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Person to Person: Caryl Gronwall

18 March 2021

Editor's Note: This story accompanies the Science Journal feature article "HETDEX and the 'Quest for the Rest.'"


While everyone in the HETDEX collaboration appears to wear more than one hat, Caryl Gronwall also wears several outside that particular project.

She specializes in studying star formation and the evolution of both nearby and distant galaxies, using ground- and space-based observatories and looking at a wide range of wavelengths (including X-ray, ultraviolet, and visible light). Gronwall is also a founding member of HETDEX, leading the project’s study of high-redshift galaxies known as Lyman-alpha emitters (LAEs)—young, distant star-forming galaxies that are thought to be the progenitors of galaxies like our own Milky Way.

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Caryl Gronwall in Stockholm, Sweden, where was attending a conference on Lyman-alpha-emitting galaxies.
Caryl Gronwall in Stockholm, Sweden, where was attending 
a conference on Lyman-alpha-emitting galaxies.

“I’ve been studying high-redshift galaxies since grad school,” she says, “but I started studying LAEs when Robin Ciardullo and his grad student John Feldmeier asked me where to point the telescope to figure out more about these background galaxies that were contaminating their planetary nebulae samples, emitting at the same wavelength. At the time, around 2002, Lyman-alpha-emitting galaxies were just being discovered in large numbers, so it was an interesting, new area of research that was opening up.”

LAEs are also a focus of the Multiwavelength Survey by Yale-Chile (known colloquially as the MUSYC Collaboration), where Gronwall leads their study, as well. “Lyman alpha emitting galaxies tend to be younger, probably forming their first generations of stars, and so they're sort of building blocks of today's galaxies,” she explains. “We want to know where they are, what types of environments we find them in, how they are formed, how they grow up to be bigger galaxies like the Milky Way.”

HETDEX, in turn, will provide Gronwall and others with a massive dataset on these galaxies. “Overall, we have a sample of 1,000 or 2,000 Lyman-alpha-emitting galaxies, and HETDEX is going to give us about a million,” she says. “So, basically, all the cool science you could do with a few thousand you can do again with a few million, but then you can also start to address questions that you couldn't really answer because you didn't have a big enough sample.”

Using the Neil Gehrels Swift Observatory, Gronwall has also been observing nearby star-forming galaxies, and she is developing a unique synergy between spectroscopic data collected by HETDEX’s VIRUS instrument and observations with Swift’s ultraviolet/optical telescope (UVOT). “It turns out that the VIRUS instrument observed a nearby galaxy, M101, a very pretty spiral galaxy called the Pinwheel Galaxy, so now we have this great emission-line data of M101,” she says. “And a graduate student who's working with me, Alex Belles, will observe with the Swift UVOT and combine the ultraviolet data from UVOT with the emission-line information from VIRUS to study star formation properties and extinction.”

This breadth of interests and diverse array of projects, all seeming to converge and inform one another, begs the question of how Gronwall arrived at such a confluence, what motivated her to become an astronomer in the first place and to study the things she does.

“Well,” she says, “I didn’t know that I wanted to be an astronomer since I was child, but I was always interested in science. Both of my parents were physics majors as undergrads, and that was pretty unusual in the mid-1960s for women like my mom! But I was always encouraged in math and science. I majored in physics as an undergrad and took a few astronomy classes, and I decided that astronomy was more interesting to me than pure physics—and one needs a strong physics background to study astronomy. I was able to get some research experiences as an undergrad in astronomy, which led to me going to grad school in astronomy.” Finding her research focus, she notes, was “more serendipitous.”

The rest, as they say, is history.