Star formation 'reignited' in the galaxy Leo P after billions of years

Galaxies come in many shapes and sizes, from massive spiral galaxies like the Milky Way to tiny, faint dwarf galaxies. While larger galaxies often steal the spotlight, small galaxies can also provide crucial insights into the history of the universe.

Researchers led by Kristen McQuinn of the Space Telescope Science Institute (STScI) have used the James Webb Space Telescope (JWST) to investigate Leo P, an isolated dwarf galaxy located 5.3 million light-years from Earth.

Their study has revealed surprising patterns in star formation that could reshape our understanding of how galaxies evolve over cosmic time.

Leo P: A galactic time capsule

Leo P is unique among dwarf galaxies. Unlike many of its counterparts, it sits relatively isolated, away from the gravitational influence of larger galaxies like the Milky Way and Andromeda. This makes it a pristine laboratory for studying galaxy evolution without the complications of external interference.

The “P” in Leo P stands for “pristine,” a reference to its low metal content. In astronomy, “metals” refer to elements heavier than hydrogen and helium, which are produced by stars over time.

Leo P contains just 3% of the Sun’s heavy elements, making it one of the most metal-poor galaxies known. This means Leo P resembles some of the earliest galaxies that formed in the universe, offering a rare glimpse into what galaxies looked like billions of years ago.

“Leo P provides a unique laboratory to explore the early evolution of a low-mass galaxy in detail,” said McQuinn, who also leads the Nancy Grace Roman Space Telescope’s Science Operations Center.

A start-stop history of star formation

Astronomers have long studied how and when small galaxies form stars.

In most cases, dwarf galaxies begin forming stars early in their history. However, many of them stop producing stars after a few billion years – often permanently. The new observations of Leo P challenge that assumption.

Using JWST’s Near-Infrared Camera (NIRCam), McQuinn’s team measured the brightness and colors of thousands of individual stars in Leo P. This data allowed them to reconstruct the star formation history of the galaxy in great detail.

They discovered that Leo P formed stars early in cosmic history but then abruptly stopped forming new stars shortly after a period known as the Epoch of Reionization.

This was a key phase in the early universe when radiation from the first galaxies ionized the remaining neutral hydrogen gas, ending the universe’s “dark ages.” Many small galaxies lost their ability to form stars during this period and never recovered.

But Leo P was different. After a long dormancy, the galaxy reignited its star formation billions of years later – something rarely seen in other dwarf galaxies.

“We have a measurement like this for only three other galaxies that are all isolated from the Milky Way, and they all show a similar pattern,” McQuinn explained.

What makes Leo P different?

This discovery challenges long-held assumptions about why some galaxies stop forming stars while others continue.

For years, astronomers believed that a galaxy’s mass played the biggest role in whether it could sustain star formation. Smaller galaxies were thought to be more vulnerable to cosmic events like reionization, which stripped them of their gas and halted star formation.

However, Leo P tells a different story. The team compared its history with that of dwarf galaxies within the Local Group, the collection of galaxies that includes the Milky Way. Many of these galaxies ceased forming stars after the Epoch of Reionization – and never started again.

The difference? Leo P and the three other galaxies that restarted star formation are isolated, while the ones that remained dormant are satellite galaxies orbiting larger galaxies like the Milky Way.

This suggests that a galaxy’s environment, rather than just its mass, plays a crucial role in its ability to form stars over time. Galaxies that remain isolated may have a better chance of recovering and forming stars again, while those near large galaxies may lose their gas permanently.

“If the trend holds, it provides insights about the growth of low-mass structures that are not only a fundamental constraint for structure formation but a benchmark for cosmological simulations,” noted McQuinn.

A window into the early universe

Another key finding from the study is that Leo P’s extremely low metal content makes it one of the closest analogs to the earliest galaxies in the universe. The first galaxies to form after the Big Bang were extremely metal-poor, composed mainly of hydrogen and helium.

Studying galaxies like Leo P helps astronomers understand how those first galaxies evolved and produced the heavy elements that make up stars, planets, and even life itself.

What’s next? Expanding the study

Leo P is just one piece of a much larger cosmic puzzle. To confirm whether its unusual star formation history is part of a broader pattern, McQuinn’s team plans to study four more isolated, star-forming dwarf galaxies using JWST.

If they find similar trends, it could mean that many more galaxies in the early universe experienced multiple bursts of star formation instead of a single continuous phase.

These findings also have implications for cosmological simulations, which attempt to model how galaxies form and evolve over billions of years. If small galaxies like Leo P can reignite their star formation after long dormancies, then existing models may need to be revised to reflect this possibility.

Small galaxies with big secrets

The James Webb Space Telescope continues to revolutionize astronomy. Its ability to peer into the past with unprecedented clarity allows astronomers to answer some of the biggest questions about galaxy formation and evolution.

The study of Leo P is just one example of how Webb is uncovering hidden details about the universe, helping scientists piece together the story of how galaxies – and ultimately, life itself – came to be.

Leo P’s unexpectedly complex history serves as a reminder that even the smallest galaxies can hold the biggest secrets.

Image Credit: NASA, ESA, CSA, Kristen McQuinn (STScI)

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–