(photo by Andy Jibb)
Everything we thought we knew about galaxy formation was thrown into question in the 1990s after astronomers discovered two distant, massive galaxies that had completely stopped - or "quenched" - their star formation.
"The discovery meant that these galaxies [had to be] older than the age of the universe, which is physically impossible," says Jacqueline Antwi-Danso, the NSERC Banting Postdoctoral Fellow at University of Toronto's David A. Dunlap department for astronomy and astrophysics in the Faculty of Arts & Science.
"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."
Unlike familiar massive galaxies like the Milky Way, which have up to a trillion stars and are characterized by luminous, spiral-like arms of active star formation, distant, quenched galaxies are composed of old stars and look like 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.
Moreover, the distant galaxies in question 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.
Better understanding these distant galaxies is a high priority for researchers since their extreme star-formation processes are uncomfortably close to the limits permitted by current galaxy formation physics.
At U of T, Antwi-Danso is hunting the earliest distant quenched galaxies in the universe and is particularly interested in finding out how these galaxies formed and when they stopped creating stars. She is building on findings from a study she participated in as a PhD student at Texas A&M University that led to two critical discoveries. The first was the identification of two new distant quenched galaxies that confirmed current thinking on how these distant galaxies formed - "namely," Antwi-Danso says, "that these galaxies form too early and too quickly based on what theory predicts."
The study - which used the 8-meter telescope at the Gemini South Observatory based in Chile and surveyed large areas of the sky with new imaging filters - also highlighted that astronomers can reliably use ground-based telescopes to observe distant quenched galaxies as old as 12.5 billion years. Detecting galaxies any earlier than this requires space-based data, the researchers say.
Astronomers are now rethinking long-standing models of galaxy formation as they observe distant quenched galaxies with supermassive black holes at their centres that emit 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.
Harnessing the power of space-based technology
Distant galaxies are difficult to detect because the light they emit is shifted to infrared wavelengths, which is mostly blocked by the Earth's atmosphere. So, the next stages of Antwi-Danso's research will leverage the power of the James Webb Space Telescope (JWST).
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.
To further observe the two distant galaxies she discovered from Chile, Antwi-Danso will use JWST data to examine their spectra - the light emitted by these galaxies over a range of wavelengths - to reveal information like chemical composition. These and other findings will help provide a more accurate understanding of the galaxies' formation histories and can be compared with updated cosmology simulations. That, in turn, may yield new insights about potential tensions between theory and observations.
Antwi-Danso is also 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 insights 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."
