2023 Year Book | Carnegie Scientists reveal the cosmos with JWST

Carnegie Science astronomers have showcased the institution’s creativity and propensity for bold research ideas using JWST. Since the space telescope began taking research observations in the summer of 2022, Carnegie-affiliated scientists have led 13 projects, which were selected across three cycles of time allocations. These initiatives represent the breadth of astrophysical research at the Carnegie Science Observatories and the Earth and Planets Laboratory, ranging from probing star formation in ancient galaxies to elucidating exoplanet atmospheres.  

Foundational Role

Carnegie Science has been a part of JWST’s story since its inception. In the early 1990s, Observatories astronomer Alan Dressler was asked by the Associated Universities of Research in Astronomy, or AURA, to head up a committee of astronomers that would come up with questions that technological advances could help answer in the coming decades.

The group recommended an “Origins” program for  what Dressler described as a “once-in-a-species” opportunity to witness “cosmic cradles.” They indicated that this would require a space telescope bigger than Hubble that could maintain an extremely cold temperature and would be positioned very far from Earth. This would enable it to use infrared cameras to observe our cosmic beginnings.  

“We are understanding our own place in the universe and how we came to be,” said JWST Senior Project  Scientist Jane Rigby, who was a Carnegie postdoc between 2006 and 2010. She shared her JWST story with a packed house at The Huntington Library, Art  Museum, and Botanical Gardens in the spring of 2023  as part of the Observatories’ Astronomy Lecture Series.  “The Webb telescope was built to do four things and  we showed that it could do all four of those on the first  day that we released images.” 

Galaxies

Observatories astronomer Andrew Newman has had two projects on star formation in ancient galaxies selected for JWST telescope time. In the first round of allocations, he led a project that pointed the space telescope at a galaxy about 10 billion light-years from us in order to probe the long-standing question of why some galaxies stopped forming stars very early on, even though the universe at the time was a very active place and most galaxies were just bursting with star formation.  

Then, earlier this year, he and recently departed postdoc Meng Gu—who started a new position at the University of Hong Kong—were selected to use  JWST to study four giant galaxies from the early universe that grew very quickly for 1 to 2 billion years and then suddenly stopped. The ejection of gas from these galaxies—potentially accelerated by enormous black holes at their centers—could be the key to understanding why star formation ended. If Newman and Gu are able to map this gas and measure the masses of these black holes, it could provide a major breakthrough in understanding how galaxies evolve. 

Observatories astronomer Gwen Rudie also used JWST to probe star formation in early galaxies. She and former Observatories postdoc Allison Strom—now a professor at Northwestern University—used JWST to observe a carefully selected set of 33 “teenaged” galaxies that experienced remarkable growth in the universe’s youth.

In 2023, they shared their findings, revealing that galaxies that formed just 2 to 3 billion years after the  Big Bang are unusually hot and glow with light from surprising elements like nickel. They accomplished this by taking spectra of these distant galaxies— separating their light into its component wavelengths.

Looking at the light in this way helps astronomers measure the temperature and chemical composition of cosmic sources. By averaging together spectra from all 33 galaxies, they were able to create an atlas of sorts that will inform future JWST observations of very distant objects. 

Using the spectra, the researchers were able to identify eight distinct elements: Hydrogen, helium, nitrogen,  oxygen, silicon, sulfur, argon, and nickel. Although their presence was not a surprise, JWST’s ability to measure them was. By revealing the presence of certain elements in these early galaxies, astronomers like Rudie and Strom can learn about how star formation changes over the course of their evolution.   

Exoplanets

On the exoplanetary side of things, Earth and Planets Laboratory astronomer Johanna Teske has had three JWST proposals selected. Most recently, she and current postdoc Nicole Wallack were chosen for an archival project that is seeking to account for and eliminate sources of “noise” in observations of small exoplanet atmospheres that could be caused by JWST’s suite of highly engineered instruments.  

This was based on Wallack’s work with the data from  Teske’s first round of JWST observations, which really pushed the space telescope’s capabilities to their limits. In cases like theirs—looking for the signals from small planets—it’s absolutely crucial to glean every drop of information from the data to confidently answer questions about which small planets have atmospheres, and what are their compositions.  

Wallack worked very hard to accomplish this with the observations in Teske’s previous JWST program, co-led with Natasha Batalha of NASA Ames, which aimed to improve our understanding of the most common type of planets in the Milky Way—called super-Earths or sub-Neptunes. But now, after the first two cycles of JWST, there will be almost 30 such small planet atmosphere datasets taken by the entire community,  which will be publicly available for further analysis.  Their plan is to conduct a deep dive into all of this information to both potentially improve the precision, as well as look for trends in the “noise” properties in data that could be useful for planning future observations.  

Earth and Planets Laboratory astronomer Peter Gao also led two projects that deployed JWST to probe exoplanet atmospheres. The first focused on understanding a rare type of ultra-low-density planets, which have masses of only a few times that of Earth, but sizes like those of the giant planets in the Solar System.  

For his Cycle 2 program, Gao undertook a theoretical project to help interpret JWST data related to photochemical hazes. These are structures in planetary atmospheres composed of small particles that originate from photochemistry or being  “pulverized” by ultraviolet light from the host star.  Previous observations have shown that many of the exoplanets that JWST will study are hazy. So, Gao’s work will be useful to astronomers who are looking to better understand their data about these planets or to put it into models.  

Teske, Wallack, and Gao are all members of the revolutionary space telescope’s Transiting Exoplanet Community Early Release Science Team—along with several other current and past Carnegie postdoctoral fellows. This group has made many exoplanet breakthroughs, including detecting carbon dioxide,  water vapor, and photochemistry-derived sulfur dioxide in the atmospheres of distant worlds.

Looking Ahead

The infrared instruments that are enabling Carnegie  Science researchers to conduct exciting experiments using JWST will be complementary to the instrument suites being developed for the next generation of ground-based telescopes, including the Giant Magellan Telescope (GMT), currently under construction at the Carnegie Science’s Las Campanas Observatory. 

The difference in mirror size between a space-based and ground-based telescope will enable astronomers to use GMT to characterize the first stars and galaxies detected by JWST in greater detail. Likewise, optical capabilities of GMT—as compared to the infrared instruments deployed on JWST—will enhance astronomers’ capacity to detect biosignatures such as oxygen in exoplanet atmospheres.