June 4, 2014
New Study Claims That The Two Types Of Blazars May Actually Be One Hybrid
redOrbit Staff & Wire Reports - Your Universe Online
Astronomers using the NASA Fermi Gamma-ray Space Telescope to study what they thought were two different classes of black-hole-powered galaxies known as blazars now believe that they may be one and the same.
“We can think of one blazar class as a gas-guzzling car and the other as an energy-efficient electric vehicle,” lead researcher and Clemson University astrophysicist Marco Ajello explained Tuesday in a statement. “Our results suggest that we're actually seeing hybrids, which tap into the energy of their black holes in different ways as they age.”
By analyzing how blazars are distributed throughout the universe, Ajello and his colleagues have come to the conclusion that the two distinctive sets of properties believed to define each class actually reflect changes in how the energy is extracted by the galaxies from their central black holes.
An active galaxy possesses an extremely luminous core that is powered by black holes containing at least millions of times more mass than our sun, they explained. As gas falls towards these supermassive black holes, it settles into an accretion disk and heats up. Near the black hole’s brink, the disk ejects some gas in jets that move in opposite directions at close to the speed of light, though the exact processes behind this phenomenon are unclear.
Blazars are the highest-energy type of active galaxies, and according to the authors, the light they emit covers the entire spectrum from radio waves to gamma rays. Blazars represent over half of the more than 1,000 discrete gamma-ray sources cataloged thus far by Fermi's Large Area Telescope, and astronomers believe they appear to be extremely intense because their position is such that one of the jets is close to our line of sight.
“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,” NASA explained.
Scientists have identified two different types of blazar: one known as flat-spectrum radio quasars (FSRQs) and the other known as BL Lacs. The “gas guzzling” FSRQs have active accretion disks, are more luminous, possess smaller black hole masses and lower particle acceleration in the jets, the researchers said. BL Lacs, on the other hand, are dominated by the jet emission, with higher energy particles and weak or absent accretion disk emissions.
On Tuesday, Ajello explained that he and his colleagues set out to investigate how the distribution of blazars has changed throughout the history of the universe. However, they found it challenging to locate solid distance information for large numbers of gamma-ray-producing BL Lac objects, since their weak disk emissions make it extremely difficult to measure their redshift (an essential tool for determining distance in the deeper cosmos).
To change that, the astronomers launched an extensive program of optical observations using multiple instruments to measure the redshifts of roughly 200 BL Lac objects that had been detected by Fermi, making it the largest and most comprehensive collection of this type of data to date. With access to that data, they were able to compare the distribution of BL Lacs to a similar sample-size of FSRQs.
“What emerged suggests that, starting around 5.6 billion years ago, FSRQs began to decline while BL Lacs underwent a steady increase in numbers. The rise is particularly noticeable among BL Lacs with the most extreme energies, which are known as high-synchrotron-peaked blazars based on a particular type of emission,” NASA said. The researchers believe that this indicates a switch from one type of energy extraction to another.
Collisions and mergers involving smaller galaxies led to the growth of larger ones, and those collisions provided enough gas to help the galaxies continue to grow. In addition, they kept the gas stirred up so that it could easily reach the central black hole, where it formed an accretion disk that was large, hot and bright like those seen in FSRQs.
“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 space agency explained. “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,” NASA added. “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.”
If the hybrid blazar concept proves to be true, it would mean that the luminosity of BL Lacs would decrease over time, since the black hole will begin losing energy and spinning down. Ajello’s team said that they are eager to test their notion with larger blazar samples obtained by Fermi’s ongoing sky survey efforts.
Their research was presented Tuesday at the American Astronomical Society meeting in Boston.