The known universe requires black holes!
If you are smart enough to run a simulation of the birth of galaxies, you are smart enough to count, and when you count, you end up with surplus stars in your model, compared to reality.
Theorists have cooked up a grab-bag of explainers for the discrepancy, but until now there was no evidence to select the real cause. Now a group of astrophysicists has fingered a giant, super-speed wind bursting from a supermassive black hole found at the center of a giant galaxy.
The study was performed on galaxy IRAS F111191 +3257, which is about 2 billion light years away, and therefore 2 billion years old. This ultra-bright, ultra-big galaxy (we won't repeat that name!) formed through the merger of two predecessors houses a “super-massive” black hole with the mass of about 1.7 million suns.
Black holes are unimaginably dense structures that lurk at the center of galaxies, where they are obscured by gas, dust and stars. And because its gravity is too intense for light to escape, a black hole is invisible, by definition.
As a disk of matter accumulates near the hole, a maelstrom of energy is released, causing an outflow of hot gas.
That much was known. But was this outflow strong enough to strip away gas further out in the galaxy that would otherwise congeal into stars? That would bring theoretical star numbers closer to actual ones.
Getting the X-ray picture
First author Francesco Tombesi and colleagues observed the galaxy with satellites that see either X-rays or infrared light. To focus on events near the black hole, the group looked at extremely ionized iron, with all but one or two of the 26 electrons stripped away. That would be a telltale for some insanely strong conditions – exactly as expected near a black hole.
"Material in the inner region of a galaxy is attracted by a black hole and forms a vortex, called the accretion disk," explains Tombesi, an astrophysicist at the University of Maryland and NASA. "It’s squeezed, crunched by the black hole and becomes very hot because of friction and it produces a lot of light."
This radiation is strong enough to push material and create a wind. And sure enough, Tombesi's group identified signs of ionized iron with a telltale Doppler-shift toward blue, meaning the ions were moving toward us – at about one-quarter of the speed of light.
Such a flow, observed at a distance of 1,000 light-years from the black hole, could push hydrogen gas and prevent it from congealing into stars, explaining the 20 percent shortfall in observations, compared to computer models.
Consequential, and common
In effect, the winds from the black hole are “feedback channels,” says Tombesi. “The galaxy provides food – in the form of gas – which flows through the inner region toward the black hole." Some of the gas gets sucked into the black hole and vanishes eternally, but some is returned toward its source in the outer galaxy.
Such a process may occur in such an “active galactic nucleus” for several million years, very much more or less.
"Galaxies all have collisions if you wait long enough," and the feedback process will apply to every galaxy after a collision, Tombesi says. The collision throws a lot of material into the inner region, and "Our Milky Way is on a collision path with Andromeda, and when the two collide something similar might happen."
The project began with observations that refused to line up with computer simulations, says Tombesi. "The simulations were always over-estimating the amount of stars in massive galaxies. There was a need to find why the observations and theory did not match."
Although there was suspicion that giant black holes restrained star formation, "this is the first case where we have the observational proof," Tombesi says. "In the future, when people do computer simulations of galactic evolution, they must take into account the super-massive black hole at the center. People had said, 'Maybe it’s important or maybe not,' but now, it must be taken into account.”
– David J. Tenenbaum
Kevin Barrett, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer
- Wind from the black-hole accretion disk driving a molecular outflow in an active galaxy, F. Tombesi et al, Nature, 26 March 2015. ↩
- Astronomers discover a supermassive black hole dating to cosmic dawn. ↩
- Winds of supermassive black holes can shape galaxy-wide star formation. ↩
- Astronomy: supermassive black hole is a puzzle. ↩