February 26, 2014
Red Dead Redemption: Black Holes Bully Galaxies Into Submission
Brett Smith for redOrbit.com - Your Universe Online
Typically filled with only the oldest stars, which are relatively low in mass and appear red, giant elliptical galaxies have long baffled astronomers. These galaxies are mysteriously shut down with respect to star-forming activity – a status scientists refer to as “red and dead.”
These galaxies are thought to be low on cold gas, an important raw material for star forming, but a new study in the Monthly Notices of the Royal Astronomical Society journal reports that some of these giant elliptical galaxies have plenty of cold gas.
The study authors went on to say that stars do not form readily in giant elliptical galaxies because a supermassive black hole at their center heats or stirs up the gas and prevents it from turning into stars.
Previous theories had posited that these galaxies had somehow gotten rid of their cold gas or had used it all up. The new study was able to show that this was not the case by using observations from the European Space Agency's Herschel Space Observatory.
“We looked at eight giant elliptical galaxies that nobody had looked at with Herschel before and we were delighted to find that, contrary to previous belief, six out of eight abound with cold gas,” said study author Norbert Werner, of Stanford University.
The observatory was able to sense the existence of cold gas in these far-off galaxies by picking up far-infrared emissions derived from carbon and oxygen in the gas.
“While we see cold gas, there is no sign of ongoing star formation,” said co-author Raymond Oonk from ASTRON, the Netherlands Institute for Radio Astronomy. “This is bizarre: with plenty of cold gas at their disposal, why aren't these galaxies forming stars?”
After identifying several target galaxies, the researchers were able to scan them across the electromagnetic spectrum for evidence of gas at different temperatures that gives off light at different wavelengths; optical images were used to look for warm gas and X-ray data was used to search for hot gas.
“In the six galaxies that are rich in cold gas, the X-ray data show tell-tale signs that the hot gas is cooling,” Werner said.
The study noted that the cooling process had mysteriously stopped in these giant elliptical galaxies, preventing cold gas from condensing into stars. In two of the targeted galaxies, the hot gas doesn’t even appear to be cooling.
“The contrasting behavior of these galaxies may have a common explanation: the central supermassive black hole,” Oonk said.
In the two galaxies without cold gas, the black holes at their center are rapidly drawing in matter and ejecting jets of energetic particles. These jets could be reheating the galaxy's cold gas or driving the gas completely out of the galaxy. This would explain why star formation has stalled in these galaxies, the researchers said.
“These galaxies are red, but with the giant black holes pumping in their hearts, they are definitely not dead,” Werner said.
“Once again, Herschel has detected something that was never seen before: significant amounts of cold gas in nearby red-and-dead galaxies,” added Göran Pilbratt, Herschel Project Scientist at ESA. “Nevertheless, these galaxies do not form stars, and the culprit seems to be the black hole.”