Wednesday, January 10, 2018

Winds of Change: Supermassive Black Holes Can Overpower Even the Smallest Galaxies

One of the dwarf galaxies that the team found to contain a red geyser. Its red color shows it is no longer forming new stars.  For size comparison, the dwarf galaxy is shown next to a galaxy similar to the Milky Way. The dwarf galaxy contains about 3 billion stars, while the spiral galaxy contains about 300 billion.  The inset (top right) shows a larger image of the dwarf galaxy overlain with some of the MaNGA data for this galaxy, which revealed the winds from the supermassive black hole. Darker purple regions show gas heated by winds from the galaxy’s central black hole. These winds are what stops the galaxy from forming new stars.  Image Credit: Samantha Penny (Institute of Cosmology and Gravitation, University of Portsmouth) and the SDSS collaboration

Why do galaxies stop making new stars? Today, astronomers from the Sloan Digital Sky Survey report a surprising new answer to that important question: feedback from supermassive black holes blocks star formation, even in some of the smallest galaxies.

The results, being presented at the American Astronomical Society (AAS) meeting in National Harbor, Maryland on Thursday and soon to be published in the Monthly Notices of the Royal Astronomical Society, represent a major step forward in our understanding of how dwarf galaxies — some of the smallest in our Universe — are prevented from forming stars.

“Dwarf galaxies outnumber galaxies like the Milky Way fifty to one,” says Samantha Penny of the University of Portsmouth’s Institute of Cosmology and Gravitation and lead author of the study. “So if we want to tell the full story of galaxies, we need to understand how dwarf galaxies work.”

In any galaxy, stars are born when clouds of gas collapse under the force of their own gravity. But stars don’t keep on being born forever — at some point, star formation in a galaxy shuts off. The reason for this can be different in different galaxies. Sometimes, a galaxy simply runs out of gas, exhausting its star-making fuel. Sometimes, its gas heats up so much that the excited gas defies collapse into new stars. Sometimes, its gas is pulled out of the galaxy by a gravitational interaction with a nearby galaxy.

And sometimes, the galaxy’s own central black hole is the culprit. Most galaxies have a supermassive black hole at their centers, and understanding the connections between it and the rest of the galaxy has been an important area of research for astronomers for years. Eighteen months ago, SDSS astronomers discovered a new way in which galactic black holes can shut off star formation, which they named a “red geyser.”

That discovery, as well as the results being reported today, were made possible by the SDSS’s Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. Whereas most prior surveys had looked at each galaxy as a single entity, MaNGA uses more than 1,000 optical fibers to make detailed maps of seventeen galaxies at a time, seeing each galaxy in detail all the way from its center to its outskirts. This observing strategy enables discoveries which link the central black hole to the rest of the galaxy — like red geysers.

A red geyser forms as a result of gas falling into a galaxy’s central black hole. As the gas falls in, it heats up to millions of degrees and glows brightly. But this gas infall also drives powerful winds, blowing out across the rest of the galaxy at thousands of miles per second. Kevin Bundy, the Principal Investigator of MaNGA from the University of California Santa Cruz, explains the origin of the term — “we called these features ‘red geysers’ because the sporadic wind outbursts reminded us of a geyser, and because the end of star formation has left the galaxy with only red stars.”

“When we first found red geysers, we thought they would only be found in larger galaxies,” says Penny. “We had seen active black holes in dwarf galaxies before, but we’ve never been able to see them in action. With MaNGA, we can now see their effects across a whole galaxy. And we can do it for many, many galaxies at a time.”

Over its nearly three years in operation, MaNGA has seen galaxies of all kinds, from dwarf to giant, including more than 300 dwarf galaxies. To their great surprise, Penny and her team found red geysers in about ten percent of the dwarf galaxies they saw in the MaNGA survey.

As Karen Masters, a member of the team from the University of Portsmouth and Haverford College explains, “This discovery shows that even isolated dwarf galaxies can stop forming stars if they host an active supermassive black hole. That’s not what’s written in our textbooks on galaxy evolution. It was a real surprise to see it even once, much less in one out of every ten galaxies we looked at.”

This discovery would not have been possible without the data from the MaNGA survey — both in its incredible detail and in its ability to see so many galaxies in such a short time. MaNGA has already observed more dwarf galaxies than any previous survey with this level of detail, and it will continue over the next two years. The survey has the potential to reveal many more surprises about our Universe.


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