The Andromeda (M31) galaxy lies more than two million light years away from Earth. In 2011, an international group of astronomers began a four-year program to map and study the millions of stars comprising the galaxy. With the help of the Hubble telescope, Extreme Science and Engineering Discovery Environment (XSEDE), and the Texas Advanced Computing Center (TACC), they not only produced the best Andromeda pictures ever seen, but also put the question of universal star formation to rest.
To map M31, the Panchromatic Hubble Andromeda Treasury (PHAT) looked to its namesake Hubble Space Telescope (HST). Because the HST orbits the Earth, it can provide information to astronomers that ground-based telescopes cannot. But more than just stunning pictures, each star revealed by the HST holds clues to the history of the galaxy's formation — and thus our own. For instance, by analyzing a star's color, researchers can infer its age. From its luminosity, scientists can measure its distance from Earth.
PHAT used this information to develop star formation histories for M31, which meant decoding the number of stars of each type (age, mass, chemistry) and how much dust is obscuring their light. Modeling the star formation history of 100 million stars requires powerful computation, so the team turned to the US National Science Foundation (NSF), XSEDE, and TACC.
“We had to measure over 100 million objects with 100 different parameters for every single one of them,” says Julianne Dalcanton, principal investigator on the PHAT project. “Having XSEDE resources has been absolutely fantastic because we were able to easily run the same process over and over again in parallel.”
XSEDE enables researchers to interactively share computing resources, data, and expertise. Through XSEDE, the team gained access to the Stampede supercomputer at TACC, which was essential to determining the ages of every star mapped, patterns of star formation, and how the galaxy evolved over time.
The combination of the HST observations of M31 and modeling on Stampede allowed the PHAT team to solve a decades old problem: whether stars more massive than the sun form the same way everywhere in the universe.
“It's hard to study a galaxy when you live inside of it. But it turns out that the size and chemical composition of Andromeda is a lot like the Milky Way,” says Daniel Weisz, an astronomer in the PHAT group. “By studying how Andromeda formed and evolved, we're actually getting clues to how our own galaxy has formed and changed over time.”
In their recent paper on M31's stellar initial mass function, published this February in the Astrophysics Journal, the team provided the most conclusive evidence to date that star formation appears to be a universal process. That is, no matter how different the local conditions for star formation are, the resulting numbers and masses of stars appear to be the same everywhere in M31. This finding has broad implications across all of astronomy — from how stars form to how astronomers interpret the light from distant galaxies.
With 100 billion galaxies in the universe, there is much more work to be done to understand the realm beyond our own. While the group has completed their Andromeda mapping project, they continue to chart other galaxies.
“We do similar work on other nearby galaxies. Andromeda looks most like our own, but there are hundreds of nearby galaxies that look nothing like M31,” Weisz says. “They're interesting for exactly that reason, though. We're trying to understand why galaxies have such diversity and [what the implications are for] the broader picture of how our universe has evolved.”
Read more about the PHAT team's quest to understand infinity here.