The secret lives of “fossil relic” galaxies

By mining data in a simulated universe, a team of astronomers from Arizona State University have observed so-called "fossil relics" from an earlier era aren't exactly what they seem. The findings provide a glimpse to a seldom seen type of miniature galaxy.

The rare class of dwarf galaxies, called extremely metal-deficient galaxies (XMDs), looked so young that 20th-century astronomers initially assumed they were newly formed, with stars emerging for the first time. These galaxies are believed to have formed in the early Universe, a time when heavy elements were scarce.

While previous studies used XMDs as a comparison with protogalaxies - the cloud of dust and gas that is a precursor to galaxy formation and evolution of stars, a new study with recent NASA’s James Webb Space Telescope observations has renewed interest in investigating these protogalaxies.

The study, which was published in Monthly Notices of the Royal Astronomical Society, provides astronomers with differences in star formation history between XMDs and non-XMDs. It was led by ASU co-authors, Jacqueline Monkiewicz, Faculty Associate and Tim Carleton, Assistant Research Scientist, both of the School of Earth and Space Exploration.

“We still tend to think of them as “fossil relics," said Monkiewicz.  “Because they really seem like throwbacks to the very first protogalaxies to form after the Big Bang.”

The astronomers had some questions - what were these galaxies doing for the 13 billion years between the Big Bang and today? And, how did they manage to preserve their “youthful” appearance for such a long time?

“It’s very difficult to do ‘galactic archeology’ with modern galaxies, even with powerful space telescopes such as NASA’s  James Webb Space Telescope or the Hubble Space Telescope,” said Monkiewicz. “We can see how many new stars a galaxy is cooking up right now, and which chemical elements it is using to make them. But it’s much harder to determine exactly what that galaxy looked like billions of years in the past.”

Carleton suggested that they turn to the enormous computer simulation known as IllustrisTNG. Cosmological simulations such as Illustris use intensive computing power to create virtual reality models of the real Universe. 

“These simulations include our best understanding of all of the basic processes, like gravity, gas flows, and star formation, which combine and create all the galaxies in the Universe,” said Carleton. “The Illustris simulation is particularly useful for this analysis because its creators have done such a good job of making their simulation public, and accessible. You can basically go in and grab a snapshot of the entire Universe at any point in time.”

Initially, the researchers were doubtful that this virtual Universe would even contain the type of galaxies they were looking for.

“These are tiny galaxies, really itty-bitty,” said Monkiewicz. “They are only one hundredth or even a thousandth of the size of our Milky Way galaxy. The key word is ‘resolution’: finer details require higher resolution, which requires exponentially more computing power.  Even with supercomputers, it’s difficult to simulate the entire Universe and still see galaxies this small.”

The Illustris simulation was up to the task - Carleton found over a hundred dwarf galaxies in the Illustris databases with properties matching those of the real-life XMD galaxies.

“We even have a nice comparison sample of ‘normal’ dwarf galaxies. This means we can use the simulation to ask the question: How did they even get there?”said Carleton.

Carleton and Monkiewicz found that XMD galaxies in the Illustris simulation are more complex than they initially thought. After tracing the evolution of these galaxies back through the simulation for billions of years, the ASU researchers discovered that these unusual objects had looked like completely normal dwarf galaxies for most of their lives.

“And then one day, a billion or so year ago, they simply stopped forming stars,” said. Monkiewicz.

Turning interstellar gas into stars is one of the fundamental functions of a galaxy. The XMDs in the Illustris simulation had abruptly stopped creating new stars, which gave them their artificially “youthful” appearance. 

This explanation runs counter to the most common theories for the existence of XMDs, which usually require the triggering of a “starburst” of newly formed stars.

“We were really surprised by what we found in the simulation,” said Carleton. “They really appear different than we expected.”

The research, supported by ASU’s Beus Center for Cosmic Foundations.