Mystery Jet from M87's Supermassive Black Hole Ejecting at 4 to 6 Times Speed of Light

The flare-up in a jet of matter blasting from a monster black hole this past April is giving astronomers an incredible light show. Hubble Space Telescope image shows a 5000 light-year long jet being ejected from the active nucleus of galaxy M87 (a massive radio galaxy). 

M87 has a huge population (perhaps the greatest known around any one galaxy) of globular clusters as compared to the Milky Way’s 150-200. A 2006 survey estimates that there are approximately 12,000 globulars around M87. Globular clusters, which are found in the halo of a galaxy, contain considerably more stars and are much older than the less dense galactic, or open clusters, which are found in the disk. 

In 1918, Lick Observatory astronomer Heber Curtis discovered a jet of matter coming from M87 which he described as “a curious straight ray.” This jet extends at least 5000 light-years from the nucleus of M87 and is made up of matter ejected from the galaxy, most likely by a supermassive black hole (a hypothesis made more likely by the discovery of a disk of rapidly rotating gas around the nucleus of M87). Astronomers believe that the black hole in this galaxy has a mass of approximately 6.4 billion (6.4×109) solar masses. M87 has also been found to be a strong source of X-rays. Its proximity means that it is one of the best studied radio galaxies.[citation needed]

In pictures taken by the Hubble Space Telescope in 1999, the motion of M87’s jet was measured at four to six times the speed of light.

The outburst is coming from a blob of matter, called HST-1, embedded in the jet, a powerful narrow beam of hot gas produced by a supermassive black hole residing in the core of the giant elliptical galaxy M87. HST-1 is so bright that it is outshining even M87’s brilliant core, whose monster black hole is one of the most massive yet discovered.

The glowing gas clump has taken astronomers on a rollercoaster ride of suspense. Astronomers watched HST-1 brighten steadily for several years, then fade, and then brighten again. They say it’s hard to predict what will happen next.

NASA’s Hubble Space Telescope has been following the surprising activity for seven years, providing the most detailed ultraviolet-light view of the event. Other telescopes have been monitoring HST-1 in other wavelengths, including radio and X-rays. The Chandra X-ray Observatory was the first to report the brightening in 2000. HST-1 was first discovered and named by Hubble astronomers in 1999. The gas knot is 214 light-years from the galaxy’s core.

The flare-up may provide insights into the variability of black hole jets in distant galaxies, which are difficult to study because they are too far away. M87 is located 54 million light-years away in the Virgo Cluster, a region of the nearby universe with the highest density of galaxies.

“I did not expect the jet in M87 or any other jet powered by accretion onto a black hole to increase in brightness in the way that this jet does,” says astronomer Juan Madrid of McMaster University in Hamilton, Ontario, who conducted the Hubble study. “It grew 90 times brighter than normal. But the question is, does this happen to every single jet or active nucleus, or are we seeing some odd behavior from M87?”

Hubble gives astronomers a unique near-ultraviolet view of the flare that cannot be accomplished with ground-based telescopes. “Hubble’s sharp vision allows it to resolve HST-1 and separate it from the black hole,” Madrid explains.

Despite the many observations by Hubble and other telescopes, astronomers are not sure what is causing the brightening. One of the simplest explanations is that the jet is hitting a dust lane or gas cloud and then glows due to the collision. Another possibility is that the jet’s magnetic field lines are squeezed together, unleashing a large amount of energy. This phenomenon is similar to how solar flares develop on the Sun and is even a mechanism for creating Earth’s auroras.

The disk around a rapidly spinning black hole has magnetic field lines that entrap ionized gas falling toward the black hole. These particles, along with radiation, flow rapidly away from the black hole along the magnetic field lines. The rotational energy of the spinning accretion disk adds momentum to the outflowing jet.

Madrid assembled seven years’ worth of Hubble archival images of the jet to capture changes in the HST-1’s behavior over time. Hubble’s view of the event. Some of the images came from observing programs that studied the galaxy, but not the jet.

He found data from the Space Telescope Imaging Spectrograph (STIS) that showed a noticeable brightening between 1999 and 2001. In images from 2002 to 2005, HST-1 continued to rise steadily in brightness. In 2003 the jet knot was more brilliant than M87’s luminous core. In May 2005 HST-1 became 90 times brighter than it was in 1999. After May 2005 the flare began to fade, but it intensified again in November 2006. This second outburst was fainter than the first one.

“By watching the outburst over several years, I was able to follow the brightness and see the evolution of the flare over time,” Madrid says. “We are lucky to have telescopes like Hubble and Chandra, because without them we would see the increase in brightness in the core of M87, but we would not know where it was coming from.”

Madrid hopes that future observations of HST-1 will reveal the cause of the mysterious activity. “We hope the observations will yield some theories that will give us some good explanations as to the mechanism that is causing the flaring,” Madrid says. “Astronomers would like to know if this is an intrinsic instability of the jet when it plows its way out of the galaxy, or if it is something else.”

Jason McManus

http://hubblesite.org/newscenter/archive/releases/2009/16/full/

Image Credit: David Malin @ Anglo-Australiab Observatory