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The ‘one-two punch’: A new way galaxies can evolve and die

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The ‘one-two punch’: A new way galaxies can evolve and die

Courtesy of Natalie Sanchez

A team of UW astronomers has recently discovered a new way that galaxies die. 

Natalie “Nicole” Sanchez, a fifth-year doctoral student in the astronomy department, recently first-authored a paper exploring the effects of small satellite galaxies on the evolution and star formation rates of larger, Milky Way-sized galaxies.

According to Sanchez, there are two main types of galaxies. Spiral galaxies, like the Milky Way or Andromeda, are typically younger and full of cold gas that’s crucial for star formation. These types of galaxies are often referred to as blue galaxies, due to the many young blue stars forming within them. The other type of galaxies are full of older, redder stars, and have much less cold gas. Astronomers classify these red galaxies as “quenched,” meaning that most of their gas has been used up or lost, and that their star formation has ceased.

“Galaxy evolution is still a pretty hot topic,” Sanchez said. “That distinction between a galaxy that is star-forming and blue and likely has a disk, and a galaxy that is quenched … Those two classes of galaxies are a big continuing question of astronomy in general. What’s driving these two different populations?”

Astronomers have known for a long time that galaxies often influence one another, according to Sanchez. Throughout its lifetime, various gravitational and physical interactions will change the galaxy’s shape, influencing the course of its evolution.

Astronomers call these interactions “mergers,” and there are two types: major and minor. Major mergers involve the collision of two massive galaxies of the same size. In less than 5 billion years, the Milky Way and Andromeda galaxies will combine to form a new galaxy, in a major merger.

Most research on galactic evolution in the last 30 years has focused on these major mergers, analyzing their effects on the evolution of a galaxy from blue to red. As the two massive galaxies collide, their shapes become distorted, which results in massive gas loss. By the time the final galaxy forms, most of the gas has been ejected. With no gas with which to form stars, galaxies become quenched.

However, Sanchez’s study focused on the effects of minor mergers, which are interactions that occur between galaxies that are vastly different in size. 

“The thing that is kind of cool about this set of simulations is that it’s actually not just one big merger that comes in and disrupts the disk and allows the quenching to occur, but it’s actually this interaction between these two rinky-dink galaxies, with ratios of 1-to-15 and 1-to-20,” Sanchez said.

Sanchez ran a simulation in which she evolved a galaxy for 6 billion years, letting it run until it formed a classic spiral. In a series of tests, she then caused two different types of minor mergers and varied the time between the events in each case. 

The first minor merger is what Sanchez referred to as a flyby: The satellite galaxy passed through the major galaxy, disrupting its disk and disturbing the cold gas. In many instances, the disturbed gas eventually settled back into place and was restored to its spiral disk shape. 

Sanchez said she then collided a second satellite with the main galaxy. Unlike the flyby, this interaction completely merged the two galaxies, causing the satellite to add all of its mass to that of the spiral. The excess gas was too much for the spiral to absorb, so it was pushed toward the supermassive black hole at the galactic center. 

According to Sanchez, as the black hole’s strong gravity attracted the gas, it was accelerated to speeds nearing the speed of light, causing it to shine brightly. The huge increase in activity excited the supermassive black hole, causing it to eject energy from the galaxy in two jets of high-frequency radiation. 

However, as long as the galactic disk hadn’t recently disturbed, the galaxy could maintain its stability even against the increased black hole energetics, and the galaxy was not quenched.

Sanchez said she found that the key factor in determining whether the galaxy would quench was the time between the two mergers. If they occurred within a few hundred million years of one another — a blink of an eye in astronomical terms — the combination of the disrupted gas from the flyby and the increased energy of the black hole would destabilize the galaxy and cause all of the star-forming gas to be ejected, thus sufficiently ending star formation.

While acknowledging that this so-called “one-two punch” method may not be the most likely way that new galaxies form, Sanchez said her findings reveal a whole new way that galaxies can interact.

“The fact that we are continuing to discover new ways that the evolution of galaxies can change, I think, is really cool and exciting,” she said.

Reach contributing writer Sarah Kahle at Twitter: @karahsahle

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