Forming galaxies across cosmic time: My research interests focus on understanding the processes of the formation of galaxies (large collections of stars and dark matter) through the use of large surveys and statistics. I specialize in building and fitting flexible models to images and spectroscopy of galaxies. Galaxies are complex, so these models include the full range of things that live in the night sky: stars, black holes, gas, and cosmic dust. Understanding these components better often means writing down simple or more complex (numerical) models for how the full range of the galaxy population evolves as a whole, from the Big Bang to the present day. I’ve harnessed millions of hours of supercomputer time running specialized code to build a more complete picture of how galaxies form and evolve. I frequently use Bayesian statistics and machine learning (AI) to interpret the massive galaxy surveys of today and tomorrow; this simultaneously speeds up our models while also allowing them to become more sophisticated and complex.
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Publications
Joel Leja's research group
| Name | Role | Affiliation | Phone | Office Address | Affiliated Center(s) | Research Topics(s) | ||
|---|---|---|---|---|---|---|---|---|
 | Bingjie Wang | Postdoc | ICDS, Astronomy | bwang@psu.edu | +1 814 865 0418 | 520 Davey Laboratory | IGC | Cosmic Surveys |
Joel Leja's research group news
Discovery of Massive Early Galaxies Defies Prior Understanding of the Universe
2023-02-23
Six massive galaxies discovered in the early universe are upending what scientists previously understood about the origins of galaxies in the universe.
“These objects are way more massive than anyone expected,” said Joel Leja, assistant professor of astronomy and astrophysics at Penn State, who modeled light from these galaxies. “We expected only to find tiny, young, baby galaxies at this point in time, but we’ve discovered galaxies as mature as our own in what was previously understood to be the dawn of the universe.”
Using the first dataset released from NASA’s James Webb Space Telescope, the international team of scientists discovered objects as mature as the Milky Way when the universe was only 3% of its current age, about 500-700 million years after the Big Bang. The telescope is equipped with infrared-sensing instruments capable of detecting light that was emitted by the most ancient stars and galaxies. Essentially, the telescope allows scientists to see back in time roughly 13.5 billion years, near the beginning of the universe as we know it, Leja explained.
“This is our first glimpse back this far, so it’s important that we keep an open mind about what we are seeing,” Leja said. “While the data indicates they are likely galaxies, I think there is a real possibility that a few of these objects turn out to be obscured supermassive black holes. Regardless, the amount of mass we discovered means that the known mass in stars at this period of our universe is up to 100 times greater than we had previously thought. Even if we cut the sample in half, this is still an astounding change.
In a paper published today (Feb. 22) in Nature, the researchers show evidence that the six galaxies are far more massive than anyone expected and call into question what scientists previously understood about galaxy formation at the very beginning of the universe.
“The revelation that massive galaxy formation began extremely early in the history of the universe upends what many of us had thought was settled science,” said Leja. “We’ve been informally calling these objects ‘universe breakers’ — and they have been living up to their name so far.”
Leja explained that the galaxies the team discovered are so massive that they are in tension with 99% of models for cosmology. Accounting for such a high amount of mass would require either altering the models for cosmology or revising the scientific understanding of galaxy formation in the early universe — that galaxies started as small clouds of stars and dust that gradually grew larger over time. Either scenario requires a fundamental shift in our understanding of how the universe came to be, he added.
“We looked into the very early universe for the first time and had no idea what we were going to find,” Leja said. “It turns out we found something so unexpected it actually creates problems for science. It calls the whole picture of early galaxy formation into question.”
On July 12, NASA released the first full-color images and spectroscopic data from the James Webb Space Telescope. The largest infrared telescope in space, Webb was designed to see the genesis of the cosmos, its high resolution allowing it to view objects too old, distant or faint for the Hubble Space Telescope.
“When we got the data, everyone just started diving in and these massive things popped out really fast,” Leja said. “We started doing the modeling and tried to figure out what they were, because they were so big and bright. My first thought was we had made a mistake and we would just find it and move on with our lives. But we have yet to find that mistake, despite a lot of trying.”
Leja explained that one way to confirm the team’s findings and alleviate any remaining concerns would be to take a spectrum image of the massive galaxies. That would provide the team data on the true distances, and also the gasses and other elements that made up the galaxies. The team could then use the data to model a clearer picture of what the galaxies looked like, and how massive they truly were.
“A spectrum will immediately tell us whether or not these things are real,” Leja said. “It will show us how big they are, how far away they are. What’s funny is we have all these things we hope to learn from James Webb and this was nowhere near the top of the list. We’ve found something we never thought to ask the universe — and it happened way faster than I thought, but here we are.”
The other co-authors on the paper are Elijah Mathews and Bingjie Wang of Penn State, Ivo Labbe of the Swinburne University of Technology, Pieter van Dokkum of Yale University, Erica Nelson of the University of Colorado, Rachel Bezanson of the University of Pittsburgh, Katherine A. Suess of the University of California and Stanford University, Gabriel Brammer of the University of Copenhagen, Katherine Whitaker of the University of Massachusetts and the University of Copenhagen, and Mauro Stefanon of the Universitat de Valencia.
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Junyu Zhang selected as Eberly College of Science’s Fall 2022 Student Marshal
2022-12-14
Junyu Zhang of Chongqing, China, will be honored as the student marshal for the Eberly College of Science during Penn State’s fall commencement ceremonies on Saturday, Dec. 17, on the University Park campus.
Zhang will graduate with a 4.0 grade point average and bachelor’s degrees in astronomy, physics, and mathematics. He is a Schreyer Scholar in the Schreyer Honors College and has been a member of the Dean’s List every semester. Zhang has been honored with several awards and scholarships, including the John and Elizabeth Holmes Teas Scholarship from 2021 to 2022, the Bert Elsbach Honors Scholarship from 2020 to 2021, the Evan Pugh Scholar Senior Award in 2021, and the President’s Freshman Award in 2020.
“It is my great honor to be selected as student marshal for the Eberly College of Science,” he said. “I could not have achieved such a huge accomplishment without the support of my family, my mentors, my professors, and my friends. Therefore, I would like to share this honor with them.”
While at Penn State, Zhang conducted research with both Joel Leja, assistant professor of astronomy and astrophysics, and Niel Brandt, Verne M. Willaman Professor of Astronomy & Astrophysics and professor of physics. In his work with Leja, Zhang used a computer code called Prospector to investigate various wavelengths of light that are emitted by galaxies in order to identify markers that might distinguish galaxies that normally form stars from “rejuvenating galaxies” that fully quench—stop producing stars—and then become star-forming once more. Because rejuvenating galaxies are rare in the universe, these studies may help astronomers better understand the conditions under which stars and galaxies form. With Brandt, Zhang used data from X-ray surveys to identify active galactic nuclei found at the centers of some galaxies that have high redshift—the light they give off is skewed toward the red end of the spectrum due to their distance from Earth—and to better understand how they affect their host galaxies.
“The most important lesson that I will take with me from my time at Penn State is to follow your heart,” Zhang said. “One of the bravest decisions I ever made was to transfer to Penn State and change my major. Although I lost almost all three years I spent at my original university, I am now able to study what I love and achieve such an accomplishment like becoming a student marshal.”
After graduation, Zhang plans to continue his research in astronomy by pursuing a graduate degree.
Zhang is a graduate of Chongqing Nankai Secondary School in Chongqing, China. His parents are Li Mu and Kedi Zhang.
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Bright light from early universe 'opens new chapter in astronomy'
2022-11-30
UNIVERSITY PARK, Pa. — An unexpectedly rich array of early galaxies that was largely hidden until now has been observed by researchers using data from NASA’s James Webb Space Telescope.
The researchers found two exceptionally bright galaxies that existed approximately 350 and 450 million years after the big bang. Their extreme brightness is puzzling to astronomers and challenges existing models of galaxy formation.
“These objects are remarkable because they are far brighter than we would expect from our models of how galaxies form,” said Joel Leja, assistant professor of astronomy and astrophysics at Penn State, who developed the code used to analyze light from the distant galaxies.
As a postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics, Leja developed code capable of making sense of infrared data from distant galaxies, such as those imaged by Webb, proving that they are in fact our first glimpses of the very early universe.
“The code combines models of all the things that live in galaxies and interprets the light we observe from them,” said Leja. “This includes things like stars of various ages and elemental compositions, cosmic dust that blocks the light we see from stars, emission from gaseous nebulae, and so on.”
Two research papers, one led by Marco Castellano of the National Institute for Astrophysics in Rome, Italy, and another by Rohan Naidu of the Harvard-Smithsonian Center for Astrophysics and the Massachusetts Institute of Technology with Leja as co-author, have been published in the Astrophysical Journal Letters. The two papers describe the bright celestial objects, which both teams discovered separately in quick succession just days after Webb officially started science operations.
“With Webb, we were amazed to find the most distant starlight that anyone had ever seen, just days after Webb released its first data,” Naidu said in a NASA news release.
With just four days of analysis, the researchers found two exceptionally bright galaxies. They determined the young galaxies transformed gas into stars extremely rapidly, meaning the onset of stellar birth may have started just 100 million years after the big bang, roughly 13.8 billion years ago. The researchers also determined the two galaxies existed approximately 450 and 350 million years after the big bang, though future spectroscopic measurements with Webb will help confirm their findings.
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Tracing the Origins of Rare Cosmic Explosions
2022-11-30
The galactic homes of 84 short gamma-ray bursts (sGRBs)—produced by the collision of two neutron stars—have now been pinpointed in what is the most extensive inventory to date. A team that includes astronomers from Penn State used information from several highly sensitive instruments at the W.M. Keck Observatory, Las Campanas Observatory, MMT Observatory, and Gemini Observatory, combined with some of the most sophisticated galaxy modeling ever used in the field to identify the sources of these sGRBs.
“At most, astronomers might detect about a dozen short gamma-ray bursts each year, and so far only one has a confirmed origin from a neutron star merger,” said Joel Leja, assistant professor of astronomy and astrophysics at Penn State and a member of the research team. “Their mysterious origins have led us to take a lateral approach by instead trying to understand the environments where these objects thrive. The resulting catalog greatly expands our knowledge of where sGRBs come from and will help astronomers determine their true origins.”
SGRBs are momentary flashes of intense gamma-ray light emitted when two neutron stars collide. As an homage to the fact that sGRBs are among the brightest explosions in the universe, the team calls their catalog BRIGHT (Broadband Repository for Investigating Gamma-ray burst Host Traits) with all of their data and modeling products online for community use.
“This is the largest catalog of sGRB host galaxies to ever exist, so we expect it to be the gold standard for many years to come,” said Anya Nugent, an astronomy graduate student at Northwestern University who led the research, observational efforts with Keck and the MMT observatories, and one of the two publications on the study.
While the gamma-rays from sGRBs last only seconds, the optical light that is also produced in the event can continue for hours before fading below detection thresholds, called an afterglow. SGRB afterglows were first discovered in 2005 by NASA’s Neil Gehrels Swift Observatory, whose mission operations center is located at Penn State. Since then, astronomers have spent the last 17 years trying to find out which galaxies these powerful bursts originated from, as the stars within a galaxy can give insight into the environmental conditions needed to produce these events and can connect them to their neutron star merger origins. The only sGRB with a confirmed neutron star merger origin, GRB 170817A, was detected just seconds after gravitational wave detectors observed the binary neutron star merger, GW170817.
“One way to better understand gamma ray bursts is to understand the types of ‘cosmic ecosystems’ which they tend to live in, meaning the gas and stars in the environment around them—their host galaxies,” said Leja. “We modeled the galaxies where these gamma-ray burst happened, including information about how many stars are in the galaxy, how it formed, how many heavy elements are there, and how much dust obscures our view of the galaxy. This allowed us to understand how galaxies where gamma-ray bursts explode are different than other galaxies.”
Learning about sGRB host galaxies is crucial to understanding the blasts themselves and offers clues about the types of stars that created them as well as their distance from Earth. Since neutron star mergers create heavy elements like gold and platinum, the data will also deepen scientists’ understanding of when precious metals were first created in the universe.
“Building this catalog and finally having enough host galaxies to see patterns and draw significant conclusions is exactly what the field needed to push our understanding of these fantastic events and what happens to stars after they die,” said Nugent.
“In a decade, the next generation of gravitational wave observatories will be able to detect neutron star mergers out to the same distances as we do sGRBs today. Thus, our catalog will serve as a benchmark for comparison to future detections of neutron star mergers,” added Wen-fai Fong, assistant professor of astronomy and physics at Northwestern University and lead author of one of the publications.
The first paper in the study
, published in The Astrophysical Journal, found that sGRBs occur at earlier times in the universe and at greater distances from their host galaxy’s centers than previously thought. Surprisingly, several of these explosions were found just outside their host galaxies as if they were “kicked out,” raising questions as to how they were able to travel that far.
Published in the same journal, the second research paper
in the study probed the characteristics of 69 of the identified sGRB host galaxies. The findings suggest about 85 percent of them are young, actively star-forming galaxies — a stark contrast to earlier studies that characterized the population of sGRB host galaxies as relatively old and approaching death. This means neutron star systems may form in a broad range of environments and many of them have quick formation-to-merger timescales.
Many questions remain about how neutron stars merge and how long the process takes. But observing sGRBs and their host galaxies provides one of the best perspectives to answer them and can offer more data about neutron star mergers and their hosts at much farther distances, and more frequently, than current gravitational wave detectors. This new sGRB host catalog will therefore serve as a vital reference point in the coming decade to understand the full evolution of these systems over cosmic time.
The James Webb Space Telescope (JWST) is poised to further advance our understanding of neutron star mergers and how far back in time they began, as it will be able to detect the faintest host galaxies that exist at very early times in the universe.
“This field is so young— there is so much more to learn,” said Leja. “We hope that we can learn even more about the cosmic environment of sGRBs with near-future observatories, and in this way better understand what precipitates these mysterious explosions.”
In addition to Leja, the research team at Penn State also includes Derek Fox, associate professor of astronomy and astrophysics.
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Bright Light from Early Universe 'Opens New Chapter in Astronomy'
2022-11-21
UNIVERSITY PARK, Pa. — An unexpectedly rich array of early galaxies that was largely hidden until now has been observed by researchers using data from NASA’s James Webb Space Telescope.
The researchers found two exceptionally bright galaxies that existed approximately 350 and 450 million years after the big bang. Their extreme brightness is puzzling to astronomers and challenges existing models of galaxy formation.
“These objects are remarkable because they are far brighter than we would expect from our models of how galaxies form,” said Joel Leja, assistant professor of astronomy and astrophysics at Penn State, who developed the code used to analyze light from the distant galaxies.
As a postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics, Leja developed code capable of making sense of infrared data from distant galaxies, such as those imaged by Webb, proving that they are in fact our first glimpses of the very early universe.
“The code combines models of all the things that live in galaxies and interprets the light we observe from them,” said Leja. “This includes things like stars of various ages and elemental compositions, cosmic dust that blocks the light we see from stars, emission from gaseous nebulae, and so on.”
Two research papers, one led by Marco Castellano of the National Institute for Astrophysics in Rome, Italy, and another by Rohan Naidu of the Harvard-Smithsonian Center for Astrophysics and the Massachusetts Institute of Technology with Leja as co-author, have been published in the Astrophysical Journal Letters. The two papers describe the bright celestial objects, which both teams discovered separately in quick succession just days after Webb officially started science operations.
“With Webb, we were amazed to find the most distant starlight that anyone had ever seen, just days after Webb released its first data,” Naidu said in a NASA news release.
With just four days of analysis, the researchers found two exceptionally bright galaxies. They determined the young galaxies transformed gas into stars extremely rapidly, meaning the onset of stellar birth may have started just 100 million years after the big bang, roughly 13.8 billion years ago. The researchers also determined the two galaxies existed approximately 450 and 350 million years after the big bang, though future spectroscopic measurements with Webb will help confirm their findings.
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