Using JWST to study dust-obscured galaxies halfway back to cosmic noon – and finding what slowed the Universe’s star formation.
Miriam Eleazer, an astronomical spectroscopist and a fourth year graduate student at UMass Amherst is trying to solve a mystery: ten billion years ago, the Universe experienced a “cosmic noon”; a peak in star formation. Since then, the cosmic star formation rate has continually and sharply declined. No one knows why. This cosmic noon and the subsequent decline in star formation can be clearly seen on the iconic Madau & Dickinson plot (shown below). Looking at this plot, astronomers, including Eleazer, think this mystery may be solved by studying galaxies during what is thought to be a transitional phase of galactic evolution, approximately halfway back to cosmic noon. Within this evolutionary phase, Eleazer focused their study on galaxies that formed stars hundreds of times faster than the Milky Way and were obscured by clouds of dust. They combined different astronomical diagnostics to establish a more contextual understanding of these galaxies' physical processes; “I investigated the kinematics because it informs the environment in which the stars can form.” Putting galaxies in context with their environment can help astronomers determine the reason for the decline of star formation post-cosmic noon across the Universe.
Eleazer first looked at spectral data of six target galaxies from the Mid-Infrared Instrument Medium-Resolution Spectrometer (MIRI/MRS) onboard the JWST telescope. These instruments constructed datacubes for each spatial pixel (‘spaxel’) in its field of view. When Eleazer first looked at these spectra, they noticed a double-peaked feature in the single spectrums of the galaxies, which hinted at potentially interesting dynamical processes occurring in them. To investigate further, Eleazer wrote a unique coding program to extract a spectrum for each spaxel, making kinematic maps that broke down, spatially, the velocities in the galaxy. From these maps, Eleazer hoped to gain clues about the galactic kinematic structures, as well as what process was the dominant support system for the galaxy: ordered rotation or chaotic disturbances. Eleazer directly compared these maps with images of the target galaxies from the Hubble Space Telescope (HST) (shown below) to identify which kinematic processes were happening. This could then provide insight into what is causing the change in rates of star formation seen during this evolutionary phase.
Eleazer expected to see disturbance-dominated systems for these galaxies; the types of galaxies being investigated are known to be major merger galaxies with chaotic kinematic processes. However, Eleazer’s maps showed the galaxies were rotation-dominated; they appeared ordered, like maturely settled disks. This was surprising because it’s assumed these galaxies had an actively accreting black hole in their centers injecting energy back into the galactic environment, which could then affect star formation. From the maps, though, there were no signs of major mergers and the kinematics, on the galactic scale, didn’t seem to be impacted by the active central black holes.
While Eleazer acknowledged the small sample size of galaxies do not allow a conclusive answer to be established about declining star formation, they do make it clear the results of the project provided astronomers with two incredible insights. This project was the first time this instrument mode was used to probe the kinematics of these more distant galaxies, pushing its capabilities literally to its limit. Additionally, this research showcased the power of combining kinematic and morphological characteristics of galaxies to contextualize physical processes inside them with the environment around them. For Eleazer, an approach to astronomical research like this is one they have been primed to embrace, given their non-traditional route through not only education, but also through life.
Before Eleazer became a PhD candidate at UMass Amherst, they first attended community college in Kentucky then transferred to a four-year school, and then earned their Masters in Astronomy at Wesleyan University. Before starting higher education, they were raised in southern rural poverty with the belief that the work they do now in the astronomy field would never be possible for them. “I am queer,” Eleazer says, “and growing up the way I did meant I had to find different ways to do things.” Eleazer’s approach to science today is built on a foundation of queer theory, taught to them when they were younger by older, college-aged siblings majoring in Gender Studies. This instilled in Eleazer a different perspective of science from others; a perspective Eleazer utilized at nearly every step of this research project; “[Queer theory] is exactly what I want to bring to my scientific research and to academic culture.” It's these differing perspectives that enabled Eleazer to readily queer astronomy research methods; namely, Eleazer used telescope instruments in new ways by pushing them to the limit, constructed wholly unique coding programs, and used multi-diagnostic analysis to gain a fuller context of galaxies and their environment.
With the publication of this research, Eleazer has concluded their initial research project at UMass Amherst and will continue working in the field of astronomy with an interest in spectroscopy and galactic contextualization. Eleazer is interested in using spectroscopy to “learn about the physical and chemical conditions within galaxies…to learn about the environment of the galaxy and situate [it] in its cosmic context to understand how galaxies evolve.” Eleazer especially enjoys the process of coming up with questions and finding methods that can lead to new discoveries. Eleazer also has an eye on continuing to push astronomy to new heights. “Lots of folks think things can never be other than what they are, but I think we are all learning that things change all the time. I hope, at the very least, I can help inspire others to imagine an academia based more in mutualism and that allows room for creativity, because we need to get all perspectives on an issue to really understand it.”
You can find Miriam Eleazer’s research paper here.
About the author:
Bradley Mills is a sophomore undergraduate at the College of Natural Sciences at UMass Amherst, studying Astronomy, with a concentration in Science Writing. Outside of their studies, they enjoy reading, writing, and playing Dungeons and Dragons.