Unusual galaxy in distant universe

Moire Prescott's contribution to the research was published in the scientific research journal Nature on June 22.

"My contribution was taking the speed of the stars versus position in the galaxy, then using a dynamical model to figure out the most likely velocity and orientation of the galaxy," Prescott said.

The galaxy, MACS2129-1, is described as a dead-disk galaxy, Prescott said.

"A dead-disk galaxy is a galaxy that's both disk-shaped but it's dead in the sense that it doesn't have a lot of star formation going on," she said.

The Milky Way, by contrast, is an active galaxy because it produces a lot of stars, Prescott said.

The researchers found that MACS2129-1 is about three times heavier than the Milky Way but half the size. MACS2129-1 also spins twice as fast as the Milky Way.

What makes MACS2129-1 particularly significant is the fact that, unlike many dead galaxies, which Prescott said tend to be elliptical or oval-shaped, this galaxy is disk or spiral-shaped, like the Milky Way, and its stars rotate in a flattened disk, much like the Milky Way's stars.

"My collaborators on this paper were the ones who found this galaxy," Prescott said. "It was found behind a cluster of more nearby galaxies."

Initially the Very Large Telescope (VLT), a facility operated by the European Southern Observatory in Chile, observed the galaxy in 2011.

Prescott became involved in the research in 2015, after the principal investigator, Sune Toft at the University of Copenhagen, where Prescott was before she came to NMSU, asked her to help in the study.

"The lead author asked me to carry out the dynamical modeling necessary to figure out how fast the galaxy was actually rotating while accounting for possible sources of error," she said. "I moved to NMSU in the summer of 2015, but have continued to work with the team on the paper since then."

The team used the Hubble Space Telescope to capture images of the cluster of galaxies in which MACS2129-1 is located. When they examined these images, Prescott's collaborators noted the presence of a seemingly disk-shaped galaxy.

They then employed an astronomical phenomenon known as gravitational lensing in order to zoom in on MACS2129-1.

"The trick here is to get a higher-resolution look at the galaxy by using a foreground cluster as sort of a lens in order to magnify the background galaxy," Prescott said.

In this phenomenon, light bends as it travels toward the observer as a result of the light source's gravity.

Prescott said the team does not yet know what caused star formation in MACS2129-1 to stop.

To form stars, cold gas needs to start collapsing in on itself, heat up, and eventually cause hydrogen to fuse into helium. This fusion is the birth of a star.

A likely possibility, for MACS2129-1, is that energy from its black hole -- all galaxies have a massive black hole in their center -- caused the star-forming gas to be heated or expelled from the galaxy.

It's also possible that MACS2129-1 simply became too massive for cold gas to funnel into the galaxy. As a galaxy becomes heavier, its gravitational pull becomes greater, so any cold gas attempting to enter the galaxy would heated by shocks as it gets close, making it unable to form stars.

"But we don't actually know the answer," Prescott said.

Prescott said it's possible for a dead galaxy to become active again.

"There are a lot of galaxies in the universe and they're moving around, expanding, being tugged on by other galaxies, by gravity," Prescott said. "When you have a collision of galaxies, that can cause a lot of material, a lot of gas, to channel into the center of the galaxy where it can start to collapse and cause star formation."

In the future, Prescott said, MACS2129-1 will likely evolve into an elliptical galaxy, similar to those astronomers usually see of dead galaxies.

"This may happen via collisions with small companion galaxies, which will disrupt the disk-like rotation and randomize the motions of its stars," she said.

Prescott said the discovery is important because it has changed astronomers' belief that dead galaxies could only be elliptical in shape.

"There's some nuance we don't quite understand," she said.

In the near future, Prescott said the team plans to use NASA's prospective James Webb Space Telescope to identify larger samples of dead galaxies to determine if MACS2129-1 is merely an anomaly or representative of dead galaxies.