Image: NASA/Hubble Heritage Team

Most of what we know about how galaxies evolve comes from observations of a single type: so-called Seyfert galaxies (see image). Scientists believe that supermassive black holes at the cores of these systems drive galaxy formation. New findings published in this week's issue of Astrophysical Journal Letters, however, indicates that many of these central black holes may be less powerful than previously thought.

Scientists break Seyfert galaxies into two groups: Seyfert 1 galaxies display bright energetic emissions from their black holes; Seyfert 2 galaxies do not. To explain the difference, some have suggested that a donut-shaped cloud of gas and dust surrounding the black hole simply obscures its view from Earth. But Hien Tran of Johns Hopkins University, who conducted the study, found that instead, some Seyfert 2 galaxies may actually contain black holes that are less active than those found in Seyfert 1 galaxies.

Tran looked at data from 50 Seyfert 2 galaxies and found that, based on a spectroscopic analysis of their polarized light, only half resembled Seyfert 1 galaxies. "The other half of these galaxies, though, don't seem to fit the model," he says. "The signature of the central black hole's activity doesn't appear even when you observe the galaxy in polarized light." For the time being, Tran has christened these galaxies "pure Seyfert 2 galaxies," or "pure S2s."

Tran also looked at infrared emissions from pure S2s and concluded that, on average, they were cooler than the other Seyfert galaxies. "This correlates well with the idea that pure S2s may have a less active central black hole," he says. "Less active central black holes should heat up the surrounding gas and dust less than active black holes."