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Luke Leisman and the Search for (Almost) Starless Galaxies

Luke Leisman has a very interesting job. The Grand Rapids native, now a professor of physics and astronomy at Valparaiso University in Indiana, uses some of the largest radio telescopes on earth to study faint galaxies with only a smattering of stars, and he does so with students who share his delight in a sense of discovery. The Calvin College graduate, who went on to receive a PhD from Cornell University, is also notable as a recipient of the prestigious Roger B. Chaffee Scholarship upon graduation from Grand Rapids Christian High School in 2007.

In a recent phone conversation, Leisman said that he was drawn to a question that has puzzled astronomers for a long time when he arrived at Cornell for graduate studies in 2011. Where is the "dark matter" that astronomers know is out there somewhere but cannot be seen? Could some of it be locked up in galaxies that do not contain enough stars to be picked up by optical telescopes?

Leisman wondered "Are we missing something?" He set out with others in a search for these presumably faint objects, looking for a signature coming not from visible light but through another form of radiation that sends us messages from the cosmos: radio waves.

He had a chance to be part of a team of students and faculty from Cornell and other universities participating in a systematic search for these elusive objects using a giant radio dish antenna (more than 3 football fields across!) embedded in a mountain in Puerto Rico.

"One way for a galaxy to hide in visible light is when its component stars are spread out over such a large area that they are impossible to detect with cameras attached to optical telescopes," he noted. These so called "ultra-diffuse galaxies" have very few stars, and if an inhabited planet existed within such a system, "Its night sky would be really boring," he said. "There would be few bright stars to form constellations and just a scattering of many fainter ones."

Ultra-diffuse galaxy captured by Hubble Space Telescope in 2017 lies 65 million light years away. Star density is so low that galaxies clustered in background can be seen through it.

Some of these objects would be expected to contain low temperature hydrogen clouds that are too cold to glow but which do emit radio waves, allowing detection with radio telescopes. Searches conducted by Leisman and others worldwide over the past few years have proven fruitful, as more and more ultra-diffuse galaxies are now turning up.

The ongoing search is exciting, but also puzzling. Astronomers are trying to figure out where these objects fit into the population of galaxies of all kinds. Their portion of the galactic census could tell them where some of that mysterious dark matter is located, and also provide insight into what was going on during the earliest history of the universe, billions of years ago.

Are these young galaxies in formation, where stars will eventually form from the cool hydrogen revealed by radio telescopes, or are they instead old galaxies that never had many stars in the first place? Or perhaps long ago had their stars stripped away through gravitational encounters with a much larger galaxy nearby?

A breakthrough in understanding could be coming, because thanks to the growing number of radio astronomy detections, optical astronomers now know where to point their instruments in a search for low density galaxies that may be feebly glowing in visible light.

Recently, an exceedingly faint but surprisingly large ultra-diffuse galaxy was picked up in an optical telescope pointed toward a suspicious radio source in the constellation Leo. Leo "P" (for pristine) is indeed a thinly populated cluster of dim stars, possibly held together by the gravitational influence of embedded dark matter.

Leisman said that these galaxies may be fairly common, and that radio astronomy will continue to be an important tool, not only for finding more of them, but also for studying the motions of invisible gas clouds inside them. Such studies could yield clues about how much dark matter each contains.

Leisman delights in being involved in pursuit of better understanding of elusive ultra-diffuse galaxies. "When I arrived at Cornell, I had already been studying galaxies at Calvin. Now, I had before me an opportunity to participate in a systematic search for galaxies emitting radio waves with the faculty and other students. I really wanted to work with Martha Haynes," he said of the enthusiastic astronomer leading the research group at Cornell.

"I was also drawn to radio astronomy because you can in a sense 'see' through radio waves things you cannot really see with your eye, plus you get to work with some of the largest telescopes on earth! There is something really cool and exciting about using this alternate set of tools," he said.

He went on to say that because radio telescopes inherently have lower resolution than optical telescopes (they cannot pinpoint sources with the same accuracy) radio observers have to come up with clever tricks to get around that, like using multiple telescopes all pointing in the same direction from different locations around the earth.

"We also have reasonable hours and don’t have to worry about weather," he mused. Radio waves coming from space can penetrate through clouds, and unlike the most distant sources of visible light, are accessible both day and night.

Leisman says that since coming to Valparaiso in 2017, he has gained personal satisfaction from mentoring undergraduate physics and astronomy students, recalling how Haynes and other faculty at both Cornell and Calvin had been helpful to him. "I am continually impressed by my students and how excited they can become about things; the 'aha moment,' of seeing or knowing something for the first time."

Leisman appropriately concluded our conversation by noting that "Studying the universe gives us as humans a sense of place, a sense of awe. The subjects we seek to understand are so far away, but we can still learn about them through clever techniques, like radio astronomy."

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