Jacqueline Antwi-Danso …NSERC Banting Postdoctoral Fellow at the David A. Dunlap Department for Astronomy & Astrophysics

 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.

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.

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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.

Massive galaxies like the Milky Way have up to a trillion stars and are characterised 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-metre 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.”