Six Galaxies Undergo a Rapid Monstrous Change into Ravenous Quasars

Astronomers have observed six galaxies undergoing a sudden and violent transformation — changing from quiet calm galaxies to powerful quasars blasting light across the Universe.

A new study led by University of Maryland astronomers documented six sleepy, low-ionization nuclear emission-line region galaxies (LINERs; left) suddenly transforming into blazing quasars (right), home to the brightest of all active galactic nuclei. The researchers suggest they have discovered an entirely new type of black hole activity at the centres of these six LINER galaxies. ( Left; infrared & visible light imagery): ESA/Hubble, NASA and S. Smartt (Queen’s University Belfast); (Right; artist’s concept): NASA/JPL-Caltech)

A team of astronomers led by researchers from the University of Maryland have observed six — seemingly ‘mild-mannered’ — galaxies undergo rapid changes to ravenous quasars — the brightest of all active galactic nuclei. Changes such as this are extremely rare to observe on the timescale of human experience.

The observation — detailed in a paper published in the Astrophysical Journal — could help astronomers solve the mysteries surrounding low-ionization nuclear emission-line region (LINER) galaxies. LINERS are relatively common — accounting for approximately one-third of nearby galaxies — and sit somewhere on the spectrum between ordinary and active galaxies in terms of brightness. Yet, despite how common they are, astronomers are still unsure of what the main source of LINER’s light emissions is.

In ordinary galaxies, the majority of the visible light we see is emitted by its stars — thus it can be seen spread through the galaxy. With active galaxies, however, the majority of visible light emitted from its centre — the nucleus where a supermassive black hole dwells blasting out bright light as it consumes surrounding gas and dust.

The current candidates for the majority of light emissions in LINERs include weakly active galactic nuclei or star-forming regions located outside the galactic nucleus. The newly observed monstrous transition in the behaviour of LINERs could settle this heated debate.

The team’s results suggest that LINER galaxies also host supermassive black holes which can suddenly, somehow, begin consuming surrounding gas and dust. All of the surprising changes were observed by the automated sky survey project the Zwicky Transient Facility (ZTF) during the first nine months of its operation.

Sara Frederick, a graduate student at the University of Maryland’s Department of Astronomy and the lead author of the research paper, describes the violent transformation observed by the team: “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.

“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.”

Frederick goes on to suggest that observing six of these transitions — all in relatively quiet LINER galaxies could imply that the team has successfully identified a totally new class of active galactic nucleus.

A matter of perspective?

Transitions that change the look of galaxies have been documented before, occurring most commonly in Seyfert galaxies — a type of active galaxy with quasar-like nuclei. All Seyfert galaxies have bright, active nuclei, but are classified by the amount light they emit at specific wavelengths.

Astronomers have long suspected that the differences in the emissions of

Type 1 and Type 2 Seyfert galaxies are not a quality of the galaxies themselves, but rather, the angle at which we view them from Earth.

Should this suspicion be correct, Type 1 Seyferts face Earth head-on resulting in astronomers having an unobstructed view of their galactic nuclei. Whilst Type 2 Seyferts are tilted at an oblique angle — resulting in their active galactic nuclei being obscured by a doughnut-shaped dense torus of gas and dust.

But this approach leads to a puzzle. As a galaxy’s orientation in regards to Earth is not expected to change, how can a galaxy change between a Type 1 and Type 2 Seyfert galaxy?

The team’s results challenge this view of Seyfert galaxies. Suvi Gezari, an associate professor of astronomy at UMD, a co-director of JSI and a co-author of the research paper, explains further: “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.

“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.

“Now we have a bigger issue to solve.”

The question that these observations left the team struggling to explain was how can a previously quiet and subdued galaxy suddenly transform into a bright beacon of radiation?

In order to answer this, they performed follow-up analysis on the galaxies with the Discovery Channel Telescope — operated by the Lowell Observatory. These latter observations helped identify certain characteristics to the transformations — including how each quickly transforming nucleus interacted with its host galaxy.

Frederick continues: “Our findings confirm that LINERs can, in fact, host active supermassive black holes at their centres. 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.”

Another characteristic of these metamorphosing LINER galaxies is the fact that, unlike in quasars, surrounding clouds of gas and dust far from the galactic nucleus are not lit up by its activity. Frederick and her team suspect that this is because the activity in LINERs spreads gradually from the galactic nucleus. A finding which could allow astronomers to map the development of a newborn quasar in unprecedented detail.

Frederick concludes: “It’s surprising that any galaxy can change its look on human time scales. These changes are taking place much more quickly than we can explain with current quasar theory.

“It will take some work to understand what can disrupt a galaxy’s accretion structure and cause these changes on such short order. The forces at play must be very extreme and very dramatic.”