A set of twenty-nine Hubble Space Telescope (HST) images of an exotic type of active galaxy known as a "post-starburst quasar" show that interactions and mergers drive both galaxy evolution and the growth of super-massive black holes at their centers.

Mike Brotherton, Associate Professor at the University of Wyoming, is presenting his team's findings at the American Astronomical Society meeting in St. Louis, Missouri.

Other team members include Sabrina Cales, Rajib Ganguly, and Zhaohui Shang of the University of Wyoming, Gabriella Canalizo of the University of California at Riverside, Aleks Diamond-Stanic of the University of Arizona, and Dan Vanden Berk of the Penn State University.

The result is of special interest because the images provide support for a leading theory of the evolution of massive galaxies, but also show that the situation is more complicated than previously thought. Moreover, we may be glimpsing the future of our own Milky Way galaxy.

Over the last decade, astronomers have discovered that essentially every galaxy harbors a supermassive black hole at its center, ranging from ten thousand times the mass of the sun to upwards of a billion times solar, and that there exists a close relationship between the mass of the black hole and properties of its host. When the black holes are fueled and grow, the galaxy becomes active, with the most luminous manifestation being a quasar, which can outshine the galaxy and making it difficult to observe.

In order to explain the relationships between galaxies and their central black holes, theorists have proposed detailed models in which both grow together as the result of galaxy mergers. This hierarchical picture suggests that large galaxies are built up over time through the assembly of smaller galaxies with corresponding bursts of star formation, and that this process also fuels the growth of the black holes which eventually ignite to shine as quasars. Supernova explosions and their dusty debris shroud the infant starburst until the activated quasar blows out the obscuration.

Starbursts fade as they age because the more massive and luminous stars have shorter lifetimes before exploding as supernovas. There should be a phase, however, during which the fading starburst and the quasar can be seen simultaneously.

In the late 1990s, Brotherton discovered a candidate for such a transition object, which possessed the spectral signatures of both a quasar and an older starburst.

The actual burst of star formation, equivalent to a significant fraction of a Milky Way's worth of stars, was already 400 million years old, hence the label "post-starburst"quasar. Hubble images of this single extreme and distant object showed that it was the remnant of a galaxy merger.

In order to find more of these rare post-starburst quasars, Brotherton and his Wyoming-based team turned to the Sloan Digital Sky Survey, the largest catalog of quasar and galaxy spectra in existence. Searching through a candidate list of 15,000 quasars, they identified the signatures of post-starbursts in some 600 objects. Through ground-based telescopes the objects just appear as smudges, without detail.

Brotherton and his team turned the sharp-eyed Hubble Space Telescope and its Advanced Camera for Surveys to observe a subset of these post-starburst quasars that had the strongest and most luminous stellar content. Looking at these systems 3.5 billion light-years away, Hubble, operating without the distortions of an atmosphere, can resolve sub-kiloparsec scales necessary to see nuclear structure and host galaxy morphology.

"The images started coming in and we were blown away," said Brotherton. "We see not only merger remnants as in the prototype of the class, but also post-starburst quasars with interacting companion galaxies, double nuclei, starbursting rings, and all sorts of messy structures."

Hubble snapped pictures of 29 post-starburst quasars in total, using a red-light filter that emphasized the starlight over the glare of the bluer quasar.

"These images provide us tremendous insight into the complexity of galaxy evolution," said team member Dr. Rajib Ganguly. "We see nuclear activity and post-starbursts simultaneously in systems from pre-merger to post-merger and in between."

More work remains to characterize the physics properties of each object, such as the masses and ages of the post-starbursts and the masses and fueling rates of the black holes powering the quasars.

This task will require the combination of the Hubble images with high-quality spectra from the Keck Observatory on Mauna Kea, Hawaii, which team member Gabriella Canalizo has obtained. This more detailed work should provide additional insights into this phase of galaxy evolution.

Astronomers have determined that our own Milky Way galaxy and the great spiral galaxy of Andromeda will collide three billion years from now. This event will create massive bursts of star formation and most likely fuel nuclear activity a few hundred million years later.

Hubble has imaged post-starburst quasars three and a half billion light-years away, corresponding to three and a half billion years ago, and three and a half billion years from now our own galaxy is probably going to be one of these systems.