Astronomers have been gazing at the Triangulum galaxy for centuries. But they’ve never seen it quite like this.

On Jan. 11 at the in Seattle, a team led by scientists at the 91̽�� and the Center for Computational Astrophysics unveiled results using the Panchromatic Hubble Andromeda Treasury Triangulum Extended Region — or PHATTER — survey. The endeavor is giving astrophysicists their first in-depth look at the distinct populations of stars that make up the Triangulum galaxy.

Researchers discovered that this satellite galaxy, a close companion of the much larger Andromeda galaxy, has two drastically different structures depending on the age of the stars.

“The youngest stars and the oldest stars in the Triangulum galaxy — which we can separate out using multiple wavelength filters on the Hubble Space Telescope — are organized very differently,” said , a postdoctoral researcher at the UW. “This is surprising. For a lot of galaxies, like the Milky Way and Andromeda, the stars are distributed roughly consistently, regardless of their age. That is not the case with Triangulum.”

At about 61,000 light years across, Triangulum is the third-largest galaxy in our local group, after Andromeda and our own Milky Way. In lower-resolution images it has a “flocculent” structure – with many small spiral arms radiating out from a well-defined center.

For the PHATTER survey, the Hubble Space Telescope obtained hundreds of high-resolution images of different sections of the Triangulum galaxy in 108 orbits over the course of more than a year. The team tiled these smaller-section images together to create a comprehensive, high-resolution dataset for Triangulum that for the first time showed the galaxy’s individual stars over a large region in its center.

Thanks to Hubble’s array of filters, researchers could also separate those stars by age. The distribution of younger, massive stars — those less than 1 billion years in age — were roughly in line with the “flocculent” pattern, for which the Triangulum is so renowned. But its older, redder stars are distributed in a very different pattern: two spiral arms radiating out from a rectangular bar at the galaxy’s center.

“This was a largely unknown and hidden feature of the Triangulum galaxy that was very difficult to see without this kind of detailed survey,” said Smercina

Old stars make up the majority of Triangulum’s mass, but are dimmer than their younger counterparts, according to Smercina. That could explain why the “flocculent” pattern prevails in low-resolution images of the galaxy.

The survey team also does not know why young and old stars have such divergent distributions in Triangulum. Satellite galaxies in general are an eclectic bunch, and many questions remain about their formation and evolution. Satellite galaxies come in many different shapes and can be molded by interactions with their parent galaxies. The Milky Way’s largest satellite galaxy, the Large Magellanic Cloud, for example, is similar in size and mass to Triangulum, but has an irregular and globular shape due to its proximity to our own galaxy.

The PHATTER survey’s on-going analysis should shed light on how these types of galaxies form and interact with their larger neighbors. The team plans to follow up on these initial findings by tracing the history of star formation in Triangulum, comparing different sections of the galaxy.

“A major goal of the PHATTER survey was to generate the kind of detailed, high-resolution data on this prominent satellite galaxy that will allow us to examine its structure in depth, trace its history of star formation and compare what we see to theories of galaxy formation and evolution,” said Smercina. “We’re already finding surprises.”

Other team members include , director of the Center for Computational Astrophysics in New York, a 91̽��professor of astronomy and principal investigator of the PHATTER project; 91̽��research associate professor of astronomy ; 91̽��doctoral student ; and , a postdoctoral researcher at Caltech.

For more information, contact Smercina at asmerci@uw.edu.

Scientists once thought that post-starburst galaxies scattered all of their gas and dust — the fuel required for creating new stars — in violent bursts of energy, and with extraordinary speed. Now, a team led by 91̽�� postdoctoral researcher reports that these galaxies don’t scatter all of their star-forming fuel after all. Instead, data from the Chile-based , or ALMA, reveals a more complex process at work.

After their supposed end, these dormant galaxies hold onto and compress large amounts of highly concentrated, turbulent gas. But contrary to expectation, they’re not using it to form stars.

In most galaxies, scientists expect gas to be distributed in a way similar to starlight. But for post-starburst galaxies, this isn’t the case. Post-starburst galaxies are different from other galaxies because they are born in the aftermath of violent collisions, or mergers between galaxies. Galaxy mergers typically trigger massive bursts of star formation, but in post-starburst galaxies, this outburst slows down and near-completely stops almost as soon as it begins. As a result, scientists previously thought that little or no star-forming fuel was left in these galaxies’ central star-forming factories. And until now, astronomers believed that the molecular gases had been redistributed well beyond the galaxies, either through stellar processes or by the effects of black holes. The new results, April 25 in The Astrophysical Journal, challenge this theory.

“We’ve known for some time that large amounts of molecular gas remains in the vicinity of [post-starburst galaxies] but haven’t been able to say where, which in turn, has prevented us from understanding why these galaxies stopped forming stars. Now, we have discovered a considerable amount of remaining gas within the galaxies and that remaining gas is very compact,” said Smercina, who is lead author on the paper. “While this compact gas should be forming stars efficiently, it isn’t. In fact, it is less than 10% as efficient as similarly compact gas is expected to be.”

In addition to being compact enough to make stars, the gas in the observed dormant — or quiescent — galaxies had another surprise in store for the team: The gas was often centrally located, though not always, and was surprisingly turbulent. Combined, these two characteristics led to more questions than answers for researchers.

“The rates of star formation in the [post-starburst galaxies] we observed are much lower than in other galaxies, even though there appears to be plenty of fuel to sustain the process,” said Smercina. “In this case, star formation may be suppressed due to turbulence in the gas, much like a strong wind can suppress a fire. However, star formation can also be enhanced by turbulence, just like wind can fan flames, so understanding what is generating this turbulent energy, and how exactly it is contributing to dormancy, is a remaining question of this work.”

“These results raise the question of what energy sources are present in these galaxies to drive turbulence and prevent the gas from forming new stars,” said co-author Decker French, an astronomer at the University of Illinois Urbana-Champaign. “One possibility is energy from the accretion disk of the central supermassive black holes in these galaxies.”

A clear understanding of the processes that govern the formation of stars and galaxies is key to providing context to the universe and Earth’s place in it. The discovery of turbulent, compact gas in otherwise dormant galaxies gives researchers one more clue to solving the mystery of how galaxies live, evolve and die over the course of billions of years.

“There is much about the evolution of a typical galaxy we don’t understand, and the transition from their vibrant star-forming lives into quiescence is one of the least understood periods,” said co-author J.D. Smith, an astronomer at the University of Toledo. “Although post-starbursts were very common in the early Universe, today they are quite rare. This means the nearest examples are still hundreds of millions of light-years away, but these events foreshadow the potential outcome of a collision, or merger, between the Milky Way Galaxy and the Andromeda Galaxy several billion years from now. Only with the incredible resolving power of ALMA could we peer deep into the molecular reservoirs left behind ‘after the fall.’”

“It’s often the case that we as astronomers intuit the answers to our own questions ahead of observations,” said Smercina. “But this time, we learned something completely unexpected about the universe.”

Additional co-authors are Eric Bell at the University of Michigan, Daniel Dale at the University of Wyoming, Anne Medling at the University of Toledo, Kristina Nyland at the U.S. Naval Research Laboratory, George Privon at the University of Florida, Kate Rowlands at the Space Telescope Science Institute, Fabian Walter at the Max Planck Institute for Astronomy and Ann Zabludoff at the University of Arizona. The research was funded by NASA, the Alexander von Humboldt Foundation, the Max Planck Institute for Astronomy and the National Science Foundation.

For more information, contact Smercina at asmerci@uw.edu.

Adapted from a by the National Radio Astronomy Observatory.