Do black holes limit galaxy growth? Jet of ionized gas offers a clue. (+video)
A streamer of ionized gas spotted hurtling from the heart of a galaxy nearly 245 million light-years away is suggesting to astronomers that supermassive black holes limit how big galaxies can become.
Astronomers say they have observed a powerful streamer of ionized gas hurtling from the region around a supermassive black hole at the heart of a galaxy nearly 245 million light-years away.
The observations reveal the first unambiguous evidence for a key process theorists say is required to accelerate these streamers. The results could help resolve a debate among astrophysicists over the main actor responsible for regulating a galaxy's growth.
As they formed and grew, galaxies pulled in hydrogen gas – the raw material for stars – from intergalactic space. But at some point, something restricts that inflow of gas, otherwise galaxies would be much larger than they are, explains Gerard Kriss, an associate astronomer at the Space Telescope Science Institute in Baltimore and a member of the team that observed the streamer.
One camp holds that high-speed winds of ionized gas, blowing outward from the region around a supermassive black hole's disk of infalling dust and gas are responsible. These winds are most intense and persistent when the black hole is constantly gorging and growing.
Eventually, the black hole gets so large that the winds it generates ultimately curb the inflow of gas to the galaxy. This shutdown also prevents the supermassive black hole from growing any larger.
The other camp argues that ultimately, the peak level of star formation in a galaxy's history shuts off the inflow. Additional gas from outside the galaxy is kept at bay by the intense stellar winds from large numbers of very massive stars, as well as by stellar explosions known as supernovae. Here, too, the black hole stops growing.
The new observations from the center of a galaxy known as NGC 5548, reported in the current issue of the journal Science, provide a look at the "underpinnings of how these outflows are generated and work," Dr. Kriss says, lending support to the blame-the-black-hole camp.
The impact a supermassive black hole has on its host galaxy, and how that rebounds to influence the evolution of the black hole itself, is important to understanding galaxy evolution researchers say. These processes are thought to have been working on overdrive in quasars – early, young galaxies whose central black holes emit such prodigious amounts of radiation that they are very bright even at cosmological distances.
They are too far away to study in any detail, however. So a team, led by Jelle Kaastra, a senior scientist at the Netherlands Institute for Space Research in Utrecht, tapped six space-based observatories to study NGC 5548. The galaxy is undergoing intense star formation with a very active supermassive black hole at its center. Such galaxies, known as Seyfert galaxies, are closer, less energetic than their more-distant cousins. So they are useful stand-ins for studying the processes driving quasars.
The outflow of ionized gas from a supermassive black hole's vicinity is driven by the pressure from radiation that comes from heating as dust and gas spiral in toward the black hole's event horizon, its point of no return.
Initially, the team was observing the galaxy in hopes of trying to see how far the central black hole's existing outflow of gas extended, says Dr. Kaastra.
During the observing campaign, the team noted a sudden, sharp drop in relatively low-energy x-rays from the galaxy's core, as well as a less dramatic drop in ultraviolet radiation. The team attributes the drop to a streamer, or wind, of ionized carbon, silicon, and nitrogen that is between 70 billion and 110 billion miles from the black hole and is hurtling away from it at about 7 million miles an hour. Because the black hole is spinning, the outflow is spiraling outward.
The team doesn't actually see the high-velocity wind flowing from the black hole's accretion disc, Kaastra acknowledges, but adds "there's no other place it could come from given the laws of physics."
The streamer's tendency to pass 70 percent of the ultraviolet light reaching it from near the black hole and allow only about 10 percent of the x-rays through helps explain the origin of the high-velocity winds, Kaastra says. The streamer in effect acts as a x-ray shield, blocking radiation that in essence would merely heat and ionize its surroundings. This keeps things cooler outside the shield, leaving matter there in a better state to receive a push imparted by the ultraviolet radiation that gets through the shield.
The results provide an important insight into how matter in a galaxy responds to changes in the mix of radiation it is receiving, suggests Daniel Proga, a physicist at the University of Nevada at Las Vegas, who models black hole accretion processes but is not a member of the international research team reporting the new results.
"We cannot assume that everything that is produced by a black hole will go through an entire galaxy," he says. "Some of the things will be stopped and reprocessed rapidly at small distances" from the central black hole.