Galactic Alter Ego

Imagine the galactic equivalent of the Hulk, able to suddenly transform from a mild-mannered group of stars into a raging ball of intensity. Now imagine six of them.

That’s essentially what a University of Maryland-led team of astronomers recently found, challenging established theory about the origins of quasars—bright galactic centers that emit huge amounts of energy—and their role in the evolution galaxies. They reported their findings this week in The Astrophysical Journal.

Galaxies come in a wide variety of shapes, sizes and degrees of brightness: They range from humdrum ordinary versions like our own—visible mainly because of the glow from their stars—to luminous active galaxies, which have supermassive black holes at their center that emit a steady blast of bright light as they voraciously consume nearby gas and dust.

Between them is another class, known as low-ionization nuclear emission-line region (LINER) galaxies, which account for roughly one-third of nearby galaxies. Astronomers have fiercely debated the main source of light emission from LINERs, with some pointing to weakly active galactic nuclei and others maintaining that star-forming regions farther out produce the most light.

The Maryland-led team observed six mild-mannered LINER galaxies—the Bruce Banners of this story—suddenly and surprisingly transforming into ravenous quasars, home to the brightest, most Hulk-like of all active galactic nuclei. The research could help demystify the nature of both LINERs and quasars while answering some burning questions about galactic evolution.

“For one of the six objects, we first thought we had observed a tidal disruption event, which happens when a star passes too close to a supermassive black hole and gets shredded,” said Sara Frederick, a graduate student in the Department of Astronomy and the lead author of the research paper. “But we later found it was a previously dormant black hole undergoing a transition that astronomers call a ‘changing look,’ resulting in a bright quasar. Observing six of these transitions, all in relatively quiet LINER galaxies, suggests that we’ve identified a totally new class of active galactic nucleus.”

The team observed all six of the surprising transitions during the first nine months of the Zwicky Transient Facility (ZTF), an automated sky survey project based at Caltech's Palomar Observatory near San Diego, California, which became operational in March 2018. UMD is a partner in the ZTF effort, facilitated by the Joint Space-Science Institute (JSI), a partnership between UMD and NASA’s Goddard Space Flight Center in nearby Greenbelt, Maryland.

Changing-look transitions have been documented in other galaxies—most commonly in a class known as Seyfert galaxies. By definition, they all have a bright, active galactic nucleus, but Type 1 and Type 2 Seyfert galaxies differ in the amount of light they emit at specific wavelengths. According to Frederick, many astronomers suspect that the difference results from the angle at which astronomers view the galaxies.

Type 1 Seyfert galaxies are thought to face Earth head-on, giving an unobstructed view of their nuclei, while Type 2 Seyfert galaxies are tilted at an oblique angle, such that their nuclei are partially obscured by a donut-shaped ring of dense, dusty gas clouds. Thus, changing-look transitions between these two classes present a puzzle for astronomers, since a galaxy’s orientation toward Earth is not expected to change.

Frederick and her colleagues’ new observations may call these assumptions into question.

“We started out trying to understand changing look transformations in Seyfert galaxies. But instead, we found a whole new class of active galactic nucleus capable of transforming a wimpy galaxy to a luminous quasar,” said Suvi Gezari, an associate professor of astronomy at UMD, a co-director of JSI and a co-author of the research paper. “Theory suggests that a quasar should take thousands of years to turn on, but these observations suggest that it can happen very quickly. It tells us that the theory is all wrong. We thought that Seyfert transformation was the major puzzle. But now we have a bigger issue to solve.”