Leo P, a small galaxy and a distant neighbor of the Milky Way, is lighting the way for astronomers to better understand star formation and how a galaxy grows.
In a study published in the Astrophysical Journal, a team of researchers led by Kristen McQuinn , a scientist at the Space Telescope Science Institute and an associate professor in the Department of Physics and Astronomy at the Rutgers University-New Brunswick School of Arts and Sciences, has reported finding that Leo P "reignited," reactivating during a significant period on the timeline of the universe, producing stars when many other small galaxies didn't.
By studying galaxies early in their formation and in different environments, astronomers said they may gain a deeper understanding of the universe's origins and the fundamental processes that shape it.
McQuinn and other members of the research team studied Leo P through NASA's James Webb Space Telescope, a space-based apparatus that features a large, segmented mirror and an expansive sunshield, both of which enable it to capture detailed images of distant celestial objects.
Leo P, a dwarf galaxy some 5.3 million light years from Earth, was discovered by McQuinn and other scientists in 2013. The celestial structure is far enough away from the Local Group, a clump of galaxies straddling the Milky Way, to be its neighbor without being affected by the gravitational fields of larger star systems.
The galaxy, located in the constellation Leo, is about the same size as a star cluster within the Milky Way and is about the same age as the Milky Way. The "P" in Leo P refers to "pristine," because the galaxy has so few chemical elements beside hydrogen and helium.
"Leo P provides a unique laboratory to explore the early evolution of a low-mass galaxy in detail," said McQuinn, who also is the mission head for the Science Operations Center for the Nancy Grace Roman Space Telescope at the Space Telescope Science Institute in Baltimore.
The team started by looking deeply into the past. Since the stars detected by the team with the telescope are about 13 billion years old, they can serve as "fossil records" of star formation that occurred at earlier times. "Essentially, instead of studying the stars in-situ [in their original positions] as they are forming in the early universe, we study the stars that have survived over cosmic history and use their present-day properties to infer what was occurring at earlier times," McQuinn said.
The team found that Leo P formed stars early on but then stopped making them for a few billion years. This stoppage happened during a period known as the Epoch of Reionization. It took a few billion years after the epoch for the galaxy to reignite and start forming new stars.
"We have a measurement like this for only three other galaxies – all isolated from the Milky Way – and they all show a similar pattern," McQuinn said.
Observations of the dwarf galaxies within the Local Group, however, show that, in contrast, star production disappeared during this period.
The Epoch, regarded by astronomers as a significant period in the history of the universe, occurred between about 150 million and one billion years after the Big Bang. It was during this period that the first stars and galaxies formed.
The contrast between the star production of the dwarf galaxies provides compelling evidence that it isn't just the mass of a galaxy at the time of reionization that determines whether it will be quenched, McQuinn said. 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 how the reionization of the universe may have impacted how small structures form.
"If the trend holds, it provides insights on the growth of low-mass structures that is 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 metallicity. This means that the stars of the dwarf galaxy contain 30 times fewer heavy elements than the sun, which makes Leo P similar to the primordial galaxies of the early universe.
Knowledge gleaned from these observations will help astronomers piece together the timeline of cosmic events, understand how small structures evolved over billions of years and learn about the processes that led to the creation of stars, McQuinn said.
Other scientists from Rutgers on the study included Alyson Brooks, an associate professor; Roger Cohen, a postdoctoral associate; and Max Newman, a doctoral student, all with the Department of Physics and Astronomy.
