Astronomers studying two classes of black-hole-powered galaxies monitored by NASA's Fermi Gamma-ray Space Telescope have found evidence that they represent different sides of the same cosmic coin. By unraveling how these objects, called blazars, are distributed throughout the universe, the scientists suggest that apparently distinctive properties defining each class more likely reflect a change in the way the galaxies extract energy from their central black holes.
What astronomers once thought were two blazar families may in fact be one, as shown in this artist's concept. Energy stored in the black hole during its salad days of intense accretion may later be tapped by the blazar to continue its high-energy emissions long after this gas has been depleted.
Active galaxies possess extraordinarily luminous cores powered by black holes containing millions or even billions of times the mass of the sun. As gas falls toward these supermassive black holes, it settles into an accretion disk and heats up. Near the brink of the black hole, through processes not yet well understood, some of the gas blasts out of the disk in jets moving in opposite directions at nearly the speed of light.
In the radio image at the top of the page, two jets shoot out of the center of active galaxy Cygnus A. GLAST may solve the mystery of how these jets are produced and what they are made of.
"When GLAST detects a blazar, it is monitoring violent activity from a black hole taking place in the distant past," says GLAST Interdisciplinary Scientist Charles Dermer of the Naval Research Laboratory in Washington, D.C. "Understanding gamma rays from these sources is a form of black hole archeology that reveals the high-energy history of our Universe."
Blazars are the highest-energy type of active galaxy and emit light across the spectrum, from radio to gamma rays. They make up more than half of the discrete gamma-ray sources cataloged by Fermi's Large Area Telescope, which has detected more than 1,000 to date. Astronomers think blazars appear so intense because they happen to tip our way, bringing one jet nearly into our line of sight. Looking almost directly down the barrel of a particle jet moving near the speed of light, emissions from the jet and the region producing it dominate our view.
To be considered a blazar, an active galaxy must show either rapid changes in visible light on timescales as short as a few days, strong optical polarization, or glow brightly at radio wavelengths with a "flat spectrum" — that is, one exhibiting relatively little change in brightness among neighboring frequencies.
Astronomers have identified two models in the blazar line. One, known as flat-spectrum radio quasars (FSRQs), show strong emission from an active accretion disk, much higher luminosities, smaller black hole masses and lower particle acceleration in the jets. The other, called BL Lacs, are totally dominated by the jet emission, with the jet particles reaching much higher energy and the accretion disk emission either weak or absent.
Speaking at the American Astronomical Society meeting in Boston on Tuesday, Ajello said he and his team wanted to probe how the distribution of these objects changed over the course of cosmic history, but solid distance information for large numbers of gamma-ray-producing BL Lac objects was hard to come by.
"One of our most important tools for determining distance is the movement of spectral lines toward redder wavelengths as we look deeper into the cosmos," explained team member Dario Gasparrini, an astronomer at the Italian Space Agency's Science Data Center in Rome. "The weak disk emission from BL Lacs makes it extremely difficult to measure their redshift and therefore to establish a distance."
So the team undertook an extensive program of optical observations to measure the redshifts of BL Lac objects detected by Fermi.
"This project has taken several years and simply wouldn't have been possible without the extensive use of many ground-based observatories by our colleagues," said team member Roger Romani, an astrophysicist at the Kavli Institute for Particle Astrophysics and Cosmology, a facility run jointly by Stanford University and the SLAC National Accelerator Laboratory in Menlo Park, California.
"What we think we're seeing here is a changeover from one style of extracting energy from the central black hole to another," adds Romani.
Large galaxies grew out of collisions and mergers with many smaller galaxies, and this process occurs with greater frequency as we look back in time. These collisions provided plentiful gas to the growing galaxy and kept the gas stirred up so it could more easily reach the central black hole, where it piled up into a vast, hot, and bright accretion disk like those seen in "gas-guzzling" FSRQs. Some of the gas near the hole powers a jet while the rest falls in and gradually increases the black hole's spin.
As the universe expands and the density of galaxies decreases, so do galaxy collisions and the fresh supply of gas they provide to the black hole. The accretion disk becomes depleted over time, but what's left is orbiting a faster-spinning and more massive black hole. These properties allow BL Lac objects to maintain a powerful jet even though relatively meager amounts of material are spiraling toward the black hole.
In effect, the energy of accretion from the galaxy's days as an FSRQ becomes stored in the increasing rotation and mass of its black hole, which acts much like a battery. When the gas-rich accretion disk all but disappears, the blazar taps into the black hole's stored energy that, despite a lower accretion rate, allows it to continue operating its particle jet and producing high-energy emissions as a BL Lac object.
One observational consequence of the hybrid blazar notion is that the luminosity of BL Lacs should decrease over time as the black hole loses energy and spins down.
The astronomers say they are eager to test this idea with larger blazar samples provided in part by Fermi's continuing all-sky survey. Understanding the details of this transition also will require better knowledge of the jet, the black hole mass and the galaxy environment for both blazar classes.
The Daily Galaxy via NASA/Goddard