A study of over 200 galaxy clusters, including Abell 2597 shown here, with NASA’s Chandra X-ray Observatory has revealed how an unusual form of cosmic precipitation stifles star formation.
Image Credit: NASA/CXC/DSS/Magellan
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Using NASA’s
Chandra X-ray Observatory, astronomers have found that the growth of
galaxies containing supermassive black holes can be slowed down by a
phenomenon referred to as cosmic precipitation.
Cosmic precipitation is not a weather event, as we commonly associate
the word -- rain, sleet, or snow. Rather, it is a mechanism that allows
hot gas to produce showers of cool gas clouds that fall into a galaxy.
Researchers have analyzed X-rays from more than 200 galaxy clusters, and
believe that this gaseous precipitation is key to understanding how
giant black holes affect the growth of galaxies.
“We know that precipitation can slow us down on our way to work,”
said Mark Voit of Michigan State University (MSU) in East Lansing, lead
author of the paper that appears in the latest issue of Nature. “Now we
have evidence that it can also slow down star formation in galaxies with
huge black holes.”
Astronomers have long pursued the quest to understand how
supermassive black holes, which can be millions or even billions of
times the mass of the sun, affect their host galaxies.
“We’ve known for quite some time that supermassive black holes
influence the growth of their host galaxies, but we haven’t yet figured
out all of the details,” said co-author Megan Donahue, also of MSU.
“These results get us a step closer.”
The study looked at some of the largest known galaxies lying in the
middle of galaxy clusters. These galaxies are embedded in enormous
atmospheres of hot gas. This hot gas should cool and many stars should
then form. However, observations show that something is hindering the
star birth.
The answer appears to lie with the supermassive black holes at the
centers of the large galaxies. Under specific conditions, clumps of gas
can radiate away their energy and form cool clouds that mix with
surrounding hot gas. Some of these clouds form stars, but others rain
onto the supermassive black hole, triggering jets of energetic particles
that push against the falling gas and reheat it, preventing more stars
from forming. This cycle of cooling and heating creates a feedback loop
that regulates the growth of the galaxies.
“We can say that a typical weather forecast for the center of a
massive galaxy is this: cloudy with a chance of heat from a huge black
hole,” said co-author Greg Bryan of Columbia University in New York.
Voit and his colleagues used Chandra data to estimate how long it
should take for the gas to cool at different distances from the black
holes in the study. Using that information, they were able to accurately
predict the “weather” around each of the black holes.
They found that the precipitation feedback loop driven by energy
produced by the black hole jets prevents the showers of cold clouds from
getting too strong. The Chandra data indicate the regulation of this
precipitation has been going on for the last 7 billion years or more.
“Without these black holes and their jets, the central galaxies of
galaxy clusters would have many more stars than they do today,” said
co-author Michael McDonald of the Massachusetts Institute of Technology
in Cambridge.
While a rain of cool clouds appears to play a key role in regulating
the growth of some galaxies, the researchers have found other galaxies
where the cosmic precipitation had shut off. The intense heat in these
central galaxies, possibly from colliding with another galaxy cluster,
likely “dried up” the precipitation around the black hole.
Future studies will test whether this precipitation-black hole
feedback process also regulates star formation in smaller galaxies,
including our own Milky Way galaxy.
A pre-print of the Nature study is available online. The study builds on work by Voit and Donahue that was published in the
Jan. 20 issue of The Astrophysical Journal Letters and also is available online.
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages
the Chandra program for the agency’s Science Mission Directorate in
Washington. The Smithsonian Astrophysical Observatory in Cambridge,
Massachusetts, controls Chandra's science and flight operations.
An interactive image, podcast, and video about these findings are available at:
http://chandra.si.edu