears so that astronomers can see the black holes feasting on stars and gas from the merger.
The two teams presenting the new findings believe they are seeing two distinct phases in galaxy evolution.
The first, the “tadpole stage,” represents the early-merging systems where central black holes are still enshrouded in dust. The second is a much later “variable-object phase,” in which the merged system has cleared out enough gas for the area around the black hole to become visible. The telltale sign of this is flickering light from galaxies, they added.
“The fact that these phases were almost entirely separate was a surprise, because it is commonly believed that galaxy mergers and central black-hole activity are closely related,” Windhorst explained.
“Our nearby universe, including the Milky Way galaxy in which we live, has mature galaxies, but in order to understand how they formed and evolved, we must study them over time.” The Ultra Deep Field project “provides an actual look back in time to see snapshots of early galaxies so that we can study them when they were young.”
The researchers presented their findings on Jan. 10 at the annual meeting of the American Astronomical Society in Washington, D.C.
A link between the growth of galaxies through mergers and the feeding of the central black holes has long been suspected. But the evidence has been inconclusive for years.
The Ultra Deep Field provided “the first data we could use to test this theory, since it allowed us to study about 5,000 distant galaxies” over four months, said Arizona State University’s Seth Cohen, who led one of the teams.
Another team, led by Amber Straughn of Arizona State University, searched for “tadpole galaxies,” so called because they have bright knots and tails thought to be caused by mergers. These tails arise when the galaxies lose their gravitational grip on their stars, spewing some into space. The team found about 165 tadpole galaxies.
“To our surprise, however, these tadpole objects did not show any fluctuation in brightness,” Straughn said. “The flickering light—when it is present—comes from the material swirling around an accretion disk surrounding a black hole,” the zone where material spirals inward.
“The material is heated and begins to glow. As it spirals down toward the black hole, it can rapidly change in brightness. This study of tadpole galaxies suggests that black holes in newly merging galaxies are enshrouded in dust, and therefore, we cannot see them accreting material.”
Cohen’s team studied the brightness of about 4,600 additional objects and found that about 45 non-tadpole objects fluctuated significantly in brightness over time. This result indicates that the galaxies probably contain supermassive black holes eating stars or gas, he said.
“A black hole’s typical mealtime lasts at least a few dozen million years,” Windhorst said. “This is equivalent to black holes spending no more than 15 minutes per day eating all their food—a veritable fast food diet.”
The analysis also reinforces previous studies of monster black holes in the centers of nearby, massive galaxies, the researchers said. Those studies showed a close relationship between the mass of a galaxy’s “central bulge” of stars and that of the central black hole.
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