In the 1990s, astronomers discovered two distant, massive galaxies that had completely stopped, or quenched, their star formation. The discovery marked a complete shift in everything astronomers thought they knew about how galaxies formed.
Massive galaxies like the Milky Way took several billion years to form. But those newly discovered galaxies did so in just a fraction of that time.
“The discovery meant that these galaxies were older than the age of the universe, which is physically impossible,” says Jacqueline Antwi-Danso, the NSERC Banting Postdoctoral Fellow at the David A. Dunlap Department for Astronomy & Astrophysics.
“When we look at the formation histories of these distant quenched galaxies, the observations suggest that they formed too quickly and too early compared to what we see in cosmological simulations.”
Antwi-Danso is tackling one of astronomy’s biggest challenges in her search to find the earliest distant quenched galaxies in the universe. She is particularly interested in how these galaxies formed and when they stopped forming stars.
We want to find galaxies that contain the first generations of stars.
Astronomers have discovered several more distant, quenched galaxies at increasingly earlier periods in the universe’s history. These galaxies are more massive than the Milky Way and yet formed within a billion years of the Big Bang (which happened nearly 14 billion years ago). In other words, they formed their stars extremely rapidly, unlike any galaxy observed in the present-day.
So, what does this all mean for astronomers? The extreme star formation processes implied by these observations of distant quenched galaxies are uncomfortably close to the limits permitted by galaxy formation physics. Therefore, trying to understand these objects in more detail is a high priority research area for astronomers.
The discovery of new galaxies, nearly 12.5 billion years away
Massive galaxies like the Milky Way have up to a trillion stars and are characterized by luminous, spiral-like arms of active star formation. Meanwhile, distant, quenched galaxies are composed of old stars and look like relics: small orange-red blobs. This is because their light has been “stretched out” to infrared wavelengths due to the expansion of the universe, which also makes them fainter and harder to spot.
At U of T, Antwi-Danso is building on significant findings from a study she participated in as a PhD student at Texas A&M University. Using the 8-meter telescope at the Gemini South Observatory based in Chile, the FENIKS collaboration surveyed large areas of the sky to increase the chances of finding these rare, massive galaxies. They designed and installed two new imaging filters on the telescope to push the boundary of what was possible with ground-based infrared telescopes. The survey led to two critical discoveries.
The first was the identification of two new distant quenched galaxies. The discovery confirmed existing knowledge about the formation histories of distant galaxies, “namely, that these galaxies form too early and too quickly based on what theory predicts,” Antwi-Danso explains.
The study also highlighted that astronomers can reliably use ground-based telescopes to observe distant quenched galaxies as far back as 12.5 billion years into history of the universe. To detect them at earlier times than this, space-based data is required.
Additionally, astronomers are rethinking long-standing models of galaxy formation as they observe distant quenched galaxies with supermassive black holes at their centers emitting energetic radiation. This is important, Antwi-Danso says, because the differing models for light emission from stars and supermassive black holes can affect estimates of the physical properties of these distant galaxies.
As more questions arise, there is an increasing need to ensure the accuracy of the physical properties of distant quenched galaxies derived from modeling their observations. Fortunately, there have been significant technological advancements to address this need.
Harnessing the power of space-based technology
The next stages of Antwi-Danso’s research involve further exploration of those two distant galaxies she discovered from Chile. To do so, she’s leveraging the power of the James Webb Space Telescope (JWST).
Distant galaxies are hard to detect because their emitted light is shifted to infrared wavelengths, where the earth’s atmosphere blocks most of the light. The sky in the infrared is about 10,000 times brighter than the typical distant massive galaxy. This makes it extremely difficult to detect the most distant quenched galaxies using ground-based telescopes.
The JWST — which launched in December 2021 — is about 100 times more sensitive than the largest ground-based infrared telescopes and can observe galaxies in a fraction of the time of its predecessors. In fact, it has doubled the number of spectroscopic observations of the most distant, quenched galaxies within only two years of operation. Before its launch, astronomers had spectra of only 35 of these galaxies observed within the first two billion years of the universe’s history.
To further observe those two galaxies, Antwi-Danso will use data from the JWST to examine their spectra — the light emitted by these galaxies over a range of wavelengths — which can reveal information like chemical composition. Insights will help provide a more accurate understanding of their formation histories to compare with updated cosmology simulations, and, hopefully, offer new answers about possible tensions between theory and observations.
Additionally, Antwi-Danso is part of the CAnadian NIRISS Unbiased Cluster Survey (CANUCS), a multi-institutional collaboration that uses gravitational lensing — a phenomenon where a massive object acts as a cosmic magnifying glass — to study the building blocks of the earliest galaxies. Within that collaboration, Antwi-Danso is also a researcher on the Technicolor Survey, which employs multiple filters on the JWST’s Near-Infrared Camera to observe quenched galaxies at wavelengths that are inaccessible from the ground.
“We want to find galaxies that contain the first generations of stars, and then model their observations with galaxy formation models to infer their physical properties and star formation histories,” Antwi-Danso says.
With the technological advantages provided by the JWST to push the boundaries of distant galaxy observations, Antwi-Danso’s research will provide valuable insight into understanding how early galaxies came to be.
“We’re really excited to see where the results lead and to compare those observations with current theoretical predictions for these distant massive galaxies.”