Blowing in the wind: fast molecular clouds discovered streaming out from the Galactic Center

Artist’s conception of molecular clouds entrained in the hot wind from the center of the Milky Way. NSF/GBO/P.Vosteen

Interesting things are happening in the center of the Milky Way, and scientists using radio telescopes on three continents have been making discoveries about a high energy wind that is blasting out of our galaxy.

At the heart of the Milky Way’s Galactic Center lies a supermassive black hole, and surrounding it are regions of intense star formation. The black hole, the star-forming regions, or a combination of both are driving a million-degree wind of hot gas that flows outward above and below the plane of the Milky Way in two enormous plumes. The plumes can be detected across the electromagnetic spectrum, from radio waves to x-rays and gamma-rays. 

Scientists using radio telescopes in Australia, Chile, and the United States, including the National Science Foundation’s Green Bank Telescope (GBT), have been discovering that the very hot wind contains cold clouds of gas being dragged outward at speeds of hundreds of kilometers a second. Amazingly, some of those clouds are cold and dense enough to contain molecules of carbon monoxide — CO.

As reported this week in the international science journal Nature, a group of scientists led by Dr. Enrico Di Teodoro of Johns Hopkins University and the Australian National University, used the APEX radio telescope in the mountains of northern Chile to detect signals from interstellar CO in two gas clouds that are entrained in the nuclear wind. These clouds had been known to contain atomic hydrogen from studies made with the Australia Telescope Compact Array (ATCA) and the GBT. But interstellar gas is found in many forms, and if gas clouds are cold and dense enough they might harbor complex molecules — “and that is exactly what was found,” states Dr. Felix Jay Lockman, scientist at the Green Bank Observatory, and a co-author on this paper.

“These are groundbreaking results,” said Dr. Andrew Fox, an astronomer at the Space Telescope Science Institute in Baltimore, who was not involved in the research. “Detecting molecular gas in the nuclear wind is very exciting. It also raises new questions, because the mere existence of this cold gas is a mystery. It’s as if you found a chunk of ice inside a volcano – you would wonder why it hadn’t melted.”

This discovery comes on the heels of a study reported earlier this year using new observations of interstellar hydrogen made with the GBT, the world’s largest fully steerable radio telescope. By measuring radio emission from hydrogen in the region of the nuclear wind, more than 200 high-velocity hydrogen clouds have been discovered, some reaching outflow velocities as high as 400 kilometers a second. The large number of clouds allowed scientists to examine the outflow velocity more thoroughly, and discover that the clouds are being accelerated as they leave the Milky Way, exactly as would be expected if they were being blow out in a hot, fast wind. “We see the cold clouds and we see the hot wind,” explains Dr. Lucia Armillotta of Princeton University, a member of the research team, “but how the clouds survive in such conditions remains a mystery.”

The fast hydrogen clouds were first discovered in 2013 by a group led by Dr. Naomi McClure-Griffiths of the Australian National University, who is also a co-author on the current paper. While conducting a survey of radio emission from hydrogen around the Galactic Center using the Australia Telescope Compact Array, she noticed that there were a number of hydrogen clouds in the data with very odd velocities, detached from the general gas flow. She proposed that the clouds were being dragged along in the hot wind, a view now widely thought to be correct.

Most large galaxies like the Milky Way have hot fast winds from their centers, but only in the Milky Way can we study one up close and in detail. The discovery of molecules in the Milky Way’s nuclear wind gives new information on the mass flow and energetics of the wind, and on its origin and fate. The team of scientists is now conducting more extensive studies of these fast clouds using APEX, ATCA and the GBT. 

The Green Bank Observatory is a facility of the National Science Foundation and is operated by Associated Universities, Inc. The APEX telescope is a collaboration between the Max-Planck-Institute fur Radioastronomie, The European Southern Observatory and the Onsala Space Observatory. The Australia Telescope Compact Array is part of the Australia Telescope National Facility.

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Jill Malusky, Green Bank Observatory Public Relations, ude.o1716984083arn@y1716984083ksula1716984083mj1716984083

Dr. Felix Jay Lockman, Green Bank Observatory Scientist ude.o1716984083arn@n1716984083amkco1716984083lj1716984083

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