Hunting Down Correlations Between Star Formation and Gas Turbulence: The Thesis Work of Laura Congreve Hunter

When I loaded my Zoom application on my computer that Thursday, I had no idea what to expect. It was my first ever interview as a science writer, so in typical fashion, I decided to just go with the flow and, like any good scientist, ask questions along the way. I vaguely knew that Laura Congreve Hunter’s research related to mine because she worked with IU astronomer Dr. Liese van Zee. I knew from my own summer research that Dr. van Zee worked with galaxies like my own mentor, Dr. John Salzer, did, so I could only assume that Laura did the same. Aside from that, I knew little more than that we would talk about New Mexico’s Very Large Array (VLA), which I knew Laura had recently taken observations with. I was daunted at the task of absorbing so much information, but I buckled in for the ride anyways. I had no idea as to how much my mind would be blown away in a field I already knew a lot about compared to the average citizen.

I met Laura during the fall of my junior year of my undergraduate career with Indiana University studying Astronomy. Back then, she was a first year graduate student in the astronomy department…rarefied air for astronomy enthusiasts in an era where increasingly, there were more astronomy students than graduate positions available. A self-proclaimed extrovert, I naturally introduced myself to her like I had to many of the other department members and found that we had several interests in common besides the obvious love of things beyond the Earth’s atmosphere. The department had always been a close-knit one, so like with Laura, I had easily gotten to know many of my peers, professors, and graduate students. Because of this close-knit relationship between members of the department, it had always been fairly easy to celebrate the success of others. As such, one day, while casually browsing through some department news, I saw that Laura had been awarded time on the VLA. To think that someone not that much older than myself could successfully solicit time from a well-known telescope was mind-blowing. I reached out to Laura, hoping to learn all about her opportunity first hand, and she thankfully agreed to share about her work and experiences.

First things first, you may be wondering: what is the VLA? In the astronomical community, it is a well-known telescope, but to the general public, maybe not so much. For those in my audience who are unfamiliar with the VLA, the VLA is a radio telescope in Socorro, New Mexico–little more than an hour from the New Mexican metropolis of Albuquerque. Despite its identity as a telescope, if a casual observer were to happen upon the VLA, they might not recognize it as a telescope. For one, the VLA doesn’t look like the telescope of your average backyard astronomer. There’s no long optical tube, no tripod, and definitely no eye piece. Rather than looking like a classic optical telescope, the antenna of the VLA are more like old-fashioned radio dishes literally a la the 1997 science fiction film Contact. But that’s not the only way the VLA doesn’t look like a classic telescope. As hinted at by my use of “radio dishes” in the plural, the VLA isn’t just one radio dish: there are actually twenty-seven radio dishes total arranged in a y-shaped configuration…hence the name, “array”. These twenty-seven radio dishes can be moved closer or farther apart with a minimum array size of about 1 km across and a maximum array size of about 36 km across. The combination of many radio dishes plus the size of this array effectively makes the VLA one giant telescope. This is in stark contrast to most modern telescopes as many telescopes around the world are mere meters across. This greater telescopic “diameter” of the VLA allows for a better resolution of astronomically small objects which is incredibly important given the types of galaxies in Laura’s sample.

An aerial view of the VLA. Image Credit: NRAO

Laura’s VLA sample was primarily gas-rich dwarf galaxies in the local universe. This description means that these galaxies have lots of neutral atomic hydrogen, are smaller than normal galaxies, and are within 5 Mpc (or for a more digestible unit of distance, roughly 16 million light years) of the Milky Way galaxy. For her graduate research, she chose to do a multiwavelength study of 45 such galaxies to analyze the correlation between recent star formation and turbulence in the interstellar medium (ISM). To do this, she has to study the neutral atomic hydrogen–or HI (pronounced “H one”), as astronomers call it–and its movements because HI is considered a marker of a region’s potential to form stars due to HI clouds being reserves of material for active star formation.

A scientific image showing where the HI is in the dwarf galaxy UGC 6456. Image Credit: Laura Congreve Hunter

This is where the VLA comes in. In astronomy, it is common knowledge that different ions and molecules emit light at different wavelengths. HI emits light at the wavelength of 21 cm which is in the radio part of the electromagnetic spectrum. Since 21 cm is within the  VLA’s wavelength range, this makes the VLA well suited for observing HI clouds. In order to study the turbulence of the HI, Laura and her collaborators created 2-dimensional images called moment maps. The 0th moment is a map of where the HI is in the galaxy. The 1st moment is a map of how the HI in question is moving. The 2nd moment is a map of the velocity dispersion of the said HI. If the idea of ‘moments’ is confusing, think of this simple analogy from basic physics: The 0th moment is similar to displacement. As we learn in basic physics, displacement quantifies how far an object is from the origin of the reference frame the object in question exists in. Similarly, the 0th moment shows where HI is in the galaxy in question…though it also shows how much HI there is in different parts of the galaxy. For the 1st moment, the analogy is more direct. The 1st moment is like the physics concept of velocity. Where velocity quantifies the rate of change of position for any object, the 1st moment similarly quantifies the rate of change of position for the HI in a galaxy. The 2nd moment, however, does not have such an easy analogy as the 0th and 1st moments. However, given that the 2nd moment corresponds to velocity dispersion, this means that the 2nd moment refers to how much the velocity of the HI changes from the average velocity of the gas at a specific location in the galaxy.

Top row, from left to right: an image showing the stellar distribution of UGC 6456, an image of UGC 6456 in the H-alpha filter, and an image showing the H-alpha line widths for UGC 6456. Bottom row, left to right: the 0th moment map for the HI of UGC 6456, the 1st moment map for the HI of UGC 6456, and the 2nd moment map for the HI of UGC 6456. Image Credits: Laura Congreve Hunter

Now, why study these pint-sized galaxies? If you’ve ever had the pleasure of gazing upon images of dwarf galaxies, you’ll know that they often don’t look nearly as photogenic as their gigantic cousins and that trying to locate them in scientific images is like playing a cosmic game of Where’s Waldo?. However, as it turns out, smaller galaxies have an incredibly important property that aids in studying the events surrounding star formation: solid body rotation. Solid body rotation is like riding a merry-go-round. If you and your best friend were sitting on a merry-go-round at different distances from the center, you would rotate at different speeds, and if you and your friend were also in a straight line, you would remain in that straight line. Solid body rotation therefore allows the different parts of an object to rotate together as one unit. This has important implications for studying the events of star formation in galaxies. When things rotate–especially things that aren’t solid–mixing of constituent materials often is involved. Star formation generally happens over cosmologically short time scales, and when mixing happens, regions with different star formation histories inevitably mix together. Like mixing milk into tea or creamer into coffee, it becomes harder and harder to separate out the different components of the mixed region as mixing happens. While not usually problematic when studying galaxies globally, when studying localized regions of galaxies as Laura and her collaborators do, this becomes a problem very quickly. Fortunately, solid body rotation minimizes this mixing and thus makes it easier to sleuth around a galaxy and figure out what happened.

While her current research project is still ongoing, Laura has many aspirations for the potential of her project. Her primary aspiration for the project is that it will shed more light on the evolution of galaxies. She hopes that by understanding the connections between star formation and turbulence, astronomers will be able to understand on what time scales star formation adds energy to the HI reserves of a galaxy. In addition, since stellar feedback (i.e. supernovae, hot stellar winds from post-main sequence stars) is thought to be a primary driver of turbulent motion in the ISM, she hopes to also contribute to the understanding of how stellar feedback affects the ISM. By understanding these processes, astronomers will be able to make better models of galaxies and their inner workings. However, regardless of whatever she and her collaborators end up discovering about the intra-galactic turbulence, we will be one step closer to understanding how this universe we live in works.

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