Scientists Resolve the Nature of Powerful Seyfert Galaxies
Our understanding Active Galactic Nuclei has progressed by leaps and bounds over the past several decades. Recently, debate has centered on the similarities and differences between some of these objects. Quasars, Blazars, Seyfert Galaxies, and radio galaxies are all examples of active galaxies. Active galaxies contain an accretion disk around a central black hole, with two perpendicular jets and an active galaxy's appearance to an observer on Earth depends on the orientation of the accretion disk. (Image Credit: Matt Perko, UCSB)
Scientists Resolve the Nature of Powerful Seyfert Galaxies
At the center of active galaxies are supermassive black holes of such tremendous brightness that they can outshine the rest of their galaxy by a factor of ten thousand -- that’s four orders of magnitude. Radiation ejected from these objects comes from an accretion disk of hot gas swirling around the black hole like water swirls around a drain. The gas becomes hotter and hotter as it swirls inward, so most of the emitted light comes from the inner parts of the accretion disk. Our understanding of these Active Galactic Nuclei (AGN) has progressed by leaps and bounds over the past several decades. Recently, debate has centered on the similarities and differences between some of these objects. Quasars, Blazars, Seyfert Galaxies, and Radio Galaxies are all examples of active galaxies. Active galaxies contain an accretion disk around a central black hole with two perpendicular jets. An active galaxy's appearance to an observer on Earth depends on the orientation of the accretion disk to the observer. Now, a team of researchers at the University of California - Santa Barbara has settled some important questions and exposed exciting new findings about one class of these cosmic monsters – The Seyfert Galaxies.
An active galaxy’s appearance to an observer on Earth depends on the orientation of the accretion disk. Imagine that the accretion disk is oriented edge-on with respect to you. In that case, you will have a good view of both jets, one stretching out to either side. The hot inner regions of the accretion disk, however, will be hidden away from you by the outer, cooler parts of the disk. You will see, in this case, a double-lobed radio galaxy like Cygnus A.
Now imagine that the accretion disk is tilted, so that you see it at an angle. In this case, you will see both blackbody radiation from the inner regions of the disk and synchrotron emission from the jets. (The synchrotron process is what accelerates electrons outward through the magnetic field). If the result is very luminous, you will call what you see a quasar. If it's a little less luminous, you will call it a radio galaxy.
Now imagine that you see the accretion disk face-on, so that one of the jets is pointing straight toward you, and the other one (pointing away from you) is hidden by the accretion disk. In this case, with a jet pointing directly at you, you will receive a huge quantity of synchrotron emission. The resulting radio source is what we call a Blazar.
Why should a supermassive black hole produce jets? We know, after all, that gravity is an attractive force, not a repulsive force. If gas is falling toward the black hole in the accretion disk, why should other gas be simultaneously accelerated away from the black hole in a jet? The detailed mechanism which creates jets is poorly known, but the general picture looks something like this -- The inner regions of the accretion disk is very hot. It is so hot, in fact, that gas starts to evaporate from the disk. As the hot ionized gas drifts away from the disk, it is accelerated and squeezed into a narrow jet by the disk's magnetic field. As the electrons of the ionized gas are accelerated by the magnetic field, they emit the synchrotron emission that we detect with our radio telescopes here on Earth.
Quasars, the most powerful active galactic nuclei, shine like lighthouses from their home galaxies. These beams are powered by supermassive black holes millions to billions of times the mass of the sun. “The [radius of a] black hole is one ten thousanths of the distance just to our nearest star, and yet it can produce the power of 10,000 entire galaxies,” said Robert Antonucci, a physics professor at UC Santa Barbara.
The gas around these black holes spins so fast that the color of the light it emits is stretched out. The material approaching us appears bluer, while receding gas appears redder. This stretches the normally sharp spikes in the light spectra into broad peaks.
The radiation from these systems also energizes distant gas clouds, which are less dense. Because these clouds rotate more slowly, the peaks in their light emission stay sharp. And because they are less dense, the atoms have enough time to make slower transitions between energy states without interference from neighboring atoms, so scientists see additional spikes that are due to heat rather than radiation.
Quasars are a type of active galaxy. The luminosity of a quasar's “quasi-stellar” nucleus can be from 10 to 100,000 times the luminosity of our galaxy (which is an ordinary galaxy). Quasars are powered by gas falling inward toward a central supermassive black hole. Around 11 billion years ago, quasars were much more numerous than they are now. So, where have all the quasars gone? Active galaxies are still around, but they're just less active because they aren't fed as much gas.
Blazars are a type of active galaxy which are still present today. Blazars, like quasars, were first discovered as “quasi-stellar” points of light. Blazars can vary significantly in brightness in less than a day, indicating that most of their light must come from a region less than one light-day across (about 200 AU). Long-exposure photographs of blazars reveal “fuzz” around the quasistellar source. The “fuzz” is elliptical in shape and has the absorption line spectrum typical of ordinary galaxies. This indicates that blazars are probably elliptical galaxies with highly luminous central nuclei. (The nucleus itself has neither emission lines nor absorption lines in its spectrum).
Radio galaxies are also a type of active galaxy. They are elliptical galaxies with jets of gas extending away from a central nucleus. The jets emit radio waves by the synchrotron process.
Seyfert galaxies are spiral galaxies with extremely bright nuclei. The luminosity of Seyfert galaxies ranges from 0.1 to 10 times the luminosity of our galaxy. About two percent of spiral galaxies are Seyferts. The spectra of Seyfert galaxies are characterized by emission lines from highly ionized gas. In some Seyferts, the emission lines are broadened by random motions up to 10,000 kilometers per second. Within the nuclei of Seyfert galaxies, there exists very hot gas that is swirling around very rapidly. The luminosity of the nucleus of a Seyfert galaxy can vary wildly on timescales of less than a month. This implies that the size of the nucleus is less than one light-month (approximately 5000 AU). Thus, a very large amount of energy is emerging from a very small volume. Aside from their bright nuclei, Seyfert galaxies look like ordinary spiral galaxies.
An ongoing debate related to Seyfert galaxies stems from the distinction between the two types of Seyfert galaxies. Type I Seyfert galaxies produce both of these spectra, but the light from type II Seyfert galaxies is missing the broad peaks. Before they knew about the black holes at the center, scientists had thought the two were different entities, and were puzzled over what could be powering them.
Robert Antonucci, a physics professor at UC - Santa Barbara and lead researcher of this study, had proposed that they were actually the same objects, simply seen from different perspectives. Namely, that when the broad regions were missing from the spectra, it was because we were looking at the systems side-on, and a ring of dust was obscuring the inner part of the nucleus from our view.
Antonucci noticed that, unlike normal starlight, the light from Type II Seyfert galaxies tended to slightly favor one polarization, suggesting some of it had reflected our way, like sunlight off the surface of a lake. The polarization suggested that these photons had originated near the center of the black hole and traveled along the jets of high-energy matter that stream away from its poles.
By filtering out everything except this polarization, Antonucci was able to peer into the center of the obscured object, and he found the missing broad spectra. This confirmed that there was matter swirling around the black hole emitting broad bands of light, only to have most of this blocked by the surrounding ring of dust. The two types of Seyfert galaxies were, in fact, one class of objects.
Disagreement, Answers and New Questions
Twenty or so years ago, a group of scientists suggested that some galaxies actually may not emit these broad lines, calling them “true Seyfert II galaxies.” Using x-ray surveys, they found one called NGC 3147 that had neither a dust ring nor the broad emission lines. Proponents claimed that this galaxy must be one of these objects.
Scientists in the two camps decided to work together to resolve the issue, posing the question in a bid for a slot on the Hubble Space Telescope’s busy schedule. It intrigued the scientists managing Hubble enough that they granted the team an hour of the observatory’s time. An hour was all they needed.
The team zoomed in on the center of NGC 3147 and found the broad line region. The object was so dim that the surrounding starlight had overwhelmed this feature. After two decades of debate, the idea of “true Seyfert IIs” seems to have finally been laid to rest.
In Antonucci’s opinion, this is a true triumph of the scientific method. The virtue of science is that it eventually corrects itself. “What’s most important to astronomers is pruning this dead branch,” he said.
What’s more, the team also discovered a few new features, including that NGC 3147’s broad line spectrum was stretched far more than most of them had expected. “That means the region producing the emission lines is in fact much closer to the black hole than normal,” Antonucci said. This did, however, match the predictions of one of Antonucci’s coauthors, Ari Laor at the the Technion in Israel.
The shape of this part of the spectrum also shows clear evidence of the effects of both special and general relativity. However, Antonucci is quick to point out that scores of other experiments have already established the validity of these theories.
Curiously, the researchers also found evidence of a hot, glowing accretion disk despite the fact that matter is only trickling into NGC 3147’s black hole. “According to our best theory, there shouldn’t be enough friction to convert the gravitational in-fall energy into heat,” Antonucci explained.
Hubble has granted the group roughly six hours of additional time on the telescope to follow up their observations. They plan to use this time to further probe the center of NGC 3147 in greater detail. Hopefully a closer look will help bring out the answers to this new set of questions.
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