A black hole inside a ring of dust around a Seyfert galaxy shoots a colored jet from its center


Type I and Type II Seyfert galaxies are actually the same thing, researchers report.

A Seyfert galaxy is a dim quasar, and debate among astronomers about the galaxies stems from the distinction between the two types.

Before they discovered black holes at the center of these galaxies, scientists had thought the two were different entities, and puzzled over what could be powering them.

The center of a galaxy

At the center of certain galaxies are objects of such tremendous brightness they outshine the rest of their galaxy by four orders of magnitude.

Quasars, the most powerful active of these galactic nuclei, shine like lighthouses from their home galaxies. Supermassive black holes millions to billions of times the mass of the sun power these beams.

“The [radius of this] black hole is 1/10,000th of the distance just to our nearest star, and yet it can produce the power of 10,000 entire galaxies,” says coauthor Robert Antonucci, a professor of physics at the University of California, Santa Barbara.

The nucleus of a Seyfert galaxy. (Credit: Matt Perko)

The gas around these black holes spins so fast that the color of the light it emits gets 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. Type I Seyfert galaxies produce both of these spectra, but the light from type II galaxies is missing the broad peaks.

Antonucci had proposed that the two Seyfert galaxies were actually the same object, 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.

Ending the debate over Seyfert galaxies

Antonucci noticed that, unlike normal starlight, the light from Seyfert II 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.

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.

A cloudy swirl filled with stars surrounds a bright center of the galaxy against the black background of space
The galaxy NGC 3147 is at the center of the Seyfert debate. (Credit: ESA/HUBBLE & NASA, A. RIESS ET AL.)

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 says.

More findings

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 says. 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 explains.

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 suss out the answers to this new set of questions.

Their results appear in the Monthly Notices of the Royal Astronomical Society.

Source: UC Santa Barbara



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