Postdoctoral Fellows Shine at McWilliams Center for Cosmology and Astrophysics

In the grand dance of the cosmos, stars are born, live their lives and eventually die. Sometimes they collide, sending ripples through the fabric of space and time. Other times, a galaxy’s gravitational pull may stretch stars, creating glittery trails in the night sky.

No matter what their fate, stars hold a lot of clues to solving some of the biggest mysteries of the universe — from stellar evolution to the nature of dark matter. At Carnegie Mellon’s University’s McWilliams Center for Cosmology and Astrophysics, researchers are working to unravel some of these puzzles.

Ignacio Magaña Hernandez, Brendan O’Connor, Anna O’Grady and Nora Shipp joined the McWilliams Center as McWilliams Postdoctoral Fellows. One of the center’s highest profile programs, the fellowship, a gift from the late Bruce McWilliams, provides funds to support promising scientists pursuing independent research.

“It’s a prestigious fellowship, and their success has been tremendous. They help make advancements possible,” said Tiziana DiMatteo, professor of physics and the center’s director. “Postdocs ignite the place with new ideas and excitement.”

Ignacio Magaña Hernandez

For nearly a century, astronomers have known that the universe is expanding at a rate referred to as the Hubble constant. But the two main ways to determine its value have produced very different results. Ignacio Magaña Hernandez is using a third way — gravitational waves from merging binary black holes.

When massive objects like black holes or neutron stars collide, they release gravitational waves and, in the case of neutron stars, big bursts of light. Details from these two observations enable astrophysicists to calculate the Hubble constant. Binary black hole mergers are more frequently detected than neutron star collisions, making studying black hole mergers more advantageous. The trouble is, when black holes collide, there is no second electromagnetic signal to constrain the cosmology.

“Binary black hole mergers are electromagnetically dark — they’re called dark sirens — so we need additional information to infer their redshift,” Magaña Hernandez said.

A common method is to use the information in galaxy surveys to determine a merger’s redshift. But surveys are not complete and are usually biased toward observing the brightest and most massive galaxies. Other studies have performed a ‘completeness correction’ to correct for the missing matter.

Magaña Hernandez is taking it one step further. He’s taking into account galaxy clustering and the observed large-scale structure of the universe to better model the correction, which should offer a more powerful method to constrain the Hubble constant. Magaña Hernandez is also using his survey analysis method to glean details about the astrophysical origin of the black holes and to understand which types of galaxies are most likely to host these kinds of mergers.

As a graduate student at the University of Wisconsin-Milwaukee, Magaña Hernandez joined the LIGO Scientific Collaboration and was part of the main analysis team for the second observing run, where he co-developed gwcosmo, a python-based package that estimates the Hubble constant from binary black hole mergers cross-correlated with galaxy surveys.

For Magaña Hernandez, Carnegie Mellon is the ideal place to continue his research.

“CMU is well known for its Department of Statistics and Data Science, which allows me to tap into the latest statistical and machine learning algorithms that might be required to realize my research in a practical and scalable way,” he said.

Brendan O’Connor

Brendan O’Connor studies massive but brief cosmic explosions that release huge amounts of energy in only a second or two. He’s particularly interested in short gamma-ray bursts (GRBs), bright flashes of light that occur when two neutron stars collide, merge and become a new black hole. Satellites orbiting Earth detect and pinpoint the direction of the burst, and astronomers hurry to point their telescopes in the right direction to observe the GRBs’ emission across the light spectrum.

“There are a lot of questions that we can address by studying these little blips of gamma rays, from stellar evolution and star formation to the production of heavy elements in the universe,” O’Connor said. “They're really cool objects.”

As a graduate student at George Washington University, O’Connor led a study of GRB 221009A — the most energetic and brightest GRB ever seen. Unlike short GRBs, this burst lasted for several minutes with its afterglow lasting for months. Using the Gemini South telescope, O’Connor examined the opening angle of the GRB jet, providing information about the process that emits the gamma rays.

In addition to releasing a burst of gamma rays, the collision of two neutron stars also sends out a blast of gravitational waves and electromagnetic radiation called a kilonova. O’Connor was drawn to Carnegie Mellon’s McWilliams Center’s burgeoning leadership in multi-messenger astronomy, an approach that analyzes cosmic events by combining data from different signals such as light, particles and gravitational waves. He is working with faculty who are experts in investigating gravitational waves and searching for kilonova emissions.

Along with studying GRBs that originate from outside of our galaxy, O’Connor studies transient sources within the Milky Way. He is a lead member of the Swift Deep Galactic Plane Survey, which is systematically searching the Milky Way for faint X-ray sources. O’Connor and his team spent 22 days using the X-ray telescope on-board the space-based Neil Gehrels Swift Observatory, during which they discovered 348 previously uncatalogued X-ray sources.

Anna O’Grady

Anna O’Grady studies a type of massive star called yellow supergiants. These stars exist for brief periods of time but they hold vital clues about stellar evolution. Over the past decade, astronomers have realized that the majority of massive stars — including yellow supergiants — exist in binary systems, fundamentally changing our understanding of how these celestial objects live and die.

“If you have two stars that are close to each other, they might start interacting, and that completely and utterly changes how those stars will evolve,” said O’Grady.

While other types of stars in binaries have been extensively studied, yellow supergiant binaries represent uncharted territory. Using archival data and her own observations from ground-based telescopes, O’Grady has identified a group of candidate yellow supergiant binaries that exhibit an excess of blue light that hints at the presence of a smaller, hotter companion star. She has her sights on the Hubble Space Telescope, hoping to secure ultraviolet data that will allow her to characterize these companion stars in unprecedented detail.

O’Grady works closely with theorists like Assistant Professor Katie Breivik, who creates simulations to understand how binary-star interactions shape stellar populations as they evolve. O’Grady’s observational investigations are often informed by the questions theorists ask, so being a part of the McWilliams Center, where faculty members are pushing into new realms of astronomy, was a big draw for her.

Although she studies yellow supergiants now, as a graduate student at the University of Toronto, O’Grady led a research study that confirmed the existence of a type of star called super asymptotic giant branch stars, or super-AGB stars. This class of stars exists between smaller stars like our Sun that eventually die out as nebulae, and massive stars that explode into supernovas. Studying them may shed slight on which stars will explode as supernova.

O’Grady recently returned to her undergraduate alma mater, the Memorial University of Newfoundland, to discuss searching for these new stars in the 2024 Elizabeth R. Laird Public Lecture.

Nora Shipp

Glittering threads of stars orbit the Milky Way, the remnants of satellite galaxies and star clusters that are stretched and disrupted by its gravitational forces. Nora Shipp studies these stellar streams, which provide critical insight into dark matter and the physics governing galaxy formation.

Evidence suggests that a halo of dark matter surrounds the Milky Way. When clumps of dark matter interact with stellar streams, it disrupts their gravitational dynamics and changes their observed appearance, causing kinks or gaps in the starry trails, in effect tracing out how dark matter is distributed at the outskirts of our galaxy and providing a way to study the otherwise invisible dark matter.

As a graduate student at the University of Chicago, Shipp discovered a number of new stellar streams in data from the Dark Energy Survey. She also uses data from the Gaia satellite, which provides measurements of how the stars in the Milky Way are moving in the sky, and the Southern Stellar Structure Spectroscopic Survey, which maps stellar streams with the Anglo-Australian Telescope. By combining those data sets, Shipp is working to characterize and model streams to learn about the local distribution of dark matter.

Picking out potential streams by eye is a labor-intensive process, so the upcoming Rubin Observatory Legacy Survey of Space and Time (LSST) will be a boon to Shipp’s research.

“The large volume of data Rubin will gather presents an exciting opportunity to think of new, more automated ways to identify streams,” said Shipp, also a National Science Foundation Postdoctoral Fellow. “With Rubin Observatory, we’ll be able to take indirect measurements of the Milky Way’s dark matter clumps down to masses lower than ever before, giving us really good constraints on the particle properties of dark matter.”

Shipp is the co-convener of the Dark Matter Working Group in the Rubin Observatory/LSST Dark Energy Science Collaboration, bringing together astronomers, including McWilliams Center faculty, who use different types of astrophysical observations to learn about dark matter.

Shipp chose to spend a year at the McWilliams Center before beginning her faculty position as an assistant professor at the University of Washington.

“I’m really excited about studying dark matter with large surveys like LSST,” Shipp said. “There are a lot of people here at CMU thinking about LSST and other surveys, and also thinking about using observations of satellite galaxies to constrain dark matter, so it’s a good intersection of my different science interests.”