More than thirteen years of observations from NASA's Hubble Space Telescope have allowed astronomers to assemble time-lapse movies of a 5,000-light-year-long jet of superheated gas being ejected from a supermassive black hole in the center of the giant elliptical galaxy M87.
The movies promise to give astronomers a better understanding of how active black holes shape galaxy evolution. While matter drawn completely into a black hole cannot escape its enormous gravitational pull, most infalling material drawn toward it first joins an orbiting region known as an accretion disk encircling the black hole. Magnetic fields surrounding the black hole are thought to entrain some of this ionized gas, ejecting it as very high-velocity jets.
"Central supermassive black holes are a key component in all big galaxies," said Eileen T. Meyer of the Space Telescope Science Institute (STScI) in Baltimore, Md., the Hubble study's lead author. "Most of these black holes are believed to have gone through an active phase, and black-hole-powered jets from this active phase play a key role in the evolution of galaxies. By studying the details of this process in the nearest galaxy with an optical jet, we can hope to learn more about galaxy formation and black hole physics in general."
The Hubble movies reveal for the first time that the jet's river of plasma travels in a spiral motion. This motion is considered strong evidence that the plasma may be traveling along a magnetic field, which the team thinks is coiled like a helix. The magnetic field is believed to arise from a spinning accretion disk of material around a black hole. Although the magnetic field cannot be seen, its presence is inferred by the confinement of the jet along a narrow cone emanating from the black hole.
"We analyzed several years' worth of Hubble data of a relatively nearby jet, which allowed us to see lots of details," Meyer said. "The only reason you see the distant jet in motion at all over just a few years is because it is traveling very fast."
Meyer found evidence for the magnetic field's suspected helical structure in several locations along the jet. In the outer part of the M87 jet, for example, one bright gas clump, called knot B, appears to zigzag, as if it were moving along a spiral path. Several other gas clumps along the jet also appear to loop around an invisible structure. "Past observations of black hole jets couldn't distinguish between radial motion and side-to-side motion, so they didn't provide us with detailed information of the jet's behavior," Meyer explained.
M87 resides at the center of the neighboring Virgo cluster of roughly 2,000 galaxies, located 50 million light-years away. The galaxy's monster black hole is several billion times more massive than our Sun.
In addition, the Hubble data provided information on why the jet is composed of a long string of gas blobs, which appear to brighten and dim over time.
"The jet structure is very clumpy. Is this a ballistic effect, like cannonballs fired sequentially from a cannon?" Meyer asked. "Or, is there some particularly interesting physics going on, such as a shock that is magnetically driven?"
Meyer's team found evidence for both scenarios. "We found things that move quickly," Meyer said. "We found things that move slowly. And, we found things that are stationary. This study shows us that the clumps are very dynamic sources."
The research team spent eight months analyzing 400 observations from Hubble's Wide Field Planetary Camera 2 and Advanced Camera for Surveys. The observations were taken from 1995 to 2008. Several team members, however, have been observing M87 for 20 years. Only Hubble's sharp vision allowed the research team to measure the jet's slight motion in the sky over 13 years. Meyer's team also measured features in the hot plasma as small as 20 light-years wide.
It's too soon to tell whether all black-hole-powered jets behave like the one in M87. That's why Meyer plans to use Hubble to study three more jets. "It's always dangerous to have exactly one example because it could be a strange outlier," Meyer said. "The M87 black hole is justification for looking at more jets."
The team's results will appear Aug. 22 in the online issue of The Astrophysical Journal Letters.
In addition to Eileen Meyer, other members of the science team are William Sparks, John Biretta, Jay Anderson, Sangmo Tony Sohn, and Roeland van der Marel of STScI; Colin Norman of Johns Hopkins University, Baltimore, Md.; and Masanori Nakamura of Academia Sinica, Taipei, Taiwan.
Space Telescope Science Institute, Baltimore, Md.