Green Pea Galaxies | More Than Just a Photographer of Pretty Faces

As I logged into Zoom for my interview with Samantha Brunker, I was as excited as a kid in a candy store. Samantha and I knew each other from my undergrad career at Indiana University. We were both part of what I affectionately called the “Salzer mafia”. It was a humorous term I’d coined for the group of students who did research with the incredibly iconic Dr. John Salzer.

Over my four years at Indiana University, I got to know Samantha via mutual interests in baking and extragalactic astronomy. While we hadn’t studied the exact same research questions, I knew that she had a deep passion for the “Green Peas”. I’d learned of her interest in these strange galaxies when I attended the Department of Astronomy’s honors colloquium during my junior year of college. A fellow Salzarian undergraduate had worked with Samantha to understand their properties. The following academic year, I learned that Samantha had gotten time on the Hubble Space Telescope (HST) to study the Green Peas. Since she was doing science with a famous telescope, I knew that I had to get the low down on her current research.

A picture of Hubble from a servicing mission. Image Credit: NASA

What is a “Green Pea”?

So, what is a “Green Pea”? I’m sure that the first thing that comes to your mind is the legume that sometimes gets served with meals. However, in astronomy, this term takes on a new meaning that references their nutritious namesake. Green Peas are galaxies that appear as green, compact objects without much structure in their optical images, and despite their compact nature and small appearance in ground-based images, they come in a variety of sizes, including both dwarf galaxies and normal sized galaxies. Scientists first discovered Green Peas in the well-known citizen science project called Galaxy Zoo. They started Galaxy Zoo in 2007 as a way to save professional astronomers research time by enlisting the general public to aid them in the simple task of morphologically (in layman’s terms, by form or structure) classifying hundreds of thousands of galaxies.

While participating in Galaxy Zoo, some citizen scientists discovered these apparent point sources that appeared shockingly green in their images. In astronomy, galaxies exhibiting green colors are incredibly unusual. Objects, such as stars, usually emit light in a range of wavelengths…kind of like viewing the rainbow of colors exiting a prism after shining white light onto it, with blue on the short end of the spectrum and red on the long end. The ultimate color that we see is based on where in the spectrum the peak emission exists.

However, when you have a collection of such objects (like a galaxy), you will find that the overall color of the aggregate object will usually either be red (when longer wavelengths dominate), blue (when shorter wavelengths dominate), or white (when the shorter and longer wavelengths are about equal in their dominance). As such, to find galaxies that strongly emitted green light was a surprise. The citizen scientists immediately made a chat to discuss these strange findings. Eventually, one of the astronomers on the Galaxy Zoo project decided to actually investigate the Green Peas. Subsequent research led to the discovery that strong OIII (astronomer speak for “doubly ionized oxygen”) emission–an indicator of star formation–was the main culprit behind their unusual green color.

A diagram of the entire electromagnetic spectrum showing where each type of light falls on the spectrum and its corresponding wavelength range. Image Credit: Columbia University

Love at First Sight

Samantha herself fell in love with these quirky galaxies during her undergrad tenure at the University of Kansas. One year, for her birthday, Samantha’s family gifted her a subscription to Astronomy magazine. It was here that she first discovered mention of the Green Peas in a small blurb. The small blurb was not enough to quell her curiosity. To her good fortune, the mention of the Green Peas also happened to mention an astronomy professor at the University of Kansas who studied them with the HST. During a colloquial department gathering, she approached the professor to inquire more about the Green Peas. What she learned from the professor heightened her interest in them. The information also inspired her to work with him on similar subjects for the rest of her undergrad career.

When she got to grad school, Samantha still harbored interest in the Green Peas, but she wanted to keep her options open because finding projects that exactly fit your research interests right off the bat in grad school is unlikely. However, in a stroke of amazing luck, Samantha was able to find this. While meeting with professors at Indiana University, she fortuitously discovered that Dr. Salzer had some Green-pea-like galaxies in a data set he had acquired for his KPNO International Spectroscopic Survey (KISS). These Green-Pea-like galaxies were at redshifts of 0.3-0.5, so they were slightly farther away than the original Green Peas (redshifts of 0.2-0.4). These redshifts make these KISS galaxies at least a few billion light years away. Though that’s not a cosmically far distance in the grand scheme of things, life there would see Earth as it was when bacteria was the dominant life form! Mind-blowing, right?

Dr. Salzer desired to compare these Green-Pea-like galaxies to the actual Green Peas, so the opportunity to work with galaxies like the ones that had captured Samantha’s imagination added to her eagerness to start research. Her good rapport with Dr. Salzer was the icing on the cake. They wanted to focus on studying star formation in these Green-Pea-like galaxies, so during Samantha’s first year as a grad student, they applied for Hubble time in order to get optical broadband (astronomer speak for a filter that lets in a relatively large range of wavelengths of light) imaging to better resolve the galaxies from the KISS data set and to make color gradients to potentially find out where the star formation was taking place in these galaxies.

A Telescopic Gold Mine

Acquiring Hubble time is like acquiring gold: not easy. Due to a variety of factors, applying for Hubble time is an extremely competitive process, so they didn’t end up getting Hubble time that year. Fortunately, Dr. Salzer and Samantha realized that Green Pea galaxies were also interesting in the ultraviolet (UV, for short) spectrum. For many years, astronomers have had difficulty in understanding how ionizing radiation (which usually appears in the UV) escapes from galaxies. They knew that in the early universe, much of the existing gas was neutral. The young, hot stars produced ionizing radiation that escaped their host galaxies to ionize what’s called the intergalactic medium. Unfortunately, because these early galaxies are far away, it is hard to get clear observations of them.

Fortunately, the Green Peas are simultaneously nearby and like their early universe counterparts: they have lots of star formation, are metal (in astronomer speak, any element other than hydrogen or helium) poor, and have measurable ionizing radiation leaks. By then, astronomers had confirmed that the KISS Green-Pea-like galaxies were indeed Green Pea galaxies. The KISS Green Peas thus provided a fresh set of Green Peas to study.

After their failure to secure Hubble time for their optical research ideas for the Green Peas, studying the KISS Green Peas in the UV seemed like a great avenue to pursue further research into the Green Peas. Thus, Dr. Salzer and Samantha applied again for Hubble time–but in the UV this time instead of the optical. Much to the happy surprise of both of them, Hubble’s time allocation committees granted them the high distinction of being in the 181 successful proposals out of 1,019 total proposals that year (that’s about a 17.8% acceptance rate!). 

Hubble’s Surprising Talents

Alright, I know what you’re thinking. You’re probably thinking, “Hold up a second, Madeline! Isn’t Hubble an optical telescope?” Well, you would be correct in the assertion. Stunning optical observations are what have made Hubble famous to the general public. However, I share your shock that Hubble does much more than taking many of NASA’s pretty pictures. One of Hubble’s surprisingly many talents is doing UV spectroscopy with the Cosmic Origins Spectrograph (COS) instrument. COS is a UV spectrograph, an astronomical instrument used to measure the properties of light–particularly to determine the chemical makeup of an object. This particular spectrograph provides astronomers with both UV images (used to help Hubble acquire objects for observation) and UV spectra. Astronomers can then see the object as it appears in the UV and examine the chemical makeup of the object. 

An image of the Cosmic Origins Spectrograph (COS) in the laboratory before installation on Hubble. Image Credit: NASA

Perhaps more shocking than the existence of Hubble’s UV capabilities, it’s the only US-based telescope able to make UV observations. The UV part of the electromagnetic spectrum is actually blocked by our atmosphere. While fortunate for our skin due to UV light’s ability to damage skin, this is unfortunate for UV astronomers. This scientific fact means that if astronomers want UV observations, they must be done from space. Thus, Hubble’s COS instrument fills in a much needed gap in observing capabilities.

However, Hubble is an aging telescope at 30 years young. Due to orbital decay, it is expected to reenter the atmosphere some time in the 2030s or 2040s. Astronomers across the field naturally want to make the most of this world-class telescope before it dies a fiery death. Therefore, Hubble’s unique UV capabilities and impending demise by orbital decay give astronomers with UV projects an edge in acquiring precious Hubble time.

An Astronomical A-Team

Samantha and Dr. Salzer were awarded 25 orbits of Hubble time–5 orbits each of the KISS Green Peas they wanted to observe. To make use of her Hubble time, Samantha assembled an A-Team like no other to tackle the job. First, of course, was Dr. Salzer. Besides already being familiar with the galaxies to be observed due to his work with KISS, Dr. Salzer is, among other things, an expert in extragalactic astronomy generally, optical spectra, emission line galaxies, and surveys. The emission line galaxy expertise and his specific extragalactic star formation experience was particularly useful. This is because of the OIII prominence in Green Pea spectra and their connection to extreme star formation.

Next, she invited Professor Danielle Berg of UT Austin to join the team. Professor Berg is an expert at working with COS data and UV data in general. Samantha had met Professor Berg when she gave a colloquium at Indiana University about COS while she and Dr. Salzer were writing their second Hubble proposal. They knew Professor Berg’s expertise would be invaluable in helping them decode the data from COS, so they invited her to join them.

Finally, Samantha recruited Professor Aparna Venkatesan of the University of San Francisco. Professor Venkatesan is a theorist specializing in how ionizing radiation escapes galaxies and interacts with the mediums it encounters. Samantha had also met her before as she was a collaborator of Dr. Salzer’s, but Samantha had never had the opportunity to work with her prior to this project. She and Dr. Salzer knew that including Professor Venkatesan would allow them to attack the question of how ionizing radiation escapes from observational standpoints and theoretical and modeling perspectives. 

Eyes on the Sky and Eyes on the Future

With her powerhouse trio, Samantha is currently unlocking the extragalactic secrets encoded in the UV spectra from COS. Many questions remain about the Green Peas. These questions include questions about their structure, the spatial nature of their star formation, why the OIII emission dominates, and their appearances in the cosmological present. For now, though, Samantha and her team are focusing primarily on how the UV ionizing radiation escape rate changes related to the masses, emission line strengths, and star formation rates of the Green Pea galaxies. As of this article, the team still has only gotten observations for four out of five of their galaxies. Therefore, they’ve got lots of work before they get a clearer picture of the KISS Green Peas.

However, you can stay tuned for glimpses of their Hubble research (and other ground-breaking Hubble research). You can do this by following @spacetelelive and @nasahubble on Twitter for sneak peeks into what they’re studying before they release their findings.

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