Study Stirs Old Debate about Galaxies

Using a technique that peeks over obscuring rings of dust and gas and into
the hearts of distant galaxies, a researcher has found evidence suggesting
that as many as half of the bright, active galaxies known as Seyfert 2
galaxies may have significantly less active central black holes.

Seyfert galaxies are among the most important sources of information
astronomers have on the evolution of galaxies, and the supermassive black
holes at their centers are increasingly thought to be the “engines” driving
galaxy formation, so a change in the activity levels of the central black
holes in some Seyfert 2 galaxies could have big implications.

However, Hien D. Tran, the author of the new study, acknowledges that such
a change may still be a long way off. He feels his data “strongly suggest”
that some central black holes in Seyfert 2 galaxies are less active than
previously thought, but admits that many astronomers will debate his
interpretation, and that his theory can only be proven through follow-up
observations. “This is to be expected for something that questions what
has been the standard paradigm for such a long time,” says Tran, a
research scientist in The Krieger School of Arts and Sciences at The Johns
Hopkins University. Tran’s study was published in the June 10 issue of
Astrophysical Journal Letters.

“I think if you took a survey of astronomers today, most of them would say
the signal of the very active supermassive central black hole is in there
in those galaxies, you just haven’t looked hard enough,” says Tim Heckman,
a professor of astronomy at Hopkins who has reviewed Tran’s paper. “It’s
going to be hard to prove the negative.”

Seyfert galaxies and astronomers have always been a little bit like the
proverbial blind men and the elephant. Stuck with extremely limited
points-of-view of an elephant, the blind men came away with very different
impressions of what it was.

Astronomers aren’t blind, but they are limited to observing galaxies from
Earth or its vicinity, and for an object as big as a galaxy, that amounts
to a single tightly restricted perspective. Studying more than one example
of a particular type of galaxy can yield different points of view, but
researchers have to be sure the different examples they use are the same
type of galaxy.

That’s where the distinctive features of a Seyfert galaxy can increase the
challenge. The characteristic trait of a Seyfert galaxy is a blisteringly
bright fountain of energetic emissions from the central black hole.
Astronomers currently believe a torus- or doughnut-shaped cloud of gas
and dust surrounds the central black hole in Seyfert galaxies, and that
this cloud shapes the fountain by only allowing the intense radiation
from the black hole to erupt from the cloud’s dust-free central hole.

Seyfert 1 galaxies are oriented so that the opening of the torus points
toward Earth, allowing a direct view of activity in the galactic nucleus.
However, Seyfert 2 galaxies are oriented so that the opening of the
torus is not visible from Earth, and this makes them look different to
astronomers.

Astronomers first came to think of Seyfert galaxies as one type of galaxy
seen from different views in 1983, when Tran’s future mentor, Joseph Miller
of the University of California-Santa Cruz and his student Robert Antonucci,
now at the University of California-Santa Barbara, proposed what would
become known as the unified model of Seyfert galaxies.

Miller and Antonucci based their unified model, which included the concept
of a torus of gas and dust surrounding the central black hole, on Seyferts
data gathered through a technique known as spectropolarimetry.

“Spectropolarimetry lets us detect how much of the light from a source
comes directly from the source, and how much of it is reflected light from
other sources that may be hidden,” says Tran. “Any time light is reflected,
it becomes polarized, and this technique lets us detect that.”

Feeling that a definitive test of whether the unified model applied to all
Seyfert galaxies was lacking, Tran set out to use spectropolarimetry to
conduct a study of a large number of Seyfert 2 galaxies. With funding from
NASA, the National Science Foundation, and the Department of Energy, Tran
spent seven years either observing directly or analyzing previously
acquired data from 50 Seyfert 2 galaxies.

Aaron Barth, a research fellow at the Harvard-Smithsonian Center for
Astrophysics, reviewed Tran’s study for publication and called it “the
largest survey of its kind to date.”

In about half of the Seyfert 2 galaxies Tran studied, he could clearly
detect reflected light with spectroscopic features similar to those seen
in the center of Seyfert 1 galaxies. Although the fountain of radiation
from the area around the central black hole in these galaxies points in
a direction that makes it hard to see from Earth, enough of the black
hole’s spectral signature was reflected by material around the galaxy
for Tran to detect it.

“The other half of these galaxies, though, don’t seem to fit the model,”
says Tran. “The signature of the central black hole’s activity doesn’t
appear even when you observe the galaxy in polarized light.” Tran has
tentatively decided to call these “pure S2s,” or pure Seyfert 2 galaxies.

To begin checking if anomalous factors in individual galaxies might be
obscuring the central black hole’s spectroscopic signature, Tran analyzed
several other observable characteristics, and once again found that the
pure S2s seemed unusual.

“For example, when you plot the luminosity of these galaxies in radio
wavelengths versus their flux ratio in the infrared, you can see that
the pure S2s appear on average to be cooler and less luminous in radio
waves,” Tran says. “This correlates well with the idea that pure S2s may
have a less active central black hole. Less active central black holes
should heat up the surrounding gas and dust less than active black holes.”
There are still a number of additional possible explanations to consider,
though.

“Starbursts, which are regions of intense star formation, may be muddling
the picture,” says Heckman, who suggests using X-ray observatories to
look beyond the gas and dust to get a better picture of the galaxies in
question, perhaps providing more clues as to whether they are genuinely
different.