In terrestrial terms precipitation usually refers to rain or snowfall but in the cosmos it’s the name given to a mechanism whereby clouds of hot gas produce showers of cool gas that descend into a galaxy.
Using Chandra, which, at an altitude of 139,000 kilometers (86,500 miles) can observe galaxies and stars free from any distortion caused by Earth’s atmosphere, researchers examined X-rays from over 200 galaxy clusters. They postulate that gaseous precipitation is crucial to understanding the influence of giant black holes on 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), lead author of the paper published in the latest issue of Nature. “Now we have evidence that it can also slow down star formation in galaxies with huge black holes.”
Their findings were published just a few days after another international team of astronomers revealed their discovery of the most massive black hole yet found, as earlier reported in Digital Journal.
Understanding how supermassive black holes, bodies sometimes millions or even billions of times the size of our Sun affect their host galaxies has long been a goal for astronomers. According to Megan Donahue, also based at MSU, “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. These results get us a step closer.”
Researchers focused their survey of galaxies on some of the largest known galaxies situated in the middle of galaxy clusters.
Galaxies aren’t evenly distributed across the Universe but are found in “clumps” known as galaxy clusters. In some areas of space there are major concentrations of galaxies bound together by their own gravity. Elsewhere, almost empty areas of space, known as voids, exist.
The research team’s target galaxies are embedded in massive “atmospheres” of hot gas. In the general scheme of things, it might be assumed that as the Universe aged, this hot gas should cool causing many stars to form. But the astronomers discovered something was holding back the birth of stars.
The reason for this brake on star formation appears to be the supermassive black holes located at galactic centers. Similar to a cycle of precipitation-heating-evaporation-condensation-precipitation seen with climate systems on Earth, the local “climate,” but on a galactic scale, is a factor in the speed of star formation.
The way this works is that, given specific conditions, clumps of gas can radiate away their energy thus forming cool clouds. These cool clouds then mix with surrounding hot gas. Some of these clouds will go on to form stars but others “rain” on to the supermassive black hole. The shower of gas then triggers jets of energized particles which push against the gas raining down on the black hole. The falling gas is reheated preventing more stars from forming.
Today’s weather forecast for the galaxy is….
The process could be compared to a thundery rain-shower falling on sidewalks during a heat-wave. The drops of rain may not form rivulets but instead the hot sidewalk reenergizes the water molecules converting them back into water vapor that caused the rain in the first place.
In similar fashion, the supermassive black hole creates a cycle of cooling and heating, effectively 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.
From the data provided by NASA’s Chandra X-ray Telescope, Voit and his colleagues estimated how long it should take for the gas to cool at different distances from the black holes they looked at. As a result they could accurately predict the “weather” around each of the black holes.
Showery weather for 7 billion years
Their findings demonstrated that the precipitation feedback loop powered by energy produced by the black hole jets has a tempering effect that ensures the showers of cold clouds don’t become a monsoon. Based on the data returned by Chandra, this brake on cosmic precipitation has been in place 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,” explained co-author Michael McDonald of Massachusetts Institute of Technology.
But while a rain of cool clouds seems to be a key factor in regulating the growth of some galaxies, there are exceptions to the rule. The researchers found other galaxies that appear drought stricken. In such cases something seems to have shut off the cosmic precipitation. They advance the suggestion that the intense heat in these central galaxies, possibly resulting from a collision with another galaxy cluster, caused the precipitation around the black hole to dry up.
Further research is intended on whether the cosmic precipitation-black hole feedback process also plays a role in star formation in smaller galaxies, including our own Milky Way.
Full details of the study were published in the journal Nature Mar. 4.
