The supermassive black holes in the cores of most massive galaxies wreak havoc on their immediate surroundings. During their most active phases — when they ignite as luminous quasars — they launch extremely powerful and high-velocity outflows of gas.
These outflows can sweep up and heat material, playing a pivotal role in the formation and evolution of massive galaxies. Not only have astronomers observed them across the visible Universe, they also play a key ingredient in theoretical models.
But the physical nature of the outflows themselves has been a longstanding mystery. What physical mechanism causes gas to reach such high speeds, and in some cases be expelled from the galaxy?
A new study provides the first direct evidence that these outflows are accelerated by energetic jets produced by the supermassive black hole.
Using the Very Large Telescope in Chile, a team of astronomers led by Clive Tadhunter from Sheffield University, observed the nearby active galaxy IC 5063. At locations in the galaxy where its jets are impacting regions of dense gas, the gas is moving at extraordinary speeds of over 600,000 miles per hour.
“Much of the gas in the outflows is in the form of molecular hydrogen, which is fragile in the sense that it is destroyed at relatively low energies,” said Tadhunter in a press release. “I find it extraordinary that the molecular gas can survive being accelerated by jets of highly energetic particles moving at close to the speed of light.
As the jets travel through the galactic matter, they disrupt the surrounding gas and generate shock waves. These shock waves not only accelerate the gas, but also heat it. The team estimates the shock waves heat the gas to temperatures high enough to ionize the gas and dissociate the molecules. Molecular hydrogen is only formed in the significantly cooler post-shock gas.
“We suspected that the molecules must have been able to reform after the gas had been completely upset by the interaction with a fast plasma jet,” said Raffaella Morganti from the Kapteyn Institute Groningen University. “Our direct observations of the phenomenon have confirmed that this extreme situation can indeed occur. Now we need to work at describing the exact physics of the interaction.”
In interstellar space, molecular hydrogen forms on the surface of dust grains. But in this scenario, the dust is likely to have been destroyed in the intense shock waves. While it is possible for molecular hydrogen to form without the aid of dust grains (as seen in the early Universe) the exact mechanism in this case is still unknown.
The research helps answer a longstanding question — providing the first direct evidence that jets accelerate the molecular outflows seen in active galaxies — and asks new ones.
The results were published in Nature and are available online.
Fine, now what accelerates the jets?
I think there’s some math wrong somewhere. The phrase “the gas is moving at extraordinary speeds of over 600,000 miles per hour” and the later sentence “I find it extraordinary that the molecular gas can survive being accelerated by jets of highly energetic particles moving at close to the speed of light.” are inconsistent.
Speed of light in miles per hour: 670 616 629 MPH.
So, at 600 000 MPH the gas is moving about 10 000 times faster than my Prius, but only 1/10 of 1% of the speed of light. Hardly “close to the speed of light”.
I checked the press release and found the same error … it says the gas moves at 1 million KPH, which is a little over 600 000 MPH, and also incorrectly suggests that is close to the speed of light.
But the paper uses a figure of 600 kilometers per second, or 2 160 000 KPH, or about 1 342 160 MPH. That about twice the velocity reported in the press release, but still well less than 1% of the speed of light.
So I don’t see where the idea of relativistic outflows comes from. I’m likely missing something, but what?