“It’s surprising that any galaxy can change its look on human time scales,” said astronomer Sara Frederick at the University of Maryland. “These changes are taking place much more quickly than we can explain with current quasar theory. The forces at play must be very extreme and very dramatic.”
A team of astronomers observed that six mild-mannered galaxies suddenly and surprisingly transformed into ravenous quasars — home to the brightest of all active galactic nuclei. The team reported their observations, which could help demystify the nature of both low-ionization nuclear emission-line region (LINER) galaxies and quasars while answering some burning questions about galactic evolution. The rapid evolution suggests they discovered an entirely new way that matter accretes onto the central supermassive black holes of these once dormant galaxies.
Sitting somewhere on the spectrum between ordinary and active galaxies is another class, known as low-ionization nuclear emission-line region (LINER) galaxies. While LINERs are relatively common, accounting for roughly one-third of all nearby galaxies, astronomers have fiercely debated the main source of light emission from LINERs. Some argue that weakly active galactic nuclei are responsible, while others maintain that star-forming regions outside the galactic nucleus produce the most light.
A Previously Dormant Black Hole
“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 Frederick, 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.”
All six of the surprising transitions were observed 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 began observations in March 2018.
Seyfert Galaxies–The Puzzle
Evolving activity levels have been documented in other galaxies — most commonly in a class of active galaxies known as Seyfert galaxies. By definition, Seyfert galaxies 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, a transformation in activity levels between these two classes presents a puzzle for astronomers, since a galaxy’s orientation towards Earth is not expected to change.
“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.”
Something Altogether Different Going On
Frederick and her colleagues want to understand how a previously quiet galaxy with a calm nucleus can suddenly transition to a bright beacon of galactic radiation. To learn more, they performed follow-up observations on the objects with the Discovery Channel Telescope, which is operated by the Lowell Observatory. These observations helped to clarify aspects of the transitions, including how the rapidly transforming galactic nuclei interacted with their host galaxies.
“Our findings confirm that LINERs can, in fact, host active supermassive black holes at their centers,” Frederick said. “But these six transitions were so sudden and dramatic, it tells us that there is something altogether different going on in these galaxies. We want to know how such massive amounts of gas and dust can suddenly start falling into a black hole. Because we caught these transitions in the act, it opens up a lot of opportunities to compare what the nuclei looked like before and after the transformation.”
Unlike most quasars, which light up the surrounding clouds of gas and dust far beyond the galactic nucleus, the researchers found that only the gas and dust closest to the nucleus had been turned on. Frederick, Gezari and their collaborators suspect that this activity gradually spreads from the galactic nucleus—and may provide the opportunity to map the development of a newborn quasar.
X-Ray Space Telescopes Seeking Now Clues
“We are now using X-ray space telescopes to look at a larger set of these objects to investigate light emitted from the innermost material flowing into the supermassive black hole years after we have witnessed these changes,” wrote Sara Frederick in an email on 5/20/21 to The Daily Galaxy, “to seek clues as to how that feeding process is proceeding compared to what is established about other known Seyfert-like objects observed in the X-rays. There has not yet been a publication of that research that we can point to at the moment, but that paper is forthcoming later this year.”
Image credit top of page: ESO/L. Calçada –Quasars are the ultra-bright cores of distant active galaxies, spewing out energetic jets of particles and radiation, visible over billions of light-years.