Science: Black Holes Emit Surprisingly Strong Jets
Black holes release more energy into their host galaxies than previously thought, a new study suggests. This finding, reported in the 28 February issue of the journal Science, may help astronomers better understand black holes' effects on their host galaxies.
A black hole grows as gas from space flows onto it. It then releases two kinds of energy from that gas: radiation energy in the form of photons and kinetic energy in the form of wind jets. The more matter falls in, the more energy is released. "The effect of a black hole on its environment depends on how much energy it injects into it," said lead author Roberto Soria, senior research fellow at the International Centre for Radio Astronomy Research at Australia's Curtin University.
Artist's impression of a microquasar. Gas falls from a donor star towards the black hole, spreading to form a hot accretion disk. As gas spirals down towards the hole, gravitational energy is radiated partly as photons and partly used to launch a pair of jets. | Courtesy of T.D. Russell, ICRAR-Curtin; Using the BINSIM visualization code by R. Hynes, LSU
"Astronomers often assumed the growth rates of powerful black holes could be inferred from the photon power alone, because the jet power was negligible," explained Soria. "Now, we show the jet power is not negligible at all."
Indeed, the authors show that in some cases the jet power actually exceeds the so-called Eddington limit. According to this limit, the radiation energy flowing out of a black hole cannot surpass a certain amount (one based on the black hole's mass), or it will blow the gathering gas away.
Whether a black hole's kinetic energy is constrained in the same way has been unclear. But, an object discovered by Soria and colleagues in a galaxy called M83 may provide some answers. The team is evaluating the energy associated with an object known as a microquasar, a tremendously energetic stellar environment with a black hole at its core.
"Understanding how quasars radiate such immense power has been an important problem," said co-author Frank Winkler, research professor of astrophysics at Middlebury College in Vermont. "Bona fide quasars are so distant that it's hard to observe many of their properties. Microquasars are a lot closer, providing an opportunity to observe what we believe to be the same physical mechanism, in miniature."
Commonly known as the "southern pinwheel," M83 is located 15 million light years away in the southern constellation Hydra. Not only is it one of the closest large spiral galaxies, it also faces almost directly toward Earth and so is particularly amenable to detailed studies by astronomers.
Composite image of the spiral galaxy M83 assembled from observations made by NASA's Hubble Space Telescope and the Carnegie Institution of Washington's Magellan telescopes. | NASA; ESA; and the Hubble Heritage Team, STScI/AURA
The researchers observed the black hole and its environs with different telescopes, including NASA's Chandra X-ray Observatory and Hubble Space Telescope, and the Australia Telescope Compact Array radio complex. They did this at different times spanning several years. By analyzing X-rays produced by gas accreting onto the black hole, the scientists figured out that the black hole's mass was less than 100 times that of the Sun.
They then compared the mass of the black hole with its outgoing kinetic power, which they were able to infer in part by looking at a combination of the X-ray, radio, optical and near-infrared light energy being radiated into the surrounding region of space. This revealed that the kinetic energy from the black hole was higher than the Eddington limit for a black hole of this mass.
In other words, the kinetic energy was higher than the radiation energy, suggesting that some of the fastest growing black holes may emit more energy by way of jets than they do via light energy.
Critically, the team's observations show that the black hole has been producing this very high kinetic power for a long time, not just for short, explosive events. "If a black hole reaches super-Eddington power only for a short burst of a few seconds, it's interesting but doesn't really affect the galaxy," Soria explained. "But black holes such as this one seem to have been in a powerful state for tens of thousands of years, during which they ejected a lot of energy."
The work of this research team will help astronomers better model the evolution of black holes over time. "Super powerful black holes like this one are rare today, almost extinct," Soria explained, "but they were common in the early universe when big galaxies had a lot more gas."
"Energy production at this level for tens of thousands of years…may trigger ongoing star formation or have other large-scale effects," explained co-author William Blair, an astrophysicist and research professor in the department of physics and astronomy at Johns Hopkins University.