This Tiny Galaxy Is Answering Some Big Questions

Over cosmic time, galaxies start small and grow larger by accumulating gas and merging with each other. Many small "seed" galaxies persist into the present day, and astronomers study these dwarf galaxies to learn how they have evolved over time. Of particular interest, isolated dwarf galaxies that have been unaffected by mergers can provide a window into processes that function on a cosmic scale. A team of researchers has found that a nearby, isolated dwarf galaxy whose star formation had largely ceased early in the universe later "reignited," experiencing a rebirth that many other small galaxies didn't.

The results were presented in a press conference at the 245th meeting of the American Astronomical Society in National Harbor, Maryland. They were also published in The Astrophysical Journal.

Principal investigator Kristen McQuinn, of the Space Telescope Institute (STScI) in Baltimore, and her team studied Leo P, a dwarf galaxy some 5.3 million light years from Earth that was discovered in 2013. Leo P is far enough away from the Local Group, a gathering of galaxies that includes the Milky Way, to be a neighbor without being affected by them. The "P" in Leo P refers to "pristine," because the galaxy has so few chemical elements besides hydrogen and helium.

"Leo P provides a unique laboratory to explore the early evolution of a low-mass galaxy in detail," said McQuinn, who is also the mission head of the Nancy Grace Roman Space Telescope's Science Operations Center at STScI.

McQuinn and her team studied Leo P using NASA's James Webb Space Telescope. They imaged Leo P using Webb's NIRCam (Near-Infrared Camera) to determine the brightness and colors of thousands of stars within the dwarf galaxy. That data yielded information about the star formation history within the galaxy.

The team found that Leo P formed stars early on but then stopped making them shortly after a period known as the Epoch of Reionization, a significant period in the early history of the universe. After a few billion years the galaxy reignited and started forming new stars again.

"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 said.

Observations of dwarf galaxies within the Local Group, however, show that star production within them ceased after the Epoch of Reionization and never resumed.

The contrast between the star production of isolated dwarf galaxies versus those in the Local Group provides compelling evidence that it isn't just the mass of a galaxy at the time of reionization that determines whether its star formation will be shut down, or quenched. Its environment - meaning whether it is isolated or functioning as a satellite of a larger system - is an important factor.

McQuinn said the observations will help pin down not only when little galaxies formed their stars, but also how the reionization of the universe may have impacted how small structures form.

"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," she said.

The researchers also found that Leo P is metal-poor, possessing 3% of the Sun's heavy elements (which astronomers call metals). This makes Leo P similar to the primordial galaxies of the early universe.

The researchers plan to use Webb to study four additional isolated, star-forming dwarf galaxies to determine if similar trends in star formation rates are common among them.

The Space Telescope Science Institute is expanding the frontiers of space astronomy by hosting the science operations center of the Hubble Space Telescope, the science and mission operations centers for the James Webb Space Telescope, and the science operations center for the Nancy Grace Roman Space Telescope. STScI also houses the Barbara A. Mikulski Archive for Space Telescopes (MAST) which is a NASA-funded project to support and provide to the astronomical community a variety of astronomical data archives, and is the data repository for the Hubble, Webb, Roman, Kepler, K2, TESS missions and more. STScI is operated by the Association of Universities for Research in Astronomy in Washington, D.C.

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