Research Fellow Studies Star Power of Galaxies

A postdoctoral research fellow at Texas Tech University recently led an effort exploring the evolution of galaxies and why some evolve from ones full of life to passive, star graveyards.

One of the questions that has intrigued astronomers for decades is how galaxies go from forming hundreds of stars each year to none so quickly (if tens of millions of years can be considered quickly). Studying a small set of 12 rare galaxies using the unique capabilities of the Hubble Space Telescope and Chandra X-ray Observatory has provided new insight into how this works in some extreme cases.

Potentially powerful, monstrous black holes at the centers of galaxies are commonly considered responsible for heating up and expelling gas needed to form stars. Paul Sell, along with a group of other researchers, discovered evidence that black holes have less importance than the stars themselves in these extreme cases where formation of new stars is quickly halted.

The most important stars are the heaviest ones with violent, short lives. They burn through their fuel so fast they have a hard time remaining stable and release increasingly powerful winds as they age.  Eventually, they collapse when the fuel is used up, resulting in the catastrophic explosion of the star, called a supernova.  The combined power from the winds and supernovae of these stars heats up the gas and expels it from the galaxy.

“What we've concluded is the stars are the ones that are blowing out the gas, heating it up and stopping the formation of the stars,” said Sell, who began this project during graduate work at the University of Wisconsin-Madison. “We're not ruling out the black hole because it could be doing other things we haven't noticed. Rather, it's just not needed. There is enough energy from all the stars to do all this without needing a black hole.”

The team's findings were published in the July 11 edition of the Monthly Notices of the Royal Astronomical Society.

Two types of galaxies exist. Spiral galaxies, such as the Milky Way, are loaded with the cold gas needed to form stars. Elliptical galaxies do not have much cold gas. In order for stars to form, the cold gas condenses in a star-forming region. Once the density is high enough, stars form.

The biggest stars also are the brightest and hottest, which is why they appear blue. But they also die off very quickly, and when they do, all that is left are the smaller, dead, red stars.  Sell's team's research attempted to understand how galaxies evolve from the active, blue, star-forming spiral galaxies to the red, dead elliptical galaxies. Is it the black hole or is it the stars?

The intense star-forming episode in these galaxies is triggered by the collision of spiral galaxies. The merging galaxies are producing a galaxy much more compact than the Milky Way, but with the same mass that resembles neither spiral nor elliptical galaxies.

In each collision, the cold gas churns together and falls to the center very rapidly to produce a massive, compact core. In the core, the gas condenses to form a single, large star-forming region. There, stars form at a rapid rate, producing powerful supernovae and high-velocity winds that heat and expel the remaining gas, thus halting the formation of more stars.

The heaviest stars that initially stand out because they are the brightest are also blue because they are the hottest.  But they also die off very quickly, and when they do, all that is left are the lighter, red stars.

“If you stop the flow of cold gas to form stars, that's it,” Sell said. “The stars stop forming, and the galaxy rapidly evolves and may eventually become a red, dead elliptical galaxy. These starbursts are quite rare, however, so they may not grow into the typical giant elliptical galaxies seen in our nearby galactic neighborhood. They may, instead, be more compact.”

Sell said the discovery forces astronomers to closely examine how galaxies are evolving instead of assuming really fast outflows must be caused by the massive black hole in the center. He said more work is needed to fully understand what is going on.

“We want to understand how much gas is being blown out, how it's being blown out and whether it's going to escape or fall back in,” Sell said. “We also want to figure out what types of galaxies these turn into. We see a lot of elliptical galaxies nearby. It's not a new class of galaxies, but more that it may be a new subclass.”