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.
About Sloan Digital Sky Survey
Funding for the Sloan Digital Sky Survey IV has been provided by the
Alfred P. Sloan Foundation, the U.S. Department of Energy Office of
Science, and the Participating Institutions. SDSS acknowledges support
and resources from the Center for High-Performance Computing at the
University of Utah. The SDSS web site is www.sdss.org.
SDSS is managed by the Astrophysical Research Consortium for the
Participating Institutions of the SDSS Collaboration including the
Brazilian Participation Group, the Carnegie Institution for Science,
Carnegie Mellon University, the Chilean Participation Group, the French
Participation Group, Harvard-Smithsonian Center for Astrophysics,
Instituto de Astrofísica de Canarias, The Johns Hopkins University,
Kavli Institute for the Physics and Mathematics of the Universe (IPMU) /
University of Tokyo, Lawrence Berkeley National Laboratory, Leibniz
Institut für Astrophysik Potsdam (AIP), Max-Planck-Institut für
Astronomie (MPIA Heidelberg), Max-Planck-Institut für Astrophysik (MPA
Garching), Max-Planck-Institut für Extraterrestrische Physik (MPE),
National Astronomical Observatories of China, New Mexico State
University, New York University, University of Notre Dame, Observatório
Nacional / MCTI, The Ohio State University, Pennsylvania State
University, Shanghai Astronomical Observatory, United Kingdom
Participation Group, Universidad Nacional Autónoma de México, University
of Arizona, University of Colorado Boulder, University of Oxford,
University of Portsmouth, University of Utah, University of Virginia,
University of Washington, University of Wisconsin, Vanderbilt
University, and Yale University.
Contacts
Samantha Penny,
+44 (0)23 9284 5158
Karen Masters,
+44 (0)7590 5266005,
@KarenLMasters
Kevin Bundy,
University of California Santa Cruz,
1-831-459-3539
Jordan Raddick,
1-410-516-8889,
@raddick
